ROBERT P. SHARP Division of Geological Sciences, California Institute of Technology, Pasadena,California

Sherwin Till-Bishop Tuff Geological

Relationships, Sierra Nevada, California

Abstract: A glacial till underlies the Bishop tuff, but it has not been established heretofore onfirm geological bases that this is the Sherwin till. Relationships here described demonstrate thatthe till beneath the Bishop tuff is indeed Blackwelder's-type Sherwin.

Radiometric datings of the basal pumice of the Bishop tuff by Dalrymple and others yield anage of 710,000 years. Judging from pre-pumice weathering, the till may be still older, perhaps750,000 years. These considerations make the Sherwin till-Bishop tuff relationship an importantreference datum in the continental Pleistocene sequence of North America.

CONTENTS

Introduction 351General statement 351Physical setting 352

Acknowledgments 352Glacial drift beneath Bishop tuff 352Pumice on Sherwin till 355

Introduction 355Pumice-Sherwin till relationships along U. S. 395 355Other locations 357

Stripping of Bishop tuff 358Geometrical relationships 358No Sherwin till on Bishop tuff 359Sherwin outwash and other gravels 359Speculation on correlation 361References cited 362

Figure1. Place map of Sherwin Grade-Rock Creek area. 3532. Geological map of Sherwin Grade-Rock Creek

area 3543. Field sketch of relationships in Big Pumice Cut 3554. Field sketch of Sherwin outwash exposed in

walls of Owens River gorge 3555. Interpretive cross sections of flattopped pumice

hills 3566. Field sketch of pumice-Sherwin till relationship

in Little Pumice Cut 3577. Map of gravels on Bishop tuff along lower Rock

Creek 3608. Field sketch of old red till on old Sherwin

Grade road 361

INTRODUCTION

General Statement

As the type locality of the Sherwin till(Blackwelder, 1931, p. 895-900), the SherwinGrade-Rock Creek area has long been of localinterest. It is now attracting international atten-tion from Pleistocene chronologists because ofpotassium-argon dating of the associatedBishop tuff. Blackwelder (1931, p. 918, 899)regarded the Sherwin till as possibly Kansanand as lying above the Bishop tuff, but Gilbert(1938, p. 1860), Rinehart and Ross (1957),Putnam (1960, p. 233), and Wahrhaftig andBirman (1965, p. 310) all place the till beneaththe tuff.

The initially determined K40/A40 age of theBishop tuff was 870,000 years (Evernden andothers, 1957, p. 14). This was subsequentlyrevised to 980,000 years (Evernden and others,

1964, p. 175; Evernden and Curtis, 1965, p.355). Since he regarded the Sherwin till asIllinoian, but older than the tuff, Putnam(1962, p. 205) flatly rejected this age as toogreat. Inclusions of older rock fragments in thetuff (Gilbert, 193 8, p. 1834-1835; Rinehart andRoss, 1957; Putnam, 1960, p. 236) permit aninference of contamination, as shown byDalrymple and others (1965, p. 670-671).However, dating of sanidine crystals takenfrom pumice fragments within the tuff, thuspresumably uncontaminated, gives the stillimpressively great age of 710,000 years for thebasal pumice of the Bishop tuff volcanicepisode (Dalrymple and others, 1965, p. 670).

In view of the use being made of these dates(Ericson and others, 1964, p. 731), it is clearlydesirable to establish as firmly as possible thegeological relationship between the Bishop tuffand Pleistocene glacial deposits. If this can be

Geological Society of America Bulletin, v. 79, p. 351-364, 8 figs., March 1968351

352 R. P. SHARP—SHERWIN TILL-BISHOP TUFF, SIERRA NEVADA

done satisfactorily, and if the potassium-argonage of 710,000 years holds up, this becomes amost important datum in the Pleistocenegeological record (Rinehart and Ross, 1964, p.79)'

The geological problem, simply stated, is asfollows. Evidence is compelling that a tillunderlies the Bishop tuff. Is it the equivalent ofBlackwelder's-type Sherwin? Or are there twotills here, one older and one (the type Sherwin)younger than the Bishop tuff (Rinehart andRoss, 1964, p. 74)? Reference is to pre-Tahoetills as the Tahoe and subsequent glaciations areclearly younger than the tuff (Putnam, 1949,p. 1291). Informed readers will recall that east-side Sierra Nevada glaciations currentlyrecognized are from youngest to oldest; Tioga,Tenaya, Tahoe, Mono Basin, Sherwin, McGee,and Deadman Pass, with the first three usually-regarded as Wisconsin and the remainder asPre-Wisconsin.

The tuff-till relationship has been investi-gated in the field, at intervals, during the sum-mers of 1964, 1965, and 1966 through "handsand knees" tracing of contacts, study of ex-posures, and repeated visitations to criticallocations. The conclusion reached is the op-posite of a view, initially held, that favored thetwo-till concept. The assembled evidence showsthat Blackwelder's-type Sherwin and the tillbeneath the Bishop tuff are one and the same.The Sherwin till is older than the Bishop tuffand thus more than 710,000 years of age, if thepotassium-argon dates are valid. This has far-reaching significance with respect to Pleistocenechronologies (see discussions in Evernden andCurtis, 1965, especially by Hopkins, Howell,and Wright).

Physical Setting

The setting is a 7000-foot tableland lyingnortheast of the Sierra Nevada front in east-central California (118° 38' W., 37° 33' N.).Here Rock Creek emerges northeasterly fromits mountain canyon and turns abruptly south-easterly through a narrow gorge cut into thesouthwestern flank of the tableland (Fig. 1).Exposed Sherwin till covers a bulbous area of7 square miles bisected by this gorge. The re-gion is shown on the U. S. Geological Survey'sCasa Diablo Mountain, 15-minute quadrangle,1953.

The location, distribution, and lithologicconstitution of Sherwin drift show clearly thatit was derived from the Rock Creek drainagewith the glacier following a route directly east-

ward which departs from the present RockCreek course near the head of Whiskey Canyon(Fig. 1). Differences in thickness of the Sher-win till are considerable owing to its irregulartopography and the uneven underlying bed-rock surface. The greatest exposed thickness,500 feet, is in Rock Creek gorge.

ACKNOWLEDGMENTSWarm hospitality and assistance have been

extended by the staff of the Sierra NevadaAquatic Research Laboratory on ConvictCreek. Discussions in the field with ClydeWahrhaftig and his associates and my Caltechcolleagues have been most helpful. The manu-script has been improved by critical commentsfrom Brent Dalrymple, Charles Gilbert, andMaxwell Gage.

GLACIAL DRIFT BENEATHBISHOP TUFF

Glacial deposits beneath the Bishop tuff areknown at three sites within the map area (Fig.2). The first, most obvious, and most accessiblesite is a deep cut on U. S. Highway 395 justeast of the crossing with Rock Creek, hereaftertermed Big Pumice Cut. Exposed here fromthe top down are 10 to 12 feet of fluvial gravelwith smooth, rounded stones; 75 feet of loosewhite pumice and ash; and 45 feet of boulderytill (Fig. 3). Enclosing stones from the fluvialgravel, clastic dikes cut the pumice and ex-tend into the till, transecting boulders. Thepumice consists of two units. One is a lowerwell-layered, somewhat brownish sequence ofash and pumice, 15 feet thick, with beddingconformable to the gently inclined (10°) tillsurface. The other is a coarser, looser, poorly-bedded white pumice in horizontal attitude.The lower unit looks like an eolian deposit laiddown as a relatively uniform blanket over anuneven terrain. The upper, horizontally-beddedunit was perhaps emplaced as a series of pumiceflows. The capping of fluvial gravels is muchyounger. Beneath the pumice is an excellenttill, unbedded, poorly sorted, with a tight, fine,silty matrix, and a variety of lithologies repre-sented in the boulders. Granodiorite and quartzmonzonite are the predominating lithologies.

The Bishop tuff cliff lies 1000 feet northeastof Big Pumice Cut, but there can be littledoubt that the pumice and ash in the cut rep-resent the initial phase of the Bishop tuff vol-canic episode (Putnam, 1960, p. 235-236). Sim-ilar material is seen below the Bishop tuff inother places, and radiometric dating by Dal-

Surge Tank- = =:== Power

HouseNo.1

o

o

3CD

z,w

ffi

SO

Figure 1. Place map of Sherwin Grade-Rock Creek area.

354 R. P. SHARP-SHERWIN TILL-BISHOP TUFF, SIERRA NEVADA

PUMICE ON SHERWIN TILL

Fluv ia l gravel

355

*', 'V0 50 -100 200

^03001 |~_ nireeT

A p p r o x i m a t e Scale

Figure 3. Field sketch of relationships in Big Pumice Cut, U. S. 395 east of Rock Creek.

rymple and others (1965, p. 670) shows thepumice and cliff-making tuff to be of one age.

Beneath the Bishop tuff, glacial depositswere penetrated for some 1100 lineal feet by atunnel of the Los Angeles Department of Waterand Power on a line passing 0.75 mile north-east of Big Pumice Cut (Fig. 2). Exploratoryholes drilled for this project penetrated glacialdrift up to 400 feet thick beneath the Bishoptuff (Putnam, 1960, p. 233-234).

Nearly 4 miles east of Big Pumice Cut in thewalls of Owens River gorge upstream frompowerhouse No. 1 (Fig. 1), a lenticular bodyof glacial drift, up to 200 feet thick and 1.5miles across, is exposed (Fig. 4) beneath theBishop tuff (Gilbert, 1938, p. 1860; Ross andRinehart, 1957; Putnam, 1960, p. 234). Thesmooth surfaces and rounded shapes of stonesin this material suggested outwash. This sug-gestion was confirmed after an extended searchwithin the gorge led to an exposure adequateto establish that the matrix is water-sorted sandand fine gravel, not the tight, silty matrix of

Bishop tu f f (150ft.)(0.7 m.y.)

Sherw in outwash (100 ft.)

Ter t ia ry basa l t ( 7 5 f t . )(3.2 m.y.)

Cretaceous quartzmonzonite(300-350 ft.)

Figure 4. Field sketch of Sherwin outwash ex-posed in walls of Owens River gorge.

the till exposures on U. S. 395. Unfortunately,Putnam's descriptions (1960, p. 233-234) ofglacial drift in the Bureau of Water and Powertunnel do not distinguish between till and out-wash; nonetheless, despite his contrary state-ment (1960, p. 250), Sherwin ice need not haveextended as far east as Owens River gorge.

PUMICE ON SHERWIN TILL

Introduction

Glacial drift clearly underlies the Bishoptuff, but what is the relationship of this driftto the Sherwin till? Exposures of the till arenearby but not obviously stratigraphically be-neath the Bishop tuff. If two tills are present,they cannot be differentiated lithologically asboth consist of debris derived from the RockCreek drainage. Lithologic variations in a singleexposure of Sherwin till are as great as any dif-ferences between Sherwin deposits and the tillin Big Pumice Cut. A contact between twotills could be defined by intervening depositsor by deep weathering of the underlying unit.Both were looked for but not found. To date,fragments of pumice or Bishop tuff have neverbeen found as a constituent of Sherwin till norhas Sherwin till ever been shown to rest uponBishop tuff.

Pumice-Sherwin TillRelationships along U. S. 395

Attention is focused on a strip along U. S.395 and Rock Creek gorge extending throughsections 1, 2, and 34 (Fig. 2). At the northwestend of this strip is Big Pumice Cut exposingtill beneath pumice. At the southeast end is alarge ridge, summit elevation 7246, composedof unquestioned Sherwin till. Pumice-till rela-

356 R. P. SHARP—SHERWIN TILL-BISHOP TUFF, SIERRA NEVADA

tionships will be traced southeastward downthis strip.

Just northeast of U. S. 395, about 0.5 mileeast of Big Pumice Cut, is an isolated flat-topped hill and 0.25 mile southeast is a smallerhill rising to the same level (Figs. 1,2). Putnam(1960, p. 240, map 1) mapped these hills asSherwin till capped with fluvial gravel. Theyare gravel-capped, but they are not predom-inantly till. The smaller hill is wholly pumiceas shown by the debris on its slopes, by poleholes dug in 1964 for a powerline crossing itsflank, and by a cut on U. S. 395. The largerhill does have a band of till, with 20-footboulders, on its southern and western flanks.The till forms a topographic bench on the hill-side and is exposed in a long cut on U. S. 395.However, the hill slope above and below thetill bench is covered with pumice.

Two interpretations of these relationships areconsidered. Either a mass of post-pumice tillhas been plastered against the flank of this hill(Fig. 5B), or the hill consists of pumice thathas buried an irregular mound of till with sub-sequent dissection partially exposing the under-lying material (Fig. 5A). This last interpreta-

tion is preferred because the band of till can betraced around the west flank of the hill in theshape of a narrow finger that expands north-ward into a bulb which extends to within 100feet of the Bishop tuff cliff (Fig. 2). It seemsunlikely that a glacier could extend halfwayaround this pumice hill, depositing till in thisconfiguration on the pumice, without spread-ing debris all over the nearby landscape. Theplanimetric and cross-section relationships aresatisfactorily accounted for if pumice overliesthe till.

Southwest of these pumice hills in the south-ern part of Sec. 34 and the northern part ofSec. 2 (Fig. 2) is a broad, west trending,pumice-filled swale which is transected by theeast wall of Rock Creek gorge. The pumiceattains a maximum thickness of 50 feet andrests upon coarse bouldery till. However, is itprimary (in situ) or reworked from the pumicehills one-half mile to the north? Interpretationof the relationships is further complicated bypatches of coarse fluvial gravel and scatteredlarge crystalline boulders, both resting on thepumice, and by one small island-like till area(Fig. 2, SE1/4 sec. 34).

inenro

A

F la t t opped Hil l

F luv ia l g r a v e l ;

B ishop T u f fC l i f f

P u m i c e

WSW

ZZ^&^'Qrt o'SJfo,̂ > ENE

Approx imate Scale

Figure 5. Interpretive cross sections of flattopped pumice hills in sec. 34. A, pumice burying till;B, till plastered on pumice.

PUMICE ON SHERWIN TILL 357

That the pumice is primary is suggested byboth the present surface slope which is, to someextent, in the wrong direction for secondaryderivation from the north, and by a small knobin the northernmost part of sec. 2 that appears

fully inspected. The relationships then provedto be as sketched in Figure 6. The top layer isgrus derived by slopewash and creep from theextensive till slope above. Extensively weath-ered pumice is beneath, and partially mixed

South

North

W e a t h e r e d pumice

in situ FeetApp rox ima te Scale

Figure 6. Field sketch of pumice-Sherwin till relationships in Little Pumice Cut, U. S. 395, sec. 1.

to represent a disintegrated outcrop of co-herent pumice-tuff. This swale is thought to bean original topographic feature of the till sur-face that was buried by the erupted pumice.The pumice filling has been preserved by itslower, protected position and by a partial cap-ping of fluvial gravels.

A similar swale is followed in a southerly di-rection by U. S. 395 along the west side of sec.1. Pumice fragments lie on the swale's floorand well up on both slopes as shown in Figure2. Pumice fragments as much as 5 inches indiameter were found at the south end of theswale where it is truncated by Rock Creekgorge. Secondary deposition of the pumice inthis swale is not impossible, but the develop-ment of a pumice filling at least 30 to 40 feetdeep, which was then largely removed for noobvious reason, would be required. Morelikely this feature, too, represents a swale onthe original till surface that was buried by theinitial eruption of pumice. This possibility isstrongly supported by the evidence of LittlePumice Cut, which is described next.

One of the best and most representative ex-posures of Sherwin till in the map area is in thelong road cut on the east side of U. S. 395where it curves around the end of Ridge 7246in the SW1/4 of sec. 1. At the north end ofthis cut is a mass of brownish material initiallydismissed as weathered slopewash until care-

with, the grus. Despite weathering, disturb-ance, and mixing by surface wash, creep, andburrowing animals, enough of the integrity andcharacter of the pumice is preserved to showthat it consists of large blocks of bedded ma-terial corresponding to units identifiable in thebasal pumice-ash sequence of Big Pumice Cut.This has been established by repeated back-and-forth comparisons of the field exposuresand is confirmed by a similarity of includedforeign rock fragments. This deeply weatheredpumice and ash is essentially in place, and itrests upon several feet of brownish-gray grusthat cap the Sherwin till. The stratigraphic re-lationships duplicate those of Big Pumice Cut.

Thus, the pumice-on-till relationship can becarried 1.5 miles southeast from Big PumiceCut to Ridge 7246 which is uncontestablySherwin till. The Sherwin till is stratigraph-ically below the basal pumice-ash beds of theBishop tuff volcanic episode.

Other Locations

Small fragments of pumice have been foundon Sherwin till in other places as follows. TheSurge Tank road runs eastward from U. S. 395essentially along the Bishop tuff-Sherwin tillcontact (Figs. 1,2). The tuff makes a cliff 50to 100 feet high facing outward toward the tillwhich composes the slopes rising away from thecliff. In many places along this contact, pumice

358 R. P. SHARP-SHERWIN TILL-BISHOP TUFF, SIERRA NEVADA

fragments, mostly 1 to 2 inches in diameter butoccasionally attaining 5 inches, can be foundon the till 100 to 200 feet out from the cliffand 30 to 50 feet above (upslope from) its base.

Most significant is an acre patch of pumicefragments at elevation 7180 feet on the crest ofRidge 7246 about 0.25 mile northeast of itssummit (NW1/4, sec. 1, Fig. 2). At the eastend of this ridge, where it abuts against a lowtuff cliff, fragments of coherent tuff, up to 18inches in diameter, lie 150 feet out from thecliff and about 5 feet higher than its base.Pumice fragments up to 5 inches in diameterlie on till at the east end of a southern spur ofRidge 7246 in the NW1/4 of sec. 6, 100 yardsfrom and 60 feet above the nearest Bishop tuffcliff.

At the southern edge of the till mass, in thecenter of sec. 12 just west of U. S. 395, abun-dant fragments of pumice, some up to 6 inchesin diameter, were found on the till up to 100yards from and 75 feet above the base of a lowtuff cliff to the south.

In some situations one could perhaps arguethat the pumice fragments are small enough tohave been carried onto the till by wind. How-ever, the pumice fragments are usually accom-panied by pieces of black hornfels and basalt,common inclusions in the tuff, whose transportby wind is unlikely. All these pumice frag-ments are considered to be residual from aformer covering of Bishop tuff on those partsof the Sherwin till where they are found.

STRIPPING OF BISHOP TUFFGilbert (1938, p. 1833, 1837-1838) regards

the present margin of the Bishop tuff as essen-tially its original edge, feeling that little cliffrecession and stripping have occurred. Putnam(1960, p. 234), however, favors considerablestripping in the Sherwin Grade-Rock Creekarea, inferring that most of the Sherwin tillexposed there was once buried beneath tuff.Neither author supports his position with con-crete facts, and compelling evidence of strip-ping is hard to find. Areas of granitic rock, forexample that in the SW1/4 of sec. 36 (Fig. 2),almost certainly once buried by Bishop tuff, nolonger retain even the smallest remnant or frag-ment of tuff. Evidence of burial should be evenmore easily removed from smooth slopes of un-consolidated till than from craggy exposures ofgranitic rock. The layer of unconsolidatedpumice and ash at the base of the tuff facilitatesstripping, and the uneven distribution of thislayer promotes different degrees of stripping.

The previously described pumice fragments

on Sherwin till demonstrate less than 100 yardsof stripping in most places along the margin ofthe presently exposed till mass. Almost cer-tainly, the cover of Bishop tuff on Sherwin tillwas greater, an inference supported by thesmall patch of pumice fragments at elevation7180 feet on the crest of ridge 7246 (Fig. 2).This pumice is of the type found more abun-dantly in the cliff-making Bishop tuff than inthe basal airborne layer. Since the Bishop tuffrepresents a series of ash and pumice flows(Gilbert, 1938, p. 1851-1852), it presumablycovered the terrain up to a reasonably ac-cordant height. The pumice patch on ridge7246, with allowances for post-tuff warping andfaulting, suggests that most of the exposedSherwin till area east of Rock Creek and per-haps one third of the area west of that streamwere formerly mantled by tuff. This is some-what less than the coverage inferred by Put-nam (1960, p. 234) but possibly greater thananything envisioned by Gilbert (1938, p. 1837-1838).

GEOMETRICAL RELATIONSHIPSAlthough the map (Fig. 2) illustrates how

lobes of Sherwin till penetrate re-entrants inthe edge of the Bishop tuff, it does not showthat the tuff cliff rises abruptly above the tillat these places. Such relationships are best seenalong the Surge Tank road. Easily visible southof the road at the northwest corner of sec. 6 isa lobe of bouldery till extending north be-tween low cliffs of Bishop tuff. It would havebeen most difficult for a glacier younger thanthe tuff to have deposited the till in this rela-tionship, although a debris flow from the ice orfrom marginal glacial deposits might have donethe job. Fragments of coherent tuff, up to 18inches across, resting on the till at the head ofthis lobe support the inference, drawn fromgeometrical relationships, that the tuff isyounger.

Farther east, in the north-central part of sec.6, the Surge Tank road crosses a low mass ofbouldery till extending northeast to the foot ofthe tuff cliff. This till patch is nearly enclosedby the tuff cliff with only a limited opening tothe southwest. It would have been extremelydifficult for a glacier younger than the tuff tohave laid down this till without scattering gla-cial debris over the nearby terrain where it islacking. The inference of a pre-tuff age for thistill is confirmed in Los Angeles Bureau of Waterand Power drill hole G-1, a scant 100 feet northof the till-tuff contact (Fig. 2), that showsglacial drift beneath Bishop tuff.

SHERWIN OUTWASH AND OTHER GRAVELS 359

NO SHERWIN TILL ON BISHOP TUFF

An extended search for till or glacial erraticsresting on Bishop tuff turned up very little ofinterest. In the south central part of sec. 6, fourscattered granitic boulders, respectively 18, 16,12, and 9 inches in diameter, were found on thetuff. All were of rock types that could havebeen supplied from high-standing exposures ofWheeler Crest quartz monzonite just three-eighths of a mile west. An additional threegranitic boulders up to one foot in diameterwere found in an Indian stone circle, of whichthere is an abundance in this area. These rela-tions, plus the fact that the tuff itself occasion-ally includes granitic boulders, makes glacialemplacement of this material unlikely, espe-cially in the absence of other debris typical ofthe till.

In the south-central part of sec. 6 is an iso-lated patch of boulder gravel (Fig. 2) coveringabout half an acre and lying only 150 yards eastof exposures of Sherwin till. The deposit con-tains boulders up to 4 feet in diameter, manysmaller, worn, and rounded stones, 1 to 4 inchesacross, and finer gravel and grus. The lithologictypes present are those of the Sherwin till. Al-though the gravel patch lies downslope fromexposures of Sherwin till, it is completely sur-rounded by Bishop tuff and there is no con-necting trail of debris to the till.

Several interpretations are possible: (1) Thedebris is an outlier of Sherwin till resting ontop of Bishop tuff. (2) It is Sherwin debris,secondarily reworked and transported onto thetuff. This would be possible under existingtopographic relations. (3) It is a window ofSherwin drift exposed by removal of the over-lying Bishop tufl.

The fact that the tuff is probably, at most,10 feet thick here; that the gravel is completelysurrounded by tuff which in places rises a fewfeet above it; and, that any connection on thetuff surface to the Sherwin till 150 yards westis lacking, suggests this is probably an unroofedexposure of Sherwin drift. However, anyonewishing to argue that the Sherwin overlies theBishop tuff will be interested in visiting thislocation.

SHERWIN OUTWASHAND OTHER GRAVELS

Sherwin till-Bishop tuff geometrical relation-ships are such that, if the till were younger,Sherwin outwash should lie on the tuff or withingullies draining across the tuff. The topographicsetting is particularly favorable for this along

the eastern and southern margins of the tillbody, but no Sherwin outwash is found thereor elsewhere on the tuff. The only Sherwin out-wash recognized is that exposed in the walls ofOwens River gorge beneath the Bishop tuff(Fig. 4). Similar gravels are not found beneaththe Bishop tuff in Rock Creek gorge, suggest-ing that the direction of drainage in Sherwintime was eastward from the mountains, ratherthan southeasterly, as at present. The presentsoutheasterly slope of the land is attributed towarping after eruption of the Bishop tuff(Rinehart and Ross, 1957; Putnam, 1960, p.244-245).

A considerable area of gravel resting onBishop tuff has been mapped, mostly east oflower Rock Creek gorge, beginning 2.5 milessoutheast of the Sherwin till area (Fig. 7).These gravels rest upon the tuff and incor-porate fragments of it. Small exposures can beseen in road cuts along U. S. 395 (sees. 4, 33,Fig. 7), but the best exposed section is at thefirst curve on the old Sherwin Grade road justwest of Rock Creek at Paradise Camp (sec. 29,T. 5 S., R. 31 E., Mt. Tom Quadrangle). Hereare some 40 feet of beds, principally fluvialgravel with smoothly worn, rounded stones setin a sandy matrix. Near the top are two lenses,7 to 8 feet thick, containing larger boulders to6 feet in diameter set in a dense silty matrix.These lenses look like debris-flow deposits. Sim-ilar material is exposed in gullies along thewooden-pole powerline road west of U. S. 395.Stones in these deposits, mostly 6 to 18 inchesbut occasionally up to 8 feet in diameter, areof lithologies found in the Sherwin till andrepresented in the bedrock of upper RockCreek drainage.

These deposits have considerable antiquity,for granitic boulders are completely disinte-grated 15 feet beneath the ground surface. Thematrix is also weathered a rich yellow-brown(10YR5/4) to this depth, and the upper partsof stripped, debris-flow layers between RockCreek and U. S. 395 are heavily calichefied.The gravels are also buried by a blocky creepmantle of Bishop tuff fragments derived fromnearby higher outcrops. As much as 300 feet ofdowncutting in Rock Creek gorge has occurredsince the initial phase of gravel deposition.

Farther up the old Sherwin Grade road westof Rock Creek gorge are large areas of bouldergravel consisting wholly of Wheeler Crestquartz monzonite and associated dike rocks(Fig. 7). This material has come from the highabrupt face of Wheeler Crest to the west andnorthwest and need have no genetic relation to

360 R. P. SHARP—SHERWIN TILL-BISHOP TUFF, SIERRA NEVADA

Q

G r a v e l f romWheeler Crest

Grave l fromupper Rock Creek

0 -f

Scale in Mi les

Figure 7. Map of gravels on Bishop tuff along lower Rock Creek.

the first described gravels. It will not be con- Creek. The lower part of the gorge was not assidered further. deeply cut then as now, for the gravels spread

The distribution (Fig. 7) and constitution of eastward in shallow gullies over the Bishop tuffthe first described gravels indicate that they surface from a point about 1.5 miles above thehave been transported down the course of Rock present gorge mouth and at a level 300 feet

SPECULATION ON CORRELATION 361

higher than its present floor. However, gravelsof similar constitution are also found withinthe gorge to within 80 feet of its floor, suggest-ing that their transport extended over a con-siderable part of the gorge-cutting interval.

These gravels are related to the diversion ofRock Creek from its former northeasterly pathinto the present southeasterly course, as de-scribed by Putnam (1960, p. 249). At that timedeep dissection of the Sherwin till occurred,and a large volume of Sherwin debris must havebeen flushed down the new Rock Creek chan-nel. These gravels are regarded as remnants ofthat debris. Putnam (1960 p. 251) dates thediversion as Tahoe in age, but the gravels ap-pear much older. Since the Tahoe dating is notsupported by any direct evidence, and since theRock Creek gorge is cut 500 to 600 feet intoBishop tuff and resistant crystalline rocks, it issuggested that the Rock Creek diversion oc-curred well before Tahoe time.

These gravels are clearly younger than theBishop tuff and probably consist of debris re-worked from the Sherwin till. The possibilitythat they represent Sherwin outwash and thatthe Sherwin glaciation occurred after eruptionof the Bishop tuff is highly unlikely.

SPECULATION ON CORRELATIONThe 710,000-year potassium-argon date for

the basal pumice-ash unit of the Bishop tuftvolcanic episode (Dalrymple and others, 1965,p. 670) makes the Sherwin till surprisingly old.The following indications of erosion and weath-ering prior to extrusion of the Bishop tuff sug-gest that the Sherwin till is somewhat olderstill.

Granitic knobs capped by Bishop tuff with-out intervening glacial deposits, as in sec. 36(Fig. 2), where the Sherwin drift beneath thetuff is both extensive and thick (Tunnel 1 sec-tion, Fig. 1 in Putnam, 1960) suggest consid-erable pre-tuff erosion of the Sherwin deposits.In Big Pumice Cut a layer of soily brown grus,much like that on presently exposed slopes ofSherwin till, underlies the basal pumice and in-dicates considerable disintegration, slopewash,and creep on the Sherwin slopes before burial.The till itself is only modestly oxidized to adepth of a few feet, but disintegration of gra-nitic boulders to a depth of 25 feet below thetill-pumice contact is extensive.

The buried Sherwin till is more deeplyweathered than most Tioga moraines and ap-proaches, but does not generally attain, theweathering displayed by many Tahoe deposits

in this region. It is judged, therefore, to haveexperienced weathering for a few tens of thou-sands of years but not as much as 100,000 yearsprior to eruption of the Bishop tuff. An esti-mated age in the neighborhood of 750,000years for the Sherwin thus seems reasonable.

Large areas of Sherwin till previously havebeen recognized along the eastern Sierra Ne-vada front in the Mono and Bridgeport basinsand on West Walker River (Blackwelder, 1931,p. 895-900; Putnam, 1949, p. 1290). Althoughfirm correlation between these occurrences andthe type locality has not been established, thetopographic setting, the succession of glacia-tions, and semi-quantitative data reflecting age,support such a correlation. There is scant reasonto adopt the view of Evernden and Curtis(1965, p. 356) that the Sherwin of the type lo-cality has no established relation to otherSierra glacial deposits. Certainly, till beneaththe Bishop tuff in the Mono Basin has a corre-sponding stratigraphic position (Putnam, 1949,p. 1289).

The considerable age of the Sherwin makesone wonder about the McGee till, a presumablystill older episode of Sierra glaciation (Black-welder, 1931, p. 902-906; Putnam, 1962, p.192-195). The McGee has a topographic settingwhich is unusual, even for Sherwin, and on thatbasis alone it could well be older, especially inview of the 2.7-3.0 X 106-year age of a nearbydeposit stated to be of glacial origin (Curry,1966).

In the bottom of Rock Creek gorge along theold Sherwin grade road at the south edge of theSherwin till area (sec. 12), a road cut in tillexposes a prominent reddish zone at its core(Fig. 8). This looks like two tills separated by

FigureS. Field sketch of old red (pre-Sherwin?)till on old Sherwin Grade road, lower Rock Creek,sec. 12.

a deeply weathered zone. Gray material abovethe reddish zone has all the aspects of a first-class till and as such it is surely Sherwin. At-tempts to explain the reddish zone as a ground-

362 R. P. SHARP—SHERWIN TILL-BISHOP TUFF, SIERRA NEVADA

water phenomenon fail because some of thereddish debris is reworked into the overlyingdeposit. Deposition of the overlying materialby surface creep is not supported by its dense,tight, till-like matrix or the inclusion of an un-deformed lens of water-laid debris. A pH pro-file across the contact is of little help as it yieldsvalues of 8 to 8.5 all the way. Little veinlets ofcalcium carbonate extend from the overlyingdebris into the reddish zone which has thus hadits CaCOs restored by percolation from above.The contact is displaced by small faults in sev-eral places (Fig. 8), further evidence of somedegree of antiquity.

These relations remain something of anenigma but for the present are interpreted asthe contact between two tills. This is favoredby the location, at the bottom of a 500-footgorge near the edge of the Sherwin till area.Just possibly, one sees here the contact betweenSherwin and McGee tills.

The Sherwin till was tentatively regarded asKansan by Blackwelder (1931, p. 918). In viewof its probable age of about 750,000 years, thisseems more reasonable than the Illinoian age

urged by Putnam (1962, p. 205). Radiometricdating is currently so severely challengingtemporal correlations with continental glacialepisodes that it seems pointless at this time tourge any specific correlation of the Sherwinwith the classical midwestern glacial sequence.

As Damon (1965) notes, Pleistocene chro-nologists disagree widely about the duration ofthe glacial Pleistocene (Hopkins, in Everndenand Curtis, 1965, p. 372). Some are advocatinga duration in excess of 1 million (Evernden andCurtis, 1965, p. 343) to 1.5 million years (Eric-son and others, 1964, p. 731). A Sherwin ageof 750,000 years is not inconsistent with a gla-cial Pleistocene of this duration, but it is farout of line with the shorter glacial epochs ad-vocated by Emiliani (1958, p. 271; 1966, p.855-856; Emiliani and others, 1961, p. 687)among others. The task of bringing interpreta-tions of marine and terrestrial Pleistocenerecords into harmony remains interesting andchallenging, especially in view of Curry's (1966)recent report of a possible Sierra Nevada glaci-ation radiometrically dated at 2.7-3.0 X 106

years.

REFERENCES CITED

Blackwelder, Eliot, 1931, Pleistocene glaciation in the Sierra Nevada and Basin Ranges: Geol. Soc.America Bull, v. 42, p. 865-922.

Curry, R. R., 1966, Glaciation about 3,000,000 years ago in the Sierra Nevada: Science, v. 154, p. 770-771.

Dalrymple, G. B., Cox, Allen, and Doell, R. R., 1965, Potassium-argon age and paleomagnetism of theBishop Tuff, California: Geol. Soc. America Bull., v. 76, p. 665-674.

Damon, P. E., 1965, Pleistocene time scales: Science, v. 148, p. 1037-1038.Emiliani, Cesare, 1958, Paleotemperature analysis of core 280 and Pleistocene correlations: Jour. Geology,

v. 66, p. 264-275.1966, Isotopic paleotemperatures: Science, v. 154, p. 851-857.

Emiliani, Cesare, Mayeda, T., and Selli, R., 1961, Paleotemperature analysis of the Plio-Pleistocenesection at Le Castella, Calabria, southern Italy: Geol. Soc. America Bull., v. 72, p. 679-688.

Ericson, D. B., Ewing, Maurice, and Wollin, Goesta, 1964, The Pleistocene Epoch in deep-sea sedi-ments: Science, v. 146, p. 723-732.

Evernden, J. F., Curtis, G. H., and Kistler, R., 1957, Potassium-argon dating of Pleistocene volcanics:Quaternaria, IV, p. 13-17.

Evernden, J. F., Savage, D. E., Curtis, G. H., and James, G. T., 1964, Potassium argon dates and theCenozoic mammalian chronology of North America: Am. Jour. Sci., v. 262, p. 145-198.

Evernden, J. F., and Curtis, G. H., 1965, The potassium-argon dating of late Cenozoic rocks in EastAfrica and Italy: Current Anthropology, v. 6, p. 343-364 (with comments, p. 364-385).

Gilbert, C. M., 1938, Welded tuff in eastern California: Geol. Soc. America Bull., v. 49, p. 1829-1862.Putnam, W. C., 1949, Quaternary geology of the June Lake district, California: Geol. Soc. America

Bull., v. 60, p. 1281-1302.1960, Origin of Rock Creek and Owens River gorges, Mono County, California: Univ. Calif. Pub.

in Geol. Sci., v. 34, p. 221-280.1962, Late Cenozoic geology of McGee Mountain, Mono County, California: Univ. Calif. Pub. in

Geol. Sci., v. 40, p. 181-218.

REFERENCES CITED 363

Rinehart, C. D., and Ross, D. C., 1957, Geology of the Casa Diablo Mountain quadrangle, California:U. S. Geol. Survey Geol. Quad. Map GQ-99, scale 1:62,500.1964, Geology and mineral deposits of the Mount Morrison quadrangle, Sierra Nevada, California:

U. S. Geol., Survey Prof. Paper 385, 106 p.Wahrhaftig, Clyde, and Birman, J. H., 1965, The Quaternary of the Pacific mountain system in Cali-

fornia: p. 299-340, in Wright, H. E., and Frey, D. G., Editors, The Quaternary of the United States:Princeton, Princeton Univ. Press.

MANUSCRIPT RECEIVED BY THE SOCIETY DECEMBER 27, 1966REVISED MANUSCRIPT RECEIVED FEBRUARY 21, 1967CALIFORNIA INSTITUTE OF TECHNOLOGY DIVISION OF GEOLOGICAL SCIENCES PUB. No. 1433