Geomorphology Giles County Virginia

8/2/2019 Geomorphology Giles County Virginia

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CONTENTSPage Page

Abstract - - - - - - - . - - - - - - - - - - - - - - - - - - - - - - - - . - - - - - -Introduction - - - - - - - - - - - - - - - . - - - - - - - - - - - - - - - - - - - -Setting of the study area - - - - - - - - - - - - - - - - - - - - - - - - - - -

Bedrock geology and topography- - - - - - - - - - - - - - - - - -Present and past climate - - - - - - - - - - - - - - - - - - - - - - - -Forest vegetation . - - - - - - - - - - - - - - - - - - - - - - - - - - - -

General surficial geology and geomorphology - . . - - - - - - - - -Previous mapping of sur ficial geology in the

unglaciated Appalachians - - - - - - - - - - - - - - - - - -Surficial deposits and their distribution - - - - - - - - - - - - -Distribution of surficial deposits in four areas - - - - - - - -

Doe Creek drainage basin - - - - - . - - - . - - - - - - - - - -Little Stony Creek basin below the Cascades - - - - - -Little Stony Creek basin above the Cascades - - - - - -North slope of Big Mountain- - - - - - - - - - - - - - - - - -

Discussion - - - - - - - - - - - - - - - - - - - - - - - - - . - - - - - - - -Sedimentology of colluvium - - - - - - - - - - - - - - - - - - - - - - - . -Previous work - - - - - - - - - - - - - - - . - - - - - - - - . - - - - - - -Differentiation of colluvial environment and age by

sedimentologic and weathering criteria - - - - - - - -Particle-size analysis- - - - - - - - - - - - - - - - - - - - - - - -Clast roundness - - - - - . - - - - - - - - - - - - - - - - - - - . -Clast fabric - - . - - - - - - - - - - - - - - - - - - - - - - - - - - .Weathering indices - - - - - - - - - - - - - - - - - - - - - - - -

122255667

10101516191924242526272932

Sedimentology of colluvium-ContinuedDifferentiation of colluvial environment and age bysedimentologic and weathering criteria-Continued

Weathering rinds of Tuscarora Quartzite clasts - - . -Vertical variation in weathering characteristics - - - -

Discussion - - - - - - - - - - - - - - - - - - - - - - .. . - - - - - - - - -Hillslope evolution model and colluvium - - - - - - - - .Ages of the deposits - - - - - - - - - - - - - - - - - - - - - - - -Transport processes - - - - - - - - - - - - - - - - - - - - - - - -

Geomorphology and sedimentology of boulder streams - - - - -A genetic classification of boulder streams- - - - - - - - - - -Procedure and sedimentologic considerations - - - - - - - - -Possib le type I deposits: Products of

gravity processes - - - - - - - - - - - - - - - - - - - - - - -Possible type II deposits: Products of

periglacial processes - - - - - - - - - - - - - - . - - - - - .Possible frost-heave deposits- - - - - - - - - - - - - - - - - -Possible gelifluction deposit s - - - - - - . - - - - - - - - - -

Possible type II I deposits: Products of debris flows - - - -Observations on relative ages of the deposits - - - - - - . - -Discussion - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Surficial deposits and seismicity - - - . - - . - - - - - - - - - - - - - -Conclusions- - . - - - - - - - - - - - - . - - - - - - - - - - . . - - - - - - - - -References cited - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

ILLUSTRATIONSPage

FIGURE 1. Index map of study area - - - - - - - - - - - - - - - . - - - - - - - - - - - - - - - - - - - - - . - - - - - - - . - - - - - - - - - - - - - - - - - - - - - - 32. Bedrock map of study area and locations of study sites- . - - - - - - - - - - - - - - - - - - - - - - - . - - - - - - - - - - - - - - - - - - - - 53. Map showing surficial geology of northern part of Eggleston quadrangle- - - - - - - - - - - - - - - - - - - - - - - - - . - - - - - - - 84. Oblique aerial photograph showing crest and south slope of Bean Field Mountain - - - - - - - - . - - - - - - - - - . - - . - - - - 115. Oblique aerial photograph of upper end of Doe Creek drainage basin, showing location of various types of regolith - - 126. Photograph of road cut through shale nose capped by sandstone-rich colluvium- - - - - - - - - - - - - - - - - - - - . - - - - - - - 137. Photograph of south slope of Doe Mountain, showing large V-shaped hollows - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 148. Diagram showing boulder abundance as it relates to the location of Tuscarora Quartzite outcrops - - - - - - . - - - - - - - 14

9-12. Photographs of:9. Southwest end of Doe Mountain, showing relict sandstone-rich colluvium - - . - - - - - - . - . - - - - - . . - - . - - - - - - - 1510. Boulder stream on northwest flank of Doe Mountain - - - - - - - - - - . - - - - - - - - . - - - - - - - - - - - - - - - - - - - - - - - - 1611. South flank of Butt Mountain as viewed from crest of Doe Mountain - - - - - - - - - - - - - - - - - - - - - - - - . - - - - - - - 1712. View along steep northwest flank of Doe Mountain- - - - . - - - - - - , - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 1813. Map showing surficial geology of southern part of Interior quadrangle - - . - - - - - - - - - - - - - - - - - - - - - - - . - - - - - - - 2114. Low oblique aerial photograph showing flatiron on south (dip) slope of Big Mountain- - - . - . - - - - - - - - - - - - - - - . - - 2215-17 . Photographs of:

15. Pond Drain fan and vicinity from near crest of Big Mountain - - - - . - - - - - - - - - - - - . - - - - - - - - - - - - - - . . - - - - 2316. Road cu t through matrix-free boulder stream on north side of Big Mountain- - - - - - - - - - - - - - - - - - - - - . - - - - - 2417. Road cu t through colluvium on north slope of Big Mountain- - - - . - - . - - - - - - - . . - - - - - . - . - - . - . - - - - - . . - - 2518. Oblique aerial photograph of north end of Mountain Lake - . - - . - - . . . . ... - - - . . - - ..... - . - . . - - - - .. . - - . . . 2619. Ternary diagram showing percent sand , silt , and clay of colluvium samples . . - . . - - . . . . - ... - - - - . . .. . - - . . . . 2820. Graph showing average cumulative particle-size curves for each colluvial environment- . - - . - . . . - - - - . . - - - - . . - . 2921. Ternary diagram showing percent gravel, sand , an d silt+clay in each colluvial environment - - - - - - - - . . - - . - . - - - 3D22. Graph showing mean pebble roundness as a function of distance from mountain crest - ... - - - - - - . - - . . - . - . - - - . 3123. Composite contoured equal-area stereographic nets for each colluvial environment- - - - ...... - - - - - - ... - - . . - - - 32

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IV CONTENTS

24. Equal-area stereographic net showing the azimuth and dip of VI (the axis of maximum concentration) foreach fabric sample having a significant 51 value- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 3325. Histogram showing the distribution of fabric strength for each environment, as indicated by the 51 value,for each site - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 3426. Schematic diagram of colors an d combinations of weathering rinds observed on sandstone clasts - - - - - - - - - - - - - - - 3527. Sketch of wall of soil pi t at site 6D - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 3628. Diagrams showing the most commonly observed fabric patterns of slope deposits, plotted as contoured,equal-area stereographic nets - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 3929. Zingg diagram of boulder shapes at eight sites - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 4030. Photograph of type A boulder stream near site 156- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 4131. Surface-boulder fabrics at sites 156 and 137 (type A boulder streams), presented as contoured, equal-area

stereographic nets - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 4232. Photograph of type B1 boulder stream at site 141- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 4333. Surface-boulder fabrics at sites 100, 141, 142, and 154 (type B1 boulder streams), presented as contoured, equal-area

stereographic nets - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 4434. Photograph of boulder stream at site 100 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 4535. Longitudinal topographic profile of boulder stream at site 100, showing boulder size at 3-m intervals - - - - - - - - - - - - 4636. Transverse topographic profile of boulder stream at site 100, showing boulder size at 0.5-m intervals - - - - - - - - - - - - 4737. Sketch made from close-up photograph of exposure at site 100 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 4838. Sketch of entire exposure at site 100, showing clast fabrics at four locations - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 4839. Photograph of boulder stream at site 75 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 4940. Photograph of boulder stream having convex-up transverse t opograph ic profile (site 140)- - - - - - - - - - - - - - - - - - - - - 5041. Surface-boulder fabrics at sites 75, 140, 151, an d 153 presented as contoured equal-area stereographic nets - - - - - - - 51

TABLESPage

TABLE 1. Bedrock stratigraphic units in the study area- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --- - - - 62. Sedimentary characteristics, or values, of colluvium from five environments defined in the study area - - - - - - - - - - - 273. Significant differences between colluvium in different environments - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 274. Weathering differences between A ] and B2 soil horizons on colluvium-capped noses - - - - - - - - - - - - - - - - - - - - - - - - 355. Changes in weathering characteristics with depth at site 65 - - - - - - - - - - - - - - - - - - - - - - - --- - - - - - - - - - - - - - - - - - 366. A-B plane dip distributions in boulder fabrics (n=50)- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 457. Percent cracked boulders at selected sites- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 518. Comparison between types of boulder streams - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 52


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SURFICIAL GEOLOGY AND GEOMORPHOLOGY OF THEMOUNTAIN LAKE AREA, GILES COUNTY, VIRGINIA,

INCLUDING SEDIMENTOLOGICAL STUDIES OFCOLLUVIUM AND BOULDER STREAMS

By HUGH H. MILLS!

ABSTRACTExtensive areas of shale and limestone bedrock in a small uplandarea in the Valley and Ridge province are overlain by sandstone-richcolluvium derived from mountain caprocks. Sand stone boulders tendto be concentrated in hollows rather than on noses and are more

numerous in moister than in drier hollows. Some of the colluvium isvery old, as indicated by it s presence on hilltops or in piedmont areaswhere presently there are no sandstone outcrops upslope. The distribution of th is relict material indicates how the landscape has evolvedthrough time, particularly in regard to the retreat of sandstoneescarpments. The gross features of the topograph y are controlled bythe general structure and lithology of the bedrock. Detailed mappingof surficial deposits, however, shows more subtle influences of bedrock characteristics on topography. For example, resistant bouldersderived from sandstone outcrops affect the form of shale slopes thatthey overlie, both by influencing the form of hollows incised into theslopes and by determining the amount of retreat that the slopesundergo due to later al erosion by streams at their bases.The study of colluvium on mountain flanks in this area shows thatfive environments of colluvium deposition can be differentiated onthe basis of texture, color, clast weathering, pebble roundness, andclast fabric: upper, steeply sloping noses nearer the mountain crest;lower, gently sloping noses farther from the crest; moist hollows nearthe mountain crest; dry hollows or those farther from the crest; andside slopes or other steep slopes. Sediments in the two nose environments are distinguished from those in other environments by moreintense weathering (as indicated by finer texture, redder hue, and agreater percentage of weathered clasts), with weathering in the lowernose setting being especially intense. Sediments in the two hollowenvironments are distinguished from each other by clast fabric, withsediments in moist hollows near the moun tain crest showing muchmore consistent downslope orientation of clast long axes.The differences in degree of weathering appear to result in largepart from differences in age, the colluvium on lower noses beingoldest, that on upper noses being younger, and that in hollows and onside slopes being youngest. Age differences probably reflect themanner in which mountainslope topography evolves. The most rapiddownslope movement of colluvium occurs in hollows; colluvial caps onnoses commonly are former hollow fills that reached their prese,ltpositions by topographic reversal. Once on noses, the relict colluviumceases to move until eventually eroded by a laterally migratinghollow, and therefore at any given time it usually is older and moreweathered than the colluvium in hollows.

1 Tennessee Technological University. Department of Earth Sciences, Cookeville, TN 38505.

Differences in clast r oundness and fabric app ear to be related moreclosely to differences in transport distance and process than todifferences in weathering. Clast roundness reflects mainly the distance of transport. Consistency of particle orientation probablyindicates whether deposition was by laminar or turbulent flow.Colluvium with highly consistent particle orientatio n (indicative oflanlinar flow) may be a relict gelifluction deposit, whereas that lacking such orientation (indicative of turbulent flow) may be a debrisflow deposit produced by a catastrophic flood. Estimates of the agesof colluvial deposits based on the degree of weathering suggest thatthe hollow and side-slope colluvium is no older than late Wisconsinan,whereas at least some of the lower-nose colluvium probably is preIllinoian.

Concentrations of sandstone boulders in hollows, here referred toas boulder streams, extend as much as 1 km downslope from outcro psthat produce them. On the basis of fabric and other properties,boulder streams were divided into four types. The first type occurs onslopes having gradients of about 35, and boulders here are easilydisturbed by stepping on them. Tabular boulders tend to be inclineddownslope at an angle less than the slope of the surface on which theyrest. Boulders are probably transported by sliding and rolling, andtheir instability and angularity suggest that these deposits are quiteyoung. The second type of boulder stream rests on slopes that havegradients ranging from 8 to 30, are stable underfoot, and haveirregular surfaces containing pits probably formed by eluviation offines. Fabri c analysis shows an exceptionally large number of tabularboulders with near-vertical orientations. As frost heave is the onlyprocess known to orient large clasts in this manner, it is inferred thatsuch streams are relicts of Pleistocene periglacial regimes. Somesupport for a Pleistocene age is provided by the fact that thesedeposits generally appear to bemore weathered and stable than otherboulder deposits and also tend to be located away from moderndrainageways. The third and fourth types have a gradient rangesimilar to that of the second type bu t have a matrix of fines, arelatively smooth surface, and tabular boulders that tend to dipdownhill at an angle less than that of the slope angle. The propertiesof these boulder streams are consistent with an origin both bygelifluction and by debris flow. I t s hypothesized that the third typeof boulder stream, which is underlain by colluvium having consistently oriented clast s and which occurs near mountain crests, is relictgelifluction deposits of Pleistocene age, whereas the fourth type,which shows poorly developed fabrics and which is located far fromthe crests, is debris-flow deposits of Holocene age.

No evidence of faulting or liquefaction that might be indicative ofQuaternary seismicity was found in the regolith.

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2 SURFICIAL GEOLOGY AN D GEOMORPHOLOGY, GILES COUNTY, VIRGINIAINTRODUCTION

Compared with many other regions, the cen tral andsouthern Appalachian region has quite restricted lateCenozoic deposits. Most surficial deposits in this areahave never been mapped because of their small size andlimited economic potential. The terrace deposits of afew larger rivers have been studied by Quaternarygeologists because the lower terraces may be corre latedand often yield datable organic material. Colluvium,however, although the most widespread deposit in theregion, has received almost no attention. Geologistsgenerally have assumed that the distribution and characteri stics of colluvium can be deduced from knowledgeof the underlying bedrock and that this material haslittle significance except for soil scientists. This studyof a small area in the vicinity of Mountain Lake, Virginia, attempts to show that detailed mapping of colluviuin and other upland surficial deposits can make asignificant contribution to the understanding of landscape development.The sedimentary characteristics of colluvium andboulder streams are also examined in this paper. Colluvium has been investigated in some detail in periglacialenvironments. Because corresponding studies oncolluvium from nonperiglacial environments have beenfew, however, exactly which sedimentary propertiescan be used to identify periglacial slope deposits remainunknown, despite the importance of this question for amore complete understanding of Quaternary geology.Another important characteristic of colluvial depositsis age. Although age measurements are rare for colluvium, relative ages, determined by a study of the weathering features of deposits, can readily be obtained andare useful for the study of hillslope evolution. In thisstudy, two aspects of colluvium were examined: (1) thevariation in degree of weathering and other sedimentary properties among different topographic settingsand (2) sedimentary evidence for transport and deposition by catastrophic floods and periglacial processes.Bouldery scree is common in the study area, particularly in hollows that head below sandstone outcrops.("Scree" herein means unconsolidated rock fragmentson a mountain slope. "Talus" is a landform composed ofscree, particularly at the base of a ledge or cliff.) Concent rations of boulders in hollows are referred to hereinas "boulder streams." Many investigators have suggested that widespread scree deposits are evidence forperiglacial activity in the southern and central Appalachians during glacial stages of the Pleistocene (forexample, King, 1964; Fiedler, 1967; Michalek, 1968;Haselton, 1973; Godfrey, 1975; Hedges, 1975). Hack(1960, 1965), however, provided evidence that at leastsome scree is being produced during present climaticconditions, and he suggested that scree develops wher-

ever slopes are steep and the bedrock is mechanicallystrong and resistant to chemical weathering, regardless of climate.Boulder streams might be relics of transport by periglacial frost action or gelifluction bu t probably alsocan be deposited by catastrophic floods that occurduring rare, extremely intense rainfalls. The approachused to interpret these features was to compare thecharacteristics of boulder streams in the MountainLake area with those reported for boulder deposits ofknown origin elsewhere.

In the discussion below, "periglacial" is used in asense similar to that of Washburn (1980, p. 4) to designate processes and features of cold climates characterized by intense frost action. The assumption is madethat in southwest Virginia such conditions could nothave existed after the Pleistocene glacial climates,which ended there about 13,000 to 15,000 yr B.P.(Watts, 1979). "Gelifluction" is used in the sense ofBaulig (1957) as solifluction associated with frozenground, including seasonally frozen ground as well aspermafrost.

SETTING OF THE STUDY AREA

BEDROCK GEOLOGY AND TOPOGRAPHYThe study area is in the vicinity of Mountain Lake inGiles County, in the Valley and Ridge province of southwestern Virginia (fig. I ). A stratigraphic column for the

study area is shown in table 1, and a simplified geologicmap is presented in figure 2.Structurally, most of the study area is characterizedby broad, open folds. The southernmost fold, the JohnsCreek syncline, trends N. 60 0 E. and plunges gentlynortheast; dips are somewhat steeper in the southeastlimb. Northwest of this fold is the Mountain Lakeanticline, having a similar trend and plunge, and themore complex Butt Mountain synclinorium (Eckroade,1962). This structure trends N. 65 0 E. and near thecrest of Butt Mountain plunges 10 0 to 20 0 to thenortheast. In the study area, this structure is composedof one major depression and several minor folds. Thesynclinorium is asymmetrical along the upper reach ofLittle Stony Creek, with the northwest limb dipping35 0 and the same units on the southeast limb dipping15 o. Two high-angle faul ts repeat parts of the Juniata,'fuscarora, Rose Hill, and "Keefer" Formations (fig. 2).On the north flank of Big Mountain is a northwestwardoverturned anticline, with the southeast limb dippingabout 20 0 and the northwest limb about 45 o. Becauseof this fold and the high-angle fault, both the 'fuscaroraQuartzite and "Keefer" Sandstone are repeated three


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SETTING OF TH E STUDY AREA 3

Area ofVIRGINIA

( ' ~ i 9 > 2 A I) A:ea of'" Fig.",- - -7" ' ... GILES

, COUNTY" ............VIRGINIA " / ' Ani"a ofV Fig.28

Interior 7.5-minutequadrangle IEggleston 7.5-minute

I /quadrangle

Newport 7quadrangle

INSET01

25 MILES1------.-1 ---- ' Io 25 KILOMETERS

Blue Ridge Province

)V I R ~ : ~ ' ~ ~ ~ f ~ I"

i::!"~ ~ ~ ~ ~ ' l

---------------- --------------

oI 1o 5050

I1

100 MILESI100 KILOMETERS

FIGURE I.-Index map of study area. Stu dy area is shaded. Also shown are those parts of the study area that were mapped.

times on the north slope of Big Mountain. Near the baseof this slope is a minor syncline.The study area lies immediately northeast of the NewRiver, which constitutes the local base level. Salt PondMountain, the highest peak in the area (elev. 1,326 m),is 836 m higher than the New River. The major mountains in the area (fig. 2) are strike ridges. Numerousfirst-order valleys corrugate the flanks of these ridges.Hack and Goodlett (1960) divided such corrugatedridge-flank topography into hollows, noses, and sineslopes. Hollows are the central parts of first-order valleys, where the contours are concave downhill. Nosesare the divides between hollows, where the contours areconvex downhill. Side slopes are the areas betweennoses and hollows characterized by straight contours.On the higher mountain slopes the alternation between

hollows and noses is fairly regular bu t is somewhat lessregular on the lower slopes (fig. 2).The three largest streams in the map area are StonyCreek, Little Stony Creek, and Doe Creek (fig. 2). Onlya short reach of Stony Creek lies in the study area, sothis stream will not be discussed. The remaining twostreams flow at elevations more than 600 m below theelevations of the ridge crests. The valley of Little StonyCreek is much narrower and steeper walled than that ofDoe Creek. The reason for this difference seems to bethe very steep gradient of Little Stony Creek downstream from the waterfalls known as the Cascades (fig.2), which are formed by two prominent cliffs, anupstream one in the "Keefer" Sandstone and a downstream one in the Rose Hill Formation. Gradients thissteep are confined to the upper 2 or 3 km of Doe Creek


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OmOhOrgoSkSrhStSsOJOme

OlsOs

UD

oV'oo

SETTING OF TH E STUDY AREA 5

EXPLANATIONMillboro Shale (Devonian)Huntersville Chert (Devonian)

80"27 '30' ''

Rocky Gap Sandstone of Swartz (1929) (Devonian)Devonian units , undivided-Includes Millboro Shale ,Huntersville Chert, and Rocky Gap Sandstone"Keefer" Sandstone (Silurian)Rose Hill Formation (SilUrian)Tuscarora Quartzite (Silurian)Silurian sandstones ot her than Tuscarora QuartziteIncludes "Keefer" Sandstone and Rose Hill FormationJuniata Formation (Ordovician)Martinsburg, Eggleston, and Moccasin Formations(Ordovician)Ordovician limestones, undivided (Middle Ordovician)Ordovician sandstones, siltstones, and shales-IncludesJuniata, Martinsburg, Eggleston, and Moccasin Formations

EXPLANATION OF MAP SYMBOLSContact-Approximately locatedHigh-angle fault-Concealed , U, upthrown side; D,downthrown sideAnticline-Showing direction of plunge where knownOverturned anticline-Showing direction of dip of limbsSyncline-Showing direction of plunge where knownStrike an d dip of beddingInclined

Horizontal

AlluviumTYPES OF SAMPLE

LOCATIONSType D hollowsType M hollowsSide slopesUpper nosesLower nosesBoulder streams

FIGURE 2.-Simplified bedrock map of study area, modified fromEckroade (1962). On the north flank of Big Mountain, contacts areso closely spaced that formations have been combined. Locationsof study sites are shown; only those sites referred to individually inthe text are numbered. Often, multiple samples were collectedat one site. Sites in the Newport quadrangle, which is outsidethe study area, are shown on the inset map. A few sites in theEggleston and Interior quadrangles were outside the map areashown here.

and other streams in the region, bu t Little Stony Creekis already 10 km long where it reaches the Cascades. AsHack (1973) has stated, the power of a stream is approximately proportional to the product of the local gradient and the length of the stream, where length is measured from the d rainage divide at the head of the streamto the center of the reach for which the gradient iscomputed. Thus, the power of Litt le Stony Creek in thereach below the Cascades is much greater than thepower of Doe Creek in its headwaters reach, even thoughthe gradients of the two reaches are similar. This relatively great power enables Little Stony Creek to transport much larger boulders than Doe Creek and alsoprobably allows it to downcut more rapidly.

PRESENT ~ ~ D PAST CLIMATETemperature and precipitation records have beenmaintained at the Mountain Lake Biology Station (elev.1,180 m) on the east side of the study area. From 1972to 1979, the mean annual temperature was 8.1 C, and

the mean annual precipitation was 1,436 mm. The meantemperature for January was -3.3 C, although sustained interva ls of somewhat colder temperatures haveoccurred occasionally. For example, the mean temperature for January 1977 was -lOA C. The precipitationis fairly evenly distributed throughout the year, thedriest month being February (mean precipitation 87Amm) and the wettest, June (170.9 mm).Climate during the late Wisconsinan probably was

I much colder than at present. Watts (1979), on the basisof pollen and plant macrofossil studies, concluded thatduring this time sedge tundra covered the higher mountains of the central Appalachians. Clark (1968, and oralcommun., 1979) excavated a large-scale stone stripe onBig Mountain in the present study area and found it toresemble closely features actively forming today onlyin permafrost regions. Hence, it is likely that the meanannual temperature here at the height of the last glaciation was 0 C or below.

FOREST VEGETATIONHack and Goodlett (1960) noted that xeric (low-

moisture) tree types characterize noses, mesic


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6 SURFICIAL GEOLOGY AND GEOMORPHOLOGY , GILES COUNTY , VIRGINIATABLE I.-Bedrock stratigraphic units in the study area

Sys tem Formation

Devonian - -- Millboro ShaleDo. -- -- -- Huntersville ChertDo. -- - -- - Rocky GapSandstone ofSwartz (1929).Do. -- -- -- TonolowayLimestone.

Silurian -- - - "Keefer" SandstoneDo. - -- -- - Rose HillFormation.Do. -- -- -- 'fuscarora Quartzite

Ordovician -- Juniata FormationDo. ------ MartinsburgFormation.Do. - - - - -- Eggleston

Formation.Do. -- -- -- MoccasinFormation.Do. - - -- - - Middle Ordovicianlimestone.I From Eckroade (1962).' From Ovenshine (1961).

Lithologiccharacte r

Black fissile shaleBlocky white chertCalcareous friablesandstone.Coarse- tofine-grainedfossiliferouslimestone.OrthoquartziteHematite-cemented

sandstone andshale.Orthoquartzite,locallyconglomeratic.Sandstone, siltstone.and shale.Siltstone, 'shale, andargillaceouslimestone.Calcareous shale andshaly limestone.Mudstone andlimestone.Cherty, argillaceous,and pure limestone.

A i ~ [ o k " r ! : : t e (in meters)

160+125-30125-30

123

145145-60

' 15-45

160-1101425-490

29-32220-66

2229-396

(moderate-moisture) types the hollows, and intermediate types the side slopes. Other factors, however, including the orientation of the hollows and side slopes, thesize of the hollows, and the nature and attitude of thebedrock, also affect the forest type. Similar relationships were observed in the present setting. In moister,north-facing hollows, yellow birch (Betula aUeghani-ens is) and striped maple (Acer pensylvanicum) commonly are dominant, although in the wettest hollows,rosebay rhododendron (Rhododendron maximum) maydominate. In drier, south-facing hollows, basswood(Tilia) and sugar maple (Acer saccharum) are dominant,although in very dry hollows, oak (Quercus) and buckeye (Aesculus) may be as numerous or more numerousthan basswood, and maples may be scarce. Bogs arecharacterized by eastern hemlock (Tsuga canadensis)and, at higher altitudes, red spruce (Picea rubens).Noses are characterized mainly by oaks (Quercus) andhickories (Carya). The most common pine in the area iseastern white pine (Pinus strobus), which occurs mainlyin sandy areas.

GENERAL SURFICIAL GEOLOGYAND GEOMORPHOLOGY

The general nature and geographic distribution ofthe surficial deposits and the general geomorphology

are considered in this section. Detailed d ~ s c r i p t i o n s ofthe deposits will be undertaken in later sections.

PREVIOUS MAPPING OF SURFICIAL DEPOSITSIN TH E UNGLACIATED APPALACHIANS

Map unit differentiation of surficial rock units insettings such as that of the study area is difficult.Many types of deposits are too small in area to beshown on 1:24,000-scale maps. Consequently, on ly verygeneralized units can be used for mapping at this scale.In addition, grouping of deposits by age for the mostpart is somewhat arbitrary, as there are unlikely to bediscrete age groups such as those that occur in glacialand stream-terrace deposits, and radiocarbon dates orother age measurements commonly are not available.Pierce (1966) mapped four types of transported regolith in an area in Pennsylvania having bedrock similarto that in the present study area: (1) sandstone rubblewith a plentiful matrix on moderate slopes; (2) gravellyalluvium and sandstone rubble on valley bottoms; (3)modem fine-grained alluvium; and (4) roundstonediamicton. The latter consists of older, weathereddeposits characterized by rounded clasts in a reddishorange clayey matrix and lacking apparent sorting orstratification. Roundstone diamicton occurs on thelower slopes of mountains and on some outlying hills inPierce's study area. Pierce considered the rounds onediamicton to be highly weathered alluvium. He alsomapped residual regolith. However, although the manner in which residuum develops is an important problem, the spatial distribution of this material can bedetermined largely from bedrock maps. In contrast, thedistribution of transported regolith can be predictedonly partly from such maps. Because the distributionof transported regolith also can serve as a clue to thetransporting mechanisms and, in addition, may revealthe manner in which the landscape is changing throughtime, the study of this material was the focus of thisstudy.Fiedler (1967) mapped surficial deposits in the southeastern part of the present study area. His transportedregolith units consisted of scree, colluvium, and roundstone diamicton, the latter defined in a manner similarto the definition of Pierce (1966) . Scree, derived fromthe 'lliscarora Quartzite capping the mountains, occurson the upper slopes. Colluvium is found on the lowerslopes, and Fiedler noted that the boundary betweencolluvium and scree commonly is arbitrary. Colluviumusually occurs in hollows, whereas rounds one diamicton occurs on the lower ends of noses and locally on thetops of hills along the mountain foot slopes.

Houser (1980) mapped upland deposits in an areasoutheast of the present area using the map units


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GENERAL SURFICIAL GEOLOGY AND GEOMORPHOLOGYcolluvium, first-stage alluvium, and boulder streams.She defined colluvium as the slope deposits (mainlyboulders and cobbles) between the ridge crests and theheads of the hollows on the ridge flanks. The mapping ofthese deposits was done on the basis of topographiccriteria, using aerial photographs. First-stage alluviumconsists of those deposits, partly alluvial in nature,which occur in the upper and middle parts of the hollows; these deposits are derived from colluvium upslope.Accumulations of first-stage alluvium funneled intosteep-sided hollows are termed boulder streams.Surficial deposits in the Catoctin-South Mountainarea of Maryland as mapped by Godfrey (1975)included (1) boulder-covered slopes, (2) side-slope stonestreams, (3) valley-bottom stone streams, and (4) mountain wash at the base of the mountains. Denny (1956, pI.4) mapped the Genesee quadrangle of northern Pennsylvania. In the part of the map south of the glaciatedarea, his units included the following: (1) alluvium; (2)alluvial fans; (3) block fields; (4) thin mantles; and (5)periglacial deposits, where the latter was defined asrubble 3 m or more thick mantling lower valley walls.Hadley and Goldsmith (1963) included the followingsurficial units on their bedrock map of the Dellwoodquadrangle, North Carolina: (1) alluvium; (2) alluviumand colluvium; (3) alluvial-colluvial fan deposits(younger and older); and (4) terrace deposits (older andyounger).

From the above examples, some generalizations canbe drawn concerning surficial mapping in upland areasof the unglaciated Appalachians. First, there has beenan emphasis on boulder deposits, as these deposits arethe ones that can be most easily mapped on the basis ofsurface appearance alone. Second, mapping commonlyhas been based on a combination of topographic andsedimentary criteria. Third, mapping units on mountain foot slopes frequently have combined colluviumand alluvium, owing to the difficulty of distinguishingthem in this setting. Fourth, where age distinctionshave been made, they commonly have been ones ofrelative age, for measurement of age in years or correlation with deposits of known age elsewhere rarely hasbeen possible. Fifth , aside from those for alluvial deposits, the map units employed have been diverse. Eacharea has unique features, of course, so that units shouldvary. The great differences observed cannot simply beexplained in this manner, however. Rather, the explanation probably is that there is no obvious manner inwhich to subdivide the transported regolith. Colluviumshowing little or no fluvial influence gradually mergeswith alluvium; boulder deposits of one type commonlyshow gradual transitions to other types of boulderdeposits and also to colluvium. Subdivisions based onage are also somewhat arbitrary, as deposits often seem

to form a continuum from oldest to youngest rathethan to fall into discrete age groups. Well-defined terraces in headwater areas are rare.

An effort was made to see if soils maps would aid inthe mapping of surficial deposits. Soils maps of thestudy area (1981, Paul Swecker, U. S. Soil ConservationService, unpub. data) crudely indicate bedrock andsurficial conditions bu t are too generalized to portraymany geomorphologic ally signifi cant features. Soils omountainous areas such as the study area commonlyare mapped by the Soil Conservation Service in a generalized manner owing to their lack of agriculturapotential.

SURFICIAL DEPOSITS AND THEIR DISTRIBUTIONThe study area was selected for it s accessibility, the

variety of deposits that it appeared to contain, and thefact that previous work had been done in or near it. Therelatively high road density no t only provided accessibility bu t provided cuts which furnished informationabout the subsurface nature of the surficial sedimentsSuch exposures were particularly important since thestony nature of the soil commonly prohib ited the use ohand augers.

Map units were selected on the basis of the followingcriteria. First, units based at least partly on sedimentary properties or on other field-determined characteristics contribute more information than those basedsolely on topography. Second, relative-age classifications, even though in most cases they represent thearbitrary division of a continuum, are useful and shouldbe employed where possible. Third, the most usefuinformation to be derived from surficial deposits is thatwhich either bears on the manner in which the landscape has changed through time or provides evidence ofPleistocene glacial climates. Hence, where possible,units should be selected so as to convey information onthese topics.

Map units 1, 2, and 3 (fig. 3) indicate bedrock orresiduum. Unit 1 indicates shale bedrock or residuum,chiefly on the Martinsburg Formation, and to a lesserextent on the Millboro Shale. Unit 2 indicates limestonebedrock or residuum, chiefly on the Ordovician limestones and the lower part of the Martinsburg Formation. Unit 3 indicates sandstone-rich residuum andslightly transported colluvium on dip slopes. This material is derived from 'fuscarora Quartzite, Rose Hill Formation, "Keefer" Sandstone, and Rocky Gap Sandstone and overlies these formations as well asHuntersville Chert and Millboro Shale located downslope from them. This unit is made necessary by theareal extent of the dip slopes, where the recognition oftransported regolith is particularly difficult. This


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EXPLANATIONResidual RegolithD Shale residuum or bedrock (Unit 1)D Limestone residuum or bedrock (Unit 2)

D. .' . .[2][2] 00 .' .-.........~ : . o : :.

6080 0 0

Stof

Sandstone residuum and slightlytransported colluvium (Unit 3)

Transported RegolithColluviumYounger, less weathered sandstone-rich

colluvium (Unit 4)Older, more weathered sandstone-rich

colluvium (Unit 5)

Transported RegolithAlluviumFine-grained younger alluvium (Unit 6)Medium-grained younger alluvium (Unit 7)Coarse-grained younger alluvium (Unit 8)Older, more weathered alluvium (Unit 9)

Other SymbolsAreas having abundan t chert fragmentsBoulder streams and other dense

accumulations of large bouldersTuscarora Quartzite (Silurian)

1 KILOMETER

1 MILEJ

FIGURE 3. - Surficial geology of north ern part of Egglestonquadrangle. Concentrations of boulders associated witholder, more-weathered sandstone-ri ch colluvium are indicated by solid circles to stress rarity. Thscarora Quartziteoutcr op (St) is shown for out crop slopes only. Map units,designated numerically, are discussed in greater detail intext. Letters A -F indicate specific locations discussedin text. Southeast corner of map, south of Johns CreekMountain, is after Fiedler (1967).


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10 SURFICIAL GEOLOGY AND GEOMORPHOLOGY, GILES COUNTY, VIRGINIAsandstone-rich regolith is most easily recognized onoutcrop slopes, which are underlain primarily by shale.On dip slopes, largely underlain by sandstones, transport is more difficult to demonstra te. The texture of thesurficial mantle generally is much finer than that onoutcrop slopes; wide areas occur on dips slopes of theThscarora Quartzite where not a boulder can be seen.Locally, however, the surface may be just as boulderyas it is on outcrop slopes.

Units 4 and 5 (fig. 3), of most significance in thisstudy, indicate transported sandstone-rich diamicton,here called colluvium, identified mainly on the basis ofit s content of Thscarora or "Keefer" clasts. Unit 4 IIrepresents younger colluvium, as indicated by it s relatively unweathered appearance, its location close tomodern drainageways or on steep slopes, and it s generally bouldery surface texture. Unit 5 is older colluvium,as indicated by its weathered appearance, its locationon gently sloping noses, hilltops, or other positionsabove the modern drainageways, and it s relative paucity of surface boulders. The colluvium reaches thicknesses of at least 8 m, as shown by road cuts. Includedas unit 5 deposits are areas where the colluvium haslargely weathered away, being represented only by scattered Thscarora and "Keefer" boulders overlying shaleor limestone residuum. On soils maps such areas areshown as soil series derived from shale or limestone, bu tfor the geomorphologist they indicate areas once covered with sandstone-rich colluvium.

The colluvium probably is p roduced by creep, slopewash, debris flows, and possibly by Pleistocene gelifluction. In piedmont areas, the colluvium often showssome fluvial influence, although even there it generallyis poorly sorted and lacks stratification.

Units 6 to 9 (fig. 3) are alluvial deposits. Units 6 to 8represent flood-plain deposits or low terraces, whichhave been subdivided according to texture. Unit 6 represents fine-grained alluvium, defined as consistingmainly of sand. Unit 7 represents medium-graineddeposits defined as consisting of gravel bu t containingfew large boulders. Unit 8 represents coarse-grainedalluvium, defined by the presence of abundant largeboulders. Unit 9 is older alluvium, identified by it sheight above modern drainageways and by a greaterdegree of weathering than that shown by alluvium offlood plains and low terraces.

Chert fragment s shown on the map (fig. 3) are derivedeither from cherty Ordovician limestones or from theHuntersville Chert. Such fragments are locally abundant in both residual and transported regolith; theseareas are indicated by the triangle symbols. The circles

(fig. 3) indicate accumulations of Thscarora or "Keefer"boulders, either as boulder streams or boulder fields.Rather than mapping boulder deposits as areal units, Ishow concentrations of boulders within the colluviumunits because of the small areal extent of the depositsand because road cuts invariably show these depositsto overlie colluvium. The spacing of the circles indicatesthe continuity of the boulder cover. Where the circlesare nearly touching, the cover is nearly continuous;where the circles are farther apart, the cover is somewhat less continuous.Because the Thscarora Quartzite supplies most of thesandstone clasts on the slopes, its outcrop belt showsthe transport distance of the colluvium.

DISTRIBUTION OF SURFICIAL DEPOSITS IN FOUR AREAS

The study area was divided into four areas: DoeCreek drainage basin, Little Stony Creek basin belowthe Cascades, L ittle Stony Creek basin above the Cascades, and the north slope of B ig Mountain.DOE CREEK DRAINAGE BASIN

Included in this section is the area south of JohnsCreek Mountain, which is not actually tributary to DoeCreek.Doe Creek is located along the axis of the MountainLake anticline, flowing in a direction opposite to that ofthe plunge (figs. 2, 3). Boulders are supplied to the basinlargely by Thscarora Quartzite outcrops (fig. 4). Areasshown as Thscarora on figure 3 consist of bedrock cliffsand ledges or of sandstone blocks. The overlying RoseHill Formation also contributes rock debris to theslopes below bu t supplies few large boulders. TheJuniata Formation, beneath the Thscarora, locallycrops out as cliffs and produces boulders, bu t typicallyexposures are poor and clasts derived from this formation are in the pebble- and cobble-size ranges. TheMartinsburg Formation, because of its great thickness,underlies by far the greatest area of mountain slopesbu t generally is poorly exposed. It s contact with theJuniata Formation commonly is marked by a break inslope, the slope angle on the Martinsburg being somewhat lower. Mainly pebble- and cobble-size clasts arederived from the thin siltstones and sandstones thatconstitute the upper part of this formation, whereas themiddle part, composed of fissile shale, weathers intosmall chips.The residuum of the Martinsburg varies considerably according to the slope angle. Where angles are low,the shale weathers to a clay-rich, yellowish residuum


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12 SURFICIAL GEOLOGY AND GEOMORPHOLOGY, GILES COUNTY, VIRGINIA

FIGURE 5.- Upper end of Doe Creek drainage basin, showing location of different types of regolith, looking northeast. Doe Mountain is in leftbackground, Bean Field Mountain is in center background, an d Salt Pond is ou t of view to the right. SH, shale bedrock and residuum;LS, limestone bedrock and residuum; YC , younger sandstone-rich colluvium; OC, older sandstone-rich colluvium.exert much influence on the form of the landscape Iunderlain by limestone.

In contrast to its lack of influence on limestone slopes,sandstone-rich colluvium seems to have a large effecton the form of areas underlain by shale. A large area onthe south side of Doe Mountain is practically free ofcolluvium compared to the south flank of Bean FieldMountain which is covered by bouldery colluvium. Thehollows on the unmantled shale slopes are deep andV-shaped (figs. 3, 7), whereas those on the boulder-cladslopes are much shallower and more likely to be asymmetric. This difference probably results from the effectof large boulders in the hollows. These boulders probably can be moved only by rare, catastrophic floods.Smaller, more common floods cannot move the boulders

and are likely to cu t down between the bouldery hollowfill and the less resistant shale side slope. The floor ofthe "channel" cu t by such floods will, however, eventually be covered by boulders transported by creep fromthe adj acent boulder deposit or by the next catastrophic flood, so that subsequent lesser floods will intu m incise lateral to it. In this way the hollows migratelaterally (Mills, 1981). Where the shale is unprotectedby boulders, however, such as on the south slope of DoeMountain, the hollows are incised straight down,becoming large ravines.The density of boulders on slopes appears to becontrolled by the exact location of the 'fuscaroraQuartzite outcrop with respect to the mountain crest.Where the entire thickness of the 'fuscarora crops out


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GENERAL SURFICIAL GEOLOGY AND GEOMORPHOLOGY 13

FIGURE G.-Road cut through shale nose capped by sandstone-rich colluvium, along south flank of Bean Field Mountain.on the flank of the mountain (fig. 8A), as on Bean FieldMountain, the slopes have abundant Thscarora boulders. Where the crest is at or near the base of theThscarora, however (fig. 8B), few boulders are suppliedto the outcrop slope. For the most part, the crest of DoeMountain is like that shown in figure 8B. Which of theconfigurations shown in figure 8 prevails at a givenlocality seems to depend in part upon the resistance ofthe upper Juniata Formation at that locality. Where theJuniata is more resistant (due to thicker bedding andperhaps to a higher proportion of sandstone), the configuration is like that shown in figure 8B, for undersuch circumstances the Juniata is capable of holding upthe mountain crest; but the boulders derived from itdisintegrate a short distance below the outcrop, so thatthe hollows lack boulders. Where the Juniata Formation is less resistant, however, it is unable to hold up thecrest, and the outcrop slope retreats until it undermines the Thscarora Quartzite, which then crops outmainly on the outcrop slope, as shown in figure 8A. Inthis case, plentiful resistant boulders are supplied tothe hollows which thus tend to migrate laterally.

The phenomenon of lateral planation produced by theeffects of boulder armoring applies not only to hollows,bu t also to small stream valleys, such as the one occupied by an intermittent stream south of Pacers Gap,between Doe and Bean Field Mountains (fig. 3, loco C).Thscarora boulders are rare on the east flank of DoeMountain, except for the slopes neares t Pacers Gap. Incontrast, the southwest flank of Bean Field Mountainis covered with boulders. Because of this difference, theintermittent stream shown at locality C in figure 3apparently has migrated westward toward the base ofDoe Mountain.Topographic evidence suggests that some of thesandstone-rich colluvium must antedate the moderngeomorphic system. First, limestone hilltops capped bythis material have as many as five closed 20-ft (S.l-m)contours, meaning that at least 100 ft (30.5 m) of bedrock erosion has taken place since deposition of thecolluvium. Second, the colluvium in the southwesternpart of the map must have been derived from a formersouthwest ern extension of the Thscarora Quartzite outcrop on Doe Mountain (figs. 3, 9). Thus, for example,


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FIGURE 9. -Southwestend of Doe Mountain, showing relict sandstone-rich colluvial depos its. Dashed line shows approximatesouthwest limof Tuscarora Quartzite (St) outcrop. Southwest of gap on skyline, older colluvium (OC) has no present sandstone source upslope from iand hence must be a relict deposit from a time when there was such a source.

Doe Creek possesses a narrow flood plain throughoutit s length. Only one deposit was found that might be aterrace deposit (fig. 3, loCo E). Well-rounded clas ts werefound on the lower slopes of the hill immediately northof the stream at E and may be former Doe Creekalluvial clasts. The deposit itself, however, is colluvium,as shown by the fact that its contact with the underly- Iing residuum is steeply sloping and parallel to thehillslope surface.

LITTLE STONY CREEK BASIN BELOW TH E CASCADESAs a consequence of it s steep gradient, Little StonyCreek below the Cascades is a powerful stream. Thedifference in gradient above and below the Cascades ismarked. Above, the stream flows 5.1 km between the3,200-ft (976-m) and 3,000-ft (915-m) contours, for anaverage gradient of 0.012. Below, it flows 2.5 kmbetween the 2,800-ft (854-m) and 2,200-ft (671-m) con

tour lines, for an average gradient of 0.073, six times assteep. The upper part of the reach shown on figure 3

contains many boulders 5 m or more in diameteralthough most of these probably are only very rarelymoved. Even in the downstream part of the reach showon figure 3 boulders are much larger and more numerous than those in Doe Creek.Probably because of the steep walls of the streamvalley, the Thscarora outcrops at the crests are beingrapidly undermined, and as a result almost all the sloparea is covered with boulders and sandstone-rich colluvium. The boulder streams on the walls of this valley arethe steepest I observed; the slope of one stream wasmeasured at 36 c> (figs. 10, 11). These streams may bemoving, as some of them lack forest cover over shorreaches. At no other location in the study area did observe scree without forest cover. The boulder streamare about 15 m wide, and they die ou t near the base othe steeper, upper slopes. Below these slopes, bouldersin hollows are fewer and smaller.The southwest nose of Doe Mountain lacks 'IUscarora outcrops southwest of about the 3,380-ft (1,030m) contour (fig. 3). Southwest of this point, therefore


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FIGURE 12.-View down steep northwest flank of Doe Mountain, from crest of Doe-Mountain. Stream in valley is Little Stony Creek.

the flood plain and low terraces are composed largely ofsand. As shown in figure 13, the flood plain is somewhatbroader than that of other streams in the area, with it svery flat surface giving rise to many boggy areas.Despite the flood-plain width, however, the depth of filldoes not appear to be great, as indicated by several cutsin lower terraces that expose bedrock For example, atlocation G (fig. 13), two deposits of low terraces wereexposed by a road cut. The lower terrace is about 1.5 to2 m above the present flood plain, and the higher oneabout 3 m. The highe r terrace shows a sand layer about75 cm thick overlying a gravelly layer about 50 cmthick. Beneath the gravel is a transitional layer about30 cm thick, in which sandstone alluvial clasts aremixed with fragments of Millboro Shale, and underlying this layer is weathered Millboro Shale. The soils onthese terraces appear to be very young.Several alluvial surfaces are found south of the LittleStony Creek flood plain (fig. 15). Where Pond Draindebouches onto the floor of this valley, an alluvial fanhas been built (figs. 13, 15) which is generally sandy bu t

contains gravel. Several low, fairly young terraces canbe observed on this fan. The fan surface is characterizedby many Eastern white pines in contrast to the oldersurfaces to the east, which are covered mainly by hardwoods. The alluvial surface to the east, roughly 5 mabove the Pond Drain fan, appears to have moreweathered sediments, probably consisting of olderdeposits of Pond Drain, of the headwaters of LittleStony Creek, and of the smaller intermittent streamsbetween these streams. This surface is more gravellythan that of the Pond Drain fan . Stream gravelscemented with iron oxide were seen at one location onthis surface, the iron perhaps having been obtainedfrom the Rocky Gap Sandstone immediately upslope. Ahigher surface, at least 10 m above the Pond Drain fanand possibly alluvial in origin, occurs to the west of thefan (figs. 13, 15). This surface is about 18 to 24 m abovethe level of Little Stony Creek. I t is cu t across theHuntersville Chert and Millboro Shale, and has a gradient similar to that of Pond Drain fan, but it lacks acover of alluvium. Widely scattered rounded sandstone


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GENERAL SURFICIAL GEOLOGY AND GEOMORPHOLOGY 19clasts suggest that the surface may once have had sucha cover, however. I have classified it as an alluvialsurface mainly on the basis of its morphology and itsproximity to a younger alluvial surface. A similar, Ismaller surface to the west (fig. 13) probably is theproduct of an ephemeral stream.

Two holes were drilled with a hand auger on the fansouth of Laurel Creek (loc. H, fig. 13). The first penetrated 200 cm, and the second 240 cm, before striking asandstone cobble. The fan thus appears to consistlargely of sand, probab ly eroded from the Rocky GapSandstone upslope. Although part of this sand wastransported by Laurel Creek, much probab ly was transported downslope by small ephemeral streams. Largesandstone clasts are found on the surface only nearLaurel Creek.Near the base of the south flank of Big Mountain,fan-shaped features are located at the mouths of ephemeral or intermittent streams that flow between flatironsof Rocky Gap Sandstone (fig. 13). The surfaces of thesefans display scattered "Keefer" or Thscarora Quartziteboulders, indica ting that the fans were formed by thestreams that debouche near them. The soils on thesefans consist mainly of residuum, bu t on the basis of thesandstone boulders the fan surfaces are classified asolder, sandstone-rich colluvium (fig. 13). Several piecesof evidence suggest that these fan surfaces are largelyrelict. First, the modern streams flow along the margins of the fans o r else are incised into them (fig. 13).Second, the boulder streams between the Rocky GapSandstone flatirons, which probably supplied Thscarora or "Keefer" boulders to the fans, now terminatefar short of the fans. Third, the surfaces of some fansare as much as 15 m above the flood plain of LittleStony Creek. These fans probably are composed mainlyof shale bedrock, the protection provided by the formerbouldery mantle having preserved their surfaces.

NORTH SLOPE OF BIG MOUNTAINBecause of the extensive outcrops of ThscaroraQuartzite and "Keefer" Sandstone, the north slope of

Big Mountain is extremely bouldery and has boulderstreams larger than those elsewhere in the study area.The largest streams are in the moistest locations andare covered with dense growths of rhododendron. Generally, the boulder streams die out several hundredmeters below the lowest orthoquartzite outcrop. Boulder streams (fig. 16) and colluvium (fig. 17) are w()llexposed in cuts along Rocky Mountain Road (U.S.Forest Service 734), which runs along the north side ofBig Mountain at an elevation of about 975 m (notshown on fig. 13).The northern foot slopes of Big Mountain, thoughhaving few large boulders, are almost completely cov-

ered with a relatively young stony mantle containingmany orthoquartzite cobbles and small boulders. Th icolluvial apron extends northward all the way to StonyCreek which it has probably pushed against the northwall of it s valley, resulting in an absence of terracesThe load of Stony Creek is coarse, probably because othe proximity of the stony colluvial apron. Although byno means as bouldery as the flood plain of Little StonyCreek below the Cascades, the flood plain of StonyCreek has been classified as coarse grained (fig. 13).

DISCUSSIONI suggest herein that the older sandstone-rich colluvium overlying limestone was transported laterally to it s

present position rather than being let down verticallyas the limestone beneath it dissolved. The manner inwhich this colluvium formed bears on the age of thedeposits, for surficial sediments can be let down greavertical distances over soluble bedrock, resu lting in thepreservation of very old deposits. Pierce (1965), foexample, reported a lignitic deposit of Late Cretaceouage overlying carbonate residuum in the Great Valley osouthern Pennsylvania. Unlike Pierce's Cambrian bedrock section, however, in the present study area suchletdown seems unlikely because the Ordovician lime-

I stones upon which the colluvium rests originally wereseparated from the overlying Thscarora Quartzite and"Keefer" Sandstone by the great thickness of the Martinsburg Formation. Clasts eroded from the Siluriansandstones would have been deposited on the insolubleMartinsburg siltstones and shales. How the sandstoneclasts could have been let down vertically while hundreds of meters of clastic rock were eroded from beneaththem is very difficult to imagine. Lateral transportwould have been almost inevitable under such circumstances. A related question is why the colluvium

I appears to be preserved longer over limestone than oveshale. Because limestone undergoes mainly subsurfaceerosion and shale mainly surface erosion, deposits overlying shale are likely to be eroded away much morerapidly than those over limestone.The study of upland transported regolith reveals newways in which bedrock affects topography, par ticularlyways involving the protective role of large sandstoneboulders. For example, the forms of hollows incised intoMartinsburg shale on mountain flanks vary considerably according to whether the flank possesses a bouldermantle or not, which in turn depends upon seeminglyunimportant geologic factors near the mountain crest.In addition, as shown by the northwest flank of DoeMountain, the presence of a boulder mantle may greatlyaffect the amount of retreat that a mountain slopeundergoes; thus, the concept of the protective role of


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GENERAL SURFICIAL GEOLOGY AND GEOMORPHOLOGY

D

O ..... .

o 00 0 0St

oI

EXPLANATIONResidual Regolith

Shale residuum or bedrockSandstone residuum and slightly

transported CO l luvium

Transported RegolithColluvium

Younger, less weathered sandstone-richcolluvium

Older, more weathered sandstone-richcolluvium

Transported RegolithAlluvium

Fine-grained younger alluviumMedium-grained younger alluvium

Coarse-grained younger alluviumOlder, more weathered alluviumProbable old bedrock terrace

without alluvial coverOther Symbols

Areas having abundant chert fragmentsBoulder streams and other denseaccumulations of large boulders

Tuscarora Quartzite (Si Iurian)

1 KILOMETER

1 MILEI

FIGURE I3.-Surficial geology of southern part of Interiorquadrangle. 'fuscarora Quartzite (St) outcrop belt isshown for outcrop slopes only. Map units are same asthose for figure 3. LettersG-H ndicate specific locationsdiscussed in text.


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FIGURE l4.-South (dip) slope of Big Mountain, showing flatiron. White Pine Lodge visible in foreground.

caprocks is expanded. Previously, this role has beenperceived mainly as one of shielding the immediatelyunderlying less-resistant strata from the elements.Where the caprock produces large resistant boulders,however, these boulders extend the protection of thecaprock by armoring the outcrops of the weaker stratafar downslope from the caprock itself. These bouldersappear to play an especially important role at the baseof the mountain flank, where they may act in a mannersimilar to riprap to inhibit undermining of the mountain slope by laterally eroding streams.The origin of Mountain Lake has engaged the attention of many previous investigators. This lake lies nearthe crest of the breached Mountain Lake anticline, its Ilong axis near the trend of the fold. It rests chiefly uponthe Juniata and Martinsburg Formations, although theoutlet overlies the Tuscarora Quartzite (fig. 2). Twobasic methods of origin have been suggested for Mountain Lake. Rogers (1884), Sharp (1933), Hutchinson andPickford (1932), Eckroade (1962), and Parker and others (1975) favored damming by scree, whereas Platt and

Shoup (1950) and Marland (1967) preferred solution.Dietrich (1957) suggested a combination of these twoorigins.Solution appears to be an unlikely origin. As pointedout by Sharp (1933) and Parker and others (1975),because of the great thickness of the insoluble Martins burg Formation, it seems impossible that solution oflimestone underlying this formation could have anyeffect on the surface topography. The scree-damminghypothesis has been described most lucidly by Sharp(1933), who theorized that initially the depression occupied by Mountain Lake had been a normal streamvalley occupied by a stream flowing northward, tributary to Little Stony Creek. As long slopes of Juniataand Martinsburg were exposed just south of the outletnotch cut in the Tuscarora Quartzite, great blocks ofthis sandstone crept downward over the shale slopes,gradua lly filling the valley bottom. Weathered particlesand organic material then filled the interstices, thusmaking a water-tight dam impounding the lake. Asevidence for this theory, he noted that the depth of the


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FIGURE 15.-Pond Drain fan and vicinity as viewed from near crest of Big Mountain. A, Surface of Pond Drain fan; B, Older surface east oPond Drain fan; C, Older surface lacking an alluvial mantle, west of Pond Drain fan.

lake increases to the north, as should be expected if thedepression had been occupied originally by a streamflowing to the north. In addition, the lake bottom nearthe outlet is composed of immense blocks of 'fuscaroraQuartzite. These blocks can be traced in "unbrokenprocession" from the lake bottom to their parent ledgeof 'fuscarora Quartzite about 100 m or so northeast ofthe lake (fig. 18). Sharp also observed a lack of largeblocks west of the outlet, thus making it appear thatthe outlet stream had been crowded over to the westside of the notch in the 'fuscarora by blocks creepingfrom the east.

Although the scree-damming theory appears quiteplausible, two questions remain. First is the mechanismby which the blocks were transported. Although thisarea is noted for it s seismic activity (for example, Bollinger and Wheeler, 1982), the slope below the parentledge seems much too gentle for rockfall to have movedthe blocks. More likely is Sharp's (1933) suggestion of

creep, which may have been accelerated by gelifluctionand other periglacial processes during full-glacial climatic conditions of the Pleistocene.A second and more important question is why Mountain Lake is unique in the ungl acia ted Valley and Ridgeprovince, where breached anticlines with 'fuscaroraQuartzite outcrops are common. Possibly, several circumstances combined to allow the formation of Mountain Lake: (1) the very gentle dips of the Mountain Lakeanticline; (2) the relatively high local base level of thestream draining Mountain Lake; and (3) the relativelyhigh altitude of the lake area. None of these conditionsis uncommon in the Valley and Ridge province, bu t thecombination of all three may very well be rare, if nounique. Most lakes formed by natural dams are soondrained by destruction of the dams. Thus, althoughcreep of boulder scree into stream valleys probably hasbeen common in this region, incipient dams have inmost cases been rapidl y eliminated. At Mountain Lake


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FIGURE 16.-Road cu t through matrix-free boulder stream on north slope of Big Mountain at elevation of about 975 m.

however, circumstances have made destruction of thedam exceedingly difficult. Because the huge Thscarorablocks cannot be moved by any flow of water possible insuch a small drainage basin, the only way to drain thelake would be for the outlet stream to cu t down laterallyto the dam. Pond Drain flows for more than 2 km overthe gentle dip slopes of resistant sandstone (fig. 2),however, greatly hindering such incision. Downcuttingis made still more difficult by the fact that the floodplain of Little Stony Creek (whose elevation is controlled by the sandstone units holding up the Cascades)constitutes the local base level of Pond Drain and liesonly 236 m below the elevation of Mountain Lake itself.Hence, downcutting by Pond Drain has been minor,and there has been no opportunity for a headwarderoding nickpoint to provide the gradient below thedam necessary to cause local erosion vigorous enoughto drain the lake. Another contributing factor may havebeen the relatively high altitude of Mountain Lake

(1,181 m), making block transport by Pleistocene periglacial processes more likely than at lower altitudes.Figure 2 suggests that the most likely way for Moun

tain Lake to meet it s end is not by breaching of it s dambu t by capture resulting from the headward erosion ofDoe Creek. The headwaters reach of this stream issomewhat steeper than that of Pond Drain, and onlythe weak Martinsburg Formation stands in the way ofthe capture.

SEDIMENTOLOGY OF CO LLUVIUM

PREVIOUS WORKThe problem of whether colluvium in the unglaciated

Appalachians is the product chiefly of Pleistocene periglacial conditions or of modern hillslope processes hasbeen addressed by many previous authors. Denny


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- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

SEDIMENTOLOGY OF COLLUVIUM 2

FIGURE 17.-Road cut through matrix-rich colluvium on north slopeof Big Mountain at elevation of about 975 m. Note stony mantle atground surface above colluvium. Pick is 43 cm long.

(1951,1956), Smith (1953), Michalek (1968), Potter andMoss (1968), Haselton (1973), Godfrey (1975), Hedges(1975), and Raymond (1977) concluded that slopedeposits in this region are to a large extent relicts of Pleistocene cold periods. Other investigators, however, such asGryta and Bartholomew (1983) and Kochel and Johnson (1984), implied that modern processes, particularlydebris avalanches and flows se t off by rare, catastrophic rainfalls, can account for most colluvium onmountain foot slopes.The sedimentology of colluvium has received littleattention, an exception being the study of roundsto>J.ediamicton by Pierce (1966). Hack and Goodlett (1960),Bogucki (1970,1976), and Scott (1972) studied moderncatastrophic-flood deposits, bu t only Williams and Guy(1973) investigated sedimentary properties of thesedeposits. Outside the Appalachians, by far the mostwidely investigated slope deposits have been those in

arctic and alpine environments; many of these studiehave been reviewed by Washburn (1980). Benedic(1976) has reviewed frost-creep and gelifluction studiesRelict periglacial deposi ts in the United Kingdom havbeen intensely studied by many authors with regard tthei r sedimentology.A finding that may bear on the origin of colluvium ithe existence of two or more ages of slope deposits, aindicated by degree of weathering, reported by Hadle

and Goldsmith (1963), King (1964), Michalek (1968Gryta and Bartholomew (1983), and Mills (1977Although no dates have been obtained, Hadley anGoldsmith (1963) and Mills (1977) suggested that sucmultiple ages might reflect episodic deposition causeby Quaternary climatic cycles. Gryta and Bartholomew(1983) and Kochel and Johnson (1984), however, indcated that this age diversity may simply represensuccessive deposits from multiple catastrophic floods

DIFFERENTIATION OF COLLUVIUM ENVIRONMENT ANDAGE BY SEDIMENTOLOGIC AND WEATHERING CRITERIASedimentologic data, including particle-size, clasroundness, clast fabric, and weathering indices, clearldiscriminated colluvial deposits in five different environments. These environments are defined by topographic setting on hillslopes: Upper noses (slope angl14 0 or greater); lower noses (slope angle less than 14)side slopes (and other steep slopes with thin colluviamantles); moist, commonly north-facing, hollows withi500 m of the mountain crest (subsequently referred tas type M hollows); and other hollows (that is, dry

commonly south-facing hollows or hollows greater tha500 m from the crest, subsequently referred to as typD hollows).These categories were devised afte r enough data werobtained to indicate differences among these settingsThe lower-nose environment corresponds to the "oldermore weathered sandstone-rich colluvium" map unit ofigures 3 and 13, and the other four environments arincluded in the "younger, less-weathered sandstonerich colluvium" unit. At sites in each of these environments, measurements were made of fabric, clast roundI ness, particle-size distribution, color (Munsell colochart), and clast weathering. Sample sites, commonllocated at road cuts, are shown on figure 2. For comparison, a few samples of alluvium were also includedStudy was confined largely to outcrop slopes whersandstone-rich colluvium derived from the Rose Hill'lliscarora, and Juniata units overlies the MartinsburFormation. Here the mountain-flank topography is besdeveloped, and the regolith is most easily recognized atransported as opposed to residual. The mean and standard deviation for each sedimentologic or weatherin


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FIGURE 18.-North end of Mountain Lake. Arrow indicates large boulders that form the natural dam. Boulders were derived from ThscaroraQuartzite ledge a short distance upslope. Ou tlet is behind boathouse.

property in each environment is presented in table 2; intable 3, the properties which differ at the p:5 0.05 confidence level are shown for each pair of colluvium environments. Each property is discussed in de tail below.PARTICLESIZE ANALYSIS

The percent sand, silt, and clay of 67 samples ofcolluvium matrix are shown on a ternary diagram (fig.19) as mean values and range envelopes for each environment. The mean and standard deviation of the percent clay (in the less-than-2-mm fraction) is presentedin table 2, which shows that alluvium has the lowestpercent clay (13 percent). Colluvium of hollows and sideslopes has mean clay values ranging from 17 to 23percent. Sites on upper noses have slightly higher clayvalues (29 percent), bu t the only percentage that ismarkedly different (43 percent) belongs to the lowernose environment. The lower-nose value is significantlydifferent from that of the other four environments; the

only other significant difference in percent clay isbetween upper noses and type M hollows (table 3).A total of twenty-two 50-kg samples containingclasts :5 256 mm were sieved in the field. The results arepresented in figure 20 as average cumulative sizedistribution curves for each colluvial environment; thecurve for a single sample of alluvium is presented forcomparison. The results are roughly similar to thosefound for the fine fraction. Curves were also plotted forindividual field-sieve samples, and the graphic mean(Mz ) and inclusive graphic standard deviation (aI) (Folkand Ward, 1957) computed for each. The means andstandard deviations of these statistics for each environment are shown in table 2. Table 3 shows that withregard to Mz , colluvium from lower noses is significantly different from colluvium in hollows, bu t not fromupper noses or side slopes. Upper noses are significantly different only from type D hollows and sideslopes. With regard to aI ' colluvium from lower noses issignificantly different from that from hollows. The


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SEDIMENTOLOGY O F C O L L U V I U M 2inconsistent results of the significance tests probablyarise from the small number of samples; a greaternumber almost certainly would increase the number ofsignificant differences.Individual field-sieve results are presented in figure21, a ternary diagram showing percent gravel, sand,and silt + clay. Presentation in this fashion allows acomparison with seven samples of "roundstone diamicton sieved" by Pierce (1966), who apparently used asimilar upper-clast-size limit. (His largest sieve was 128mm.) Note that Pierce's samples generally have lowersilt + clay and higher gravel contents than the lowernose samples in this study. A t-test shows that thepercent silt+clay differs significantly between the twogroups of samples.

Many particle-size analyses have been done on periglacial slope deposits (for example, Benedict, 1970;Mottershead, 1976), and even more have been repor tedfor debris flows (for example, Bull, 1962, 1964; Landimand Frakes, 1968; Pierson, 1980). Both types of sediment, however, show such wide variation in particlesize characteristics that this property is of little valuein distinguishing them. Hence, particle-size distribution provides no clues as to the origin of the colluvium.A comparison with published data did produce oneinteresting result, however; the u, values, particularlyfor the nose sites, are higher than any reported foreither periglacial or debris-flow deposits.

CLAST RO UNDN ESSTable 3 shows clear differences in pebble roundness

between environments. Studies of pebble rounding instreams generally have shown that mean roundnessincreases as a function of transport distance (Mills,1979), suggesting that the observed roundness differences may simply reflect the fact that the deposits havebeen transported for different distances. In order tocontrol for this factor, mean roundness values in figure22 have been plotted as a function of distance from themountain crest (which serves as distance of transport) .For comparison, 17 mean roundness values of pebblesamples taken from the beds of modern streams in thestudy area and vicinity have been plotted; the solid lineis the regression equation calculated for the eigh t samples from Doe Creek. Note that roundness increasesdownstream more rapidly and consistently in DoeCreek than in the other streams, probably becaufJeconditions are more favorable for downstream rounding. First, Doe Creek leaves the outcrop of sandstonesthat supply its clasts much farther upstream than doother streams (fig. 2), flowing for most of it s length overshale or limestone. Therefore, there is less opportunityfor fresh, angular clasts to be added to the stream load.

TABLE 2.-Sedimentary characteristics, or values, of colluviumfrom five environments defined in the study area[Alluvium shown for comparison; n ... not applic.blel

EnvironmentSedimentary Upper Lower Side ~ 8 ~ w ~ ~ b O a ! roperty noses noses slopes

Percent clayMea n-- - - - - - - - - - -- 29 43 23 17 22Standard deviation - - 12 14 10 6 6Number of samples-- 9 16 14 7 15

Graphic mean offield . eve .amples (4))Mean- -- - - - - - - - - - - 1.1 3.4 0.9 -0.5 -2.1Standard deviation - - 0.5 1.8 1.3 1.7 1.5Number of samples- - 3 6 4 4 4

Inclusive graphic standard deviation offield-sieve samples (4))Mean---- - - - - - - - - - 7.3 7.5 6.6 5.5 5.5Standard deviation - - 1.4 .7 .9 .4 .7Number of samples- - 3 6 4 4 4

Huc l

Mean----- - - - - - - - - 4.2YR 5.0YR 6.7 YR 6.3 YR 7.3 YRStandard deviation - - 1.2YR 2.3 YR 1.8 YR 1.3YR 1.5YRNumber of samples-- 8 16 14 7 15

Chroma'Me an-- - - - - - - - - - - - 4.8 6.7 4.4 4.0 4.5Standard deviation - - 1.0 1.0 1.3 0.0 1.3Number of samples-- 8 16 14 7 15

Percent weathered clastsMean-- - - - - - - - - - - - 25 57 4 5 4Standard deviation - - 8 17 5 6 7Number of samples-- 8 13 2 3 14

~ I e d i a n smace clast size (4))Mea"------------- -5.8


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28 SURFICIAL GEOLOGY AND GEOMORPHOLOGY, GILES COUNTY, VIRGINIAEXPLANATION Alluvium (6) 100%

...

0D\1

'\

Colluvium in Type Mhollows (7)

Colluvium in Type 0hollows (15)

Colluvium onupper noses (9)

Colluvium onlower noses (16)

Other colluvium (14)

.. .... ...

...'\ /v

C ~ a y

100% / \100%Sand \ I V V V \I V V V V SiltFIGURE 19 .-Temary diagram showing percent sand, silt, and clay (based on grade scale of Wentworth, 1922) of

less-than-2mm fraction of colluvial samples, in terms of mean and range for each environment. Number of samplesin each environment is given in parentheses. Particle size was measured by sieve and hydrometer. Other colluviumis that on side and other steep slopes.

Additional sandstone clasts along it s course are for themost part supplied only by major tributaries whoselengths are the same order of magnitude as that of DoeCreek itself; these clasts are already substantiallyrounded before they enter Doe Creek. Second, most ofthe sandstone clasts are composed of Thscarora Quartzite or Rose Hill hematitic sandstone, tough lithologiesthat resist breaking. In contrast, other streams sampled (for example, Stony Creek and Little Stony Creek)flow over sandstone bedrock for a much greater percentage of their total length, so that fresh, angular clastsmay be added along their courses. In addition, thesestreams also contain a high percentage of weaker sandstone clasts, especially those from the Rocky Gap Sandstone, which break easily and thus decrease roundness.The regression line for Doe Creek is thus consideredto be representative of the roundness-distance relation-

ship in modern streams under favorable conditions. Infact, the slope of this line is comparable to slopes ofregression equations calculated for several other studies of downstream rounding of pebbles (Mills, 1979).The 95-percent confidence interval was calculated forthis regression, and the upper limit is shown as adashed line on figure 22. The lower confidence limit hasnot been plotted, for I believe it less meaningful thanthe upper one, since there are many obvious factorsthat can decrease mean roundness for a given distanceof transport. Two of these factors have been discussed;another is the erosion of angular clasts from colluviumalong the sides of the stream. On the other hand, nofactor that can increase the mean roundness above thatnormally seen in streams has yet been demonstrated.The data from previous studies of downstream rounding of pebbles show that there is no evidence of mean


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SEDIMENTOLOGY OF COLLUVIUM 2roundness increasing with distance faster in naturalstreams than in laboratory flumes (Mills, 1979). Hence,pebble samples having mean roundness values significant ly above the Doe Creek regression lines are of muchmore interest than are those having values significantly below the line.

Figure 22 shows that most colluvial samples fallbelow the Doe Creek regression line; alluvial samples dothe same. Figure 22 also shows, however, that the colluvial pebble samples tend to increase in mean roundnessas a function of distance, a phenomenon observed foralluvial pebble samples. This finding allows a simpleinterpretation of the differences in mean roundnessbetween colluvial samples for environments shown intables 2 and 3. The differences appear to be due simplyto the fact that the distance of transport varies withenvironment, as suggested earlier. For comparison,seven pebble-roundness samples from roundstonediamicton measured by Pierce (1966) are plotted infigure 22. The roundness-distance relationship shownby these samples is comparable to that shown by samples from the lower-nose colluvial environment.Aside from reflecting distance of transport, theroundness results probably give little information ongenesis. Pebbles in debris flows show a much smallerdownstream increase in roundness than to those instreams (Scott, 1971; Mills, 1978), which ideally mighthelp to distinguish debris-flow from fluvial deposits. Asdiscussed above, however, other factors can produce alow mean roundness. Pebbles in gelifluction depositsgenerally seem to be more angular than those in thepresent deposits (for example, Watson, 1969; Mottershead, 1976), bu t this difference may simply reflectgreater Holocene weathering in the Virginia deposits.This finding raises the question of whether weatheringcan account for the observed roundness differencebetween environments. The lower-nose environment,for example, which has the most rounded clasts, alsodisplays the most intense weathering. My impression,however, is that weathering is unlikely to be the chiefcause of rounding. Although weathering can bluntsharp edges, for granular lithologies such as sandstonethe outline of the clast's silhouette commonly becomesmore ragged with greater weathering. Additional evidence against weathering as a major factor is providedby the similarity in roundness between the upper-noseand the type M hollows (table 2), despite their differencein weathering environment. The transport distance oftheir deposits is similar, however.One final point of interest concerns the two diamondshaped symbols in figure 22. These symbols representsamples taken from a foot slope in Clover Hollow(Newport quadrangle, near intersection of Routes 601and 639; not shown) that conceivably could be an old

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SEDIMENTOLOGY OF COLLUVIUM 3

0.6

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(f ) 0.4(f )wZoz::>oa:Z

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32 SURFICIAL GEOLOGY AND GEOMORPHOLOGY, GILES COUNTY, VIRGINIAA B

E

DOWNHILL

FIGURE 23.-Composite equal area (Schmidt, lower hemisphere)nets for each colluvial environment. The downslope direction istoward the bottom. Co ntouring was done by the method of Kamb(1959) using a computer subroutine by Corbat6 (1965) at contourintervals of two standard deviations. Composite plots were madefor each environment by rotating the stereographic nets of allsamples from that environment so that the downslope directionof each was at the same position; then all the points were contoured as one net. The rationale for this procedure is analogous tosumming signals on an oscilloscope, in that when viewed individually the Schmidt net for each site may show such a weak fabricthat no trend is evident; if the nets have any common trend,however, then adding them together should result in the "noise"averagingou t and the "signal" increasing in amplitude. The blackareas represent the densest concentration of axes. On diagramswhere intervals have been numbered, higher numbers representdenser concentr ations of axes. A, Composite of 10 sites in type Dhollows. B, Composite of seven sites on upper noses. C, Composi teof 10 site s on lower noses.D, Composite of eight sites in type Mhollows. E, Composite of five site s on side or other steep slopes.

ments differ only with respect to fabric. This differencewas so dramatic that it was considered a sufficientbasis for distingui shing the two environments.Clast fabric possibly can discriminate between gelifluction and debris-flow deposits. Fabric pattern is not

very helpful because many mass-wasting depositsexhibit clasts with a preferred long-axis orientationparallel to slope and imbricated uphill (Benedict, 1976),as do the present colluvial deposits. Fabric strength, orthe consistency of long-axis orientation, may be moreuseful, however. Many investigators have reportedquite strong fabrics in gelifluction deposits (for example, Kirby, 1967; Mottershead, 1976; Brochu, 1978;Nelson, 1985), whereas debris-flow fabrics, thoughmuch less studied, commonly are relatively weak(Lindsay, 1968; Williams and Guy, 1973; Mills, 1984).This fabric difference probably results from the laminarflow of gelifluction and the turbulence of debris flows.

In the present study, the very strong fabrics of thedeposits of the type M hollows are compatible with agelifluction origin, particularly as these locations arenorth facing and high where Pleistocene climatic conditions would have been most rigorous. Other forms oflaminar flow cannot be eliminated as the cause of thefabric strength, however. Creep, for example, probablyaccounts for the relatively strong fabrics shown by theside slopes (Mills, 1983), although this process probably does not account for the hollow deposits with theirsomewhat gent ler slopes. The weakness of fabrics in thetype D hollows appears to exclude gelifluction, an exclusion which seems reasonable since the south-facinghollows would have been warmer and dryer than otherhollows during the Pleistocene glacial climates and hollows

Documents

Geomorphology Giles County Virginia