Geologic History - SMRNA · The geologic history of New Mexico is wonderfully di-verse. Exposed within the state’s boundaries are Pre-cambrian igneous and metamorphic rocks more

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New Mexico’s Environment

Geologic HistoryBarry S. Kues and Jonathan F. Callender

New Mexico in Maps, Second Edition, Edited by Jerry L. Williams

The geologic history of New Mexico is wonderfully di-verse. Exposed within the state’s boundaries are Pre-cambrian igneous and metamorphic rocks more than1.5 billion years (b.y.) old, sedimentary strata repre-senting each geologic period from Cambrian to Qua-ternary, and a variety of volcanic rocks erupted overthe past 60 million years (m.y.) to within a few hundredyears of the present. Study of these rocks and theirrelationships within the structural, tectonic, and geo-morphic framework of New Mexico’s present land-scapes has yielded much information on the geologicevolution of the state. Because of the complexities ofNew Mexico’s geology, however, only a brief outline ofespecially important aspects of the state’s geology canbe presented here. Some additional details of NewMexico stratigraphy and structure may be found in thediscussion of the state’s paleontology and young fault-ing elsewhere.

New Mexico’s Precambrian rocks are exposed predomi-nantly in the cores of mountain ranges along the eastside of the Rio Grande and in a few isolated ranges tothe west, such as the Brazos, Nacimiento, Zuni, andBarros mountains. A wide variety of metamorphic andigneous rock types is present, with much local varia-tion and many complex structures. In general the pat-tern seems to have been initial deposition and gradualburial of clastic sediments about 2 b.y. ago (probably atthe edge of an ancient continent), accompanied byseveral episodes of extrusive volcanic activity, and fol-lowed by extensive regional folding, faulting, and meta-morphism. Deformation and metamorphism wroughtdramatic changes in the volcanic and sedimentaryrocks: clastic sediments became contorted phyllites,schists, and quartzites, and the extrusive volcanics weretransformed into sheared belts of felsites and amphibo-lites. Intrusion of granitic magmas overlapped the longcontinuing tectonism and metamorphism, producingmetamorphosed gneisses, in addition to large volumesof undeformed granite. Pegmatite dikes, representingfinal crystallization of magmas and containing beryl,lepidolite, tantalite, and other rare minerals, were in-jected locally into older granites, most notably at theHarding pegmatite mine near Dixon, and in the PetacaDistrict of the Brazos Mountains.

These events appear to have begun earlier in northernNew Mexico. Precambrian rocks in the Brazos, Taos,and Nacimiento mountains have been dated at 1.7 to1.8 b.y., whereas the Precambrian cores of the Zuni,Manzano, Ladron, and Magdalena mountains are 1.3to 1.6 b.y. old, and the Sandia Granite is about 1.45 b.y.old. Farther south, in the San Andres Mountains, Pre-cambrian ages are 1.3 to 1.4 b.y., and the Precam-brian of the Franklin Mountains near El Paso is scarcely1 b.y. old. As Precambrian metamorphic and igneousactivity subsided, the landscapes began to be eroded,and, with the waxing and waning of Paleozoic seasacross New Mexico, were in most areas eventually cov-ered by Paleozoic sediments. Uplift during late Tertiarytime, associated primarily with tectonic movement alongthe Rio Grande rift, has once again exposed some ofthese Precambrian rocks.

Through the Paleozoic Era most of the state was cov-ered by vast shallow seas in which thick sequences oflimestones, sandstones, and shales accumulated. Thesedimentary record for the Cambrian through Devo-nian periods is limited to the mountain ranges in thesouth-central and southwestern part of New Mexico;erosion of early and middle Paleozoic sediments innorthern New Mexico occurred later in the Paleozoic.Through the Mississippian and Pennsylvanian periods,marine sediments were deposited in many parts of NewMexico. Renewed uplift in the Pennsylvanian createdseveral large north-south trending islands that dividedthe northern seas, and by the beginning of the Per-mian these islands had coalesced into a landmass thatshed great volumes of red clastic sediments, pushingthe shoreline inexorably southward. These events cor-responded temporally to the worldwide assembly ofsupercontinent Pangaea. In southern New Mexico thetropical seas in which the great Capitan Reef Complexgrew persisted until nearly the end of the Permian, buteventually dwindled and vanished, leaving thick se-quences of salt and potash over much of southeasternNew Mexico.

Rocks of Triassic and Jurassic age are confined mainlyto the northern half of the state and were deposited asrivers spread eroded sediments across vast continen-

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New Mexico Master Gardener Manual

tal plains toward oceans to the west. Colorful red, green,gray, brown, and white sandstones and shales of theChinle and Morrison formations represent these peri-ods in many parts of northern New Mexico.

By the last half of the Cretaceous period the seas hadreturned; New Mexico was on the western shoreline ofa great shallow ocean that covered most of central NorthAmerica. Numerous advances and retreats of the shore-line produced a great variety of marine and swampyfacies. The classic sequence in the San Juan Basin isthe best and most easily observed example, for theseCretaceous units are widely exposed today over muchof the northwestern quarter of the state. Most of NewMexico’s coal deposits formed from the lush vegeta-tion that existed in northwestern and northeastern NewMexico during this time. The sea retreated quickly outof the state at the close of the Cretaceous, the lasttime New Mexico would be covered by marine waters.About the same time, the Laramide orogeny, a pro-found mountainbuilding episode centered to the westof the state, intensified volcanic activity and uplift in theSan Juan Mountains of southwestern Colorado andneighboring areas. Large volumes of clastic sedimentswere deposited by rivers across much of New Mexico,concentrating in structural depressions such as the SanJuan and Raton basins. Local and sporadic volcanicand igneous activity also characterized some parts ofNew Mexico during the early Tertiary; the internal partsof these volcanic systems are now exposed as stocksand dikes mainly in southwestern New Mexico.

Beginning about 40 m.y. ago, much of southwesternand central New Mexico was subjected to an enormousexplosion of volcanic activity that lasted about 20 m.y.before subsiding. Great thicknesses of ash-flow tuffs,along with andesite, rhyolite, and basalt flows, origi-nated from gigantic volcanic cauldrons (some more than50 km in diameter) as a consequence of complex inter-actions between two colliding lithospheric plates alongthe western coast of North America. Many of the caul-drons are not obvious in the present landscape, havingbeen obscured by subsequent geologic events, but theyform the cores of some of the most conspicuous topo-graphic features of southwestern New Mexico, such asthe Mogollon-Datil plateau, Black Range, and Organ,Magdalena, San Mateo, and Peloncillo mountains. Hy-drothermal fluids associated with this volcanism pro-duced some of New Mexico’s most important metallicresources. Other large volcanic masses in central NewMexico (e.g., Sierra Blanca and the Capitan and Ortizmountains) formed about this time, which also wit-nessed the final uplift of the Sangre de Cristo Moun-tains. Locally, the eroded necks of isolated volcanos

which formed in the middle Tertiary, such as Shiprock,still project above the modern landscape.

Continued crustal instability, chiefly extension, was alsoresponsible for initiating, about 30 m.y. ago, the RioGrande rift, a great north-trending structural depres-sion that bisects the state. Along the eastern edge ofthe rift, fault blocks have been uplifted gradually to forma line of prominent mountain ranges, and the basinswithin the rift have accumulated thousands of metersof Miocene to Recent sediments. Continued evolutionof the rift has also assured a strong igneous imprint onthe geologic history of western and central New Mexico.In the Jemez Mountains, volcanism began about 10m.y. ago, producing a series of basaltic and rhyoliticflows. As the magma chamber underlying the area be-came depleted, explosive eruptions beginning about 1.4m.y. ago spread ash-flow tuffs and pumice across theBandelier area, and scattered ash as far east as Kan-sas. Collapse subsequent to these eruptions createdthe Valles Caldera, with a diameter of 22 km, one ofthe largest in the world. After caldera collapse, magmacontinued to be extruded until a few tens of thousandsof years ago. Extensive volcanism also began in north-eastern New Mexico about 8 m.y. ago and left morethan 100 cones as well as widespread lava flows cov-ering more than a quarter of Union and Colfax coun-ties. Volcanic activity continued here until about 4,500years ago, and some of the youngest volcanos, suchas Capulin, are virtually intact.

In west-central New Mexico the Mt. Taylor volcanic fieldflourished from about 3.5 to 2 m.y. ago; Mt. Taylor itselfwas built up over more than a million years of intermit-tent activity. The larger Zuni Bandera field southwest ofGrants is an enormous area of malpais and volcaniccones that originated about 1.5 m.y. ago, and lava ex-trusion has continued nearly to the present. TheMcCarty’s flow is one of the most voluminous volcanicflows in the world that has occurred in historical times;its eruption about A.D. 1300 has been recorded in In-dian stories. The extensive malpais near Carrizozo isnot much older (perhaps 1,000-1,500 years old). Withinabout the past million years significant volcanic activityhas also occurred southwest of Las Cruces and in sev-eral places along the west side of the Rio Grande inthe Albuquerque area. The Albuquerque volcanos andrelated structures are between 150 and 200 thousandyears old.

During the late Tertiary and Quaternary periods, sedi-ments continued to be deposited in all parts of the state.The surface of the High Plains of eastern New Mexicois of this age, and thick Pleistocene sedimentary de-

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posits have built up along the Rio Grande and manyother rivers within the state. Erosion and depositionproceed rapidly compared to most other geologic pro-cesses, and many of the most conspicuous features ofthe present landscape-from the familiar desert mesasand buttes to the intricately dissected badlands of north-west and central New Mexico to the windblown gyp-sum dunes of White Sands-are all the result of veryrecent geologic processes.

Williams, Jerry L., New Mexico in Maps, Second Edi-tion, University of New Mexico Press, Albuquerque, NM,1986, Reprinted for Educational Purposes.

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A physiographic province is a region with a particularpattern of landforms that differs significantly from thatof adjacent regions. Each province has a distinctivegeologic framework and particular combinations of to-pographic and hydrographic features that have evolvedthrough geologic time. The individual landforms (e.g.,mountains, canyons, alluvial fans) that in aggregatemake up the varied natural landscapes of a given prov-ince reflect a variety of geomorphic processes. In NewMexico these range from the action of deep-seated(hypogene) forces, including volcanism and tectonism,to surficial (epigene) processes, such as erosion andsedimentation by water or wind.

The Southern Rocky Mountain Province extends fromColorado into the north-central part of the state as atwo prong system of high ranges separated by deepstructural basins of the northern Rio Grande rift. Theeastern prong of the Southern Rockies includes sev-eral ranges of the Sangre de Cristo Mountains betweenthe San Luis and Española rift basins and the RatonSection of the Great Plains. The area east of Taos alsoincludes the high, parklike Moreno Valley between thecentral Sangre de Cristos and the Cimarron Mountains.The western mountain prong between the Navajo Sec-tion of the Colorado Plateau and the Rio Grande riftbasin includes the Brazos uplift to the north and theoutlying San Pedro, Nacimiento, and Jemez mountainsto the south. The latter highland areas, located southof the lower Chama Valley, are transitional to the south-eastern Colorado Plateau and Basin and Range prov-inces.

The Sangre de Cristo, Brazos, and SanPedro-Nacimiento ranges have cores of Precambriancrystalline rocks overlain by Paleozoic, Mesozoic, andlower Cenozoic sedimentary rocks. Cenozoic volcanicand sedimentary rocks cap the ranges in a number ofareas. Numerous glaciated peaks and alpine valleysare present in the Sangre de Cristos north of Santa Feand in the Brazos uplift near the Colorado border.Among these peaks is Wheeler Peak (13,161 ft), thehigh point of the state, northeast of Taos. Valleys drain-ing areas of alpine glaciation (e.g., Chama, Costilla,Red, Embudo-Santa Barbara, Nambe, Santa Fe, Pecos,Mora) have stepped sequences of glacial-outwash ter-races.

The Jemez Mountains west of Los Alamos (maximumelevation 11,561 ft.) are primarily a constructional fea-ture built by late Cenozoic volcanism. Eruptions of Pleis-tocene age produced the huge Toledo-Valles calderacomplex as well as the Bandelier Tuff that caps thePajarito Plateau at the western edge of the Espaholastructural basin.

The Southern Rocky Mountain Province also includesthe San Luis Valley, a Rio Grande rift basin that is tran-sitional southward to the Espahola Valley in the Basinand Range Province. At the southern end of the basin,the Rio Grande has cut a deep gorge (canyon walls upto 1,000 ft. high) in the thick accumulation of Pliocenebasalt flows that forms the central Taos Plateau. Theeastern part of the plateau includes a constructionalplain built by alluvial fans at the base of the Sangre deCristos.

The Colorado Plateau Province in northwestern NewMexico is part of a larger region (extending into Ari-zona, Utah, and Colorado) characterized by erosionallandscapes carved on relatively undeformed sequencesof sedimentary and volcanic rocks. The Zuni Mountainsbetween Gallup and Grants are the only major moun-tain uplift. The summit of the Mount Taylor volcanic cen-ter (elevation 11,301 ft) is the highest point in the NewMexico part of the province. Major landforms includescarp-bounded tablelands (plateaus, mesas, buttes, andbenches), cuestas, hogbacks, and a variety of valleyand canyon types. The province straddles the conti-nental divide (6,675 to 9,916 ft elevation range) andcontains headwaters of the Rio Chama and Rio Puerco(Rio Grande system) on the east, and the San Juanand Little Colorado rivers on the west.

The Navajo Section of the Colorado Plateau is domi-nated by two structural basins with thick sequences ofgently dipping Mesozoic and lower Cenozoic sedimen-tary rocks, mainly shale, mudstone, and sandstone withextensive coal seams. The large San Juan Basin liesbetween the Southern Rockies, the Four Corners plat-form, and the ZuniDefiance uplift. The smallerGallup-Zuni basin is located south and west of the ZuniMountains. The Chuska Mountains (maximum eleva-tion 9,370 ft), along the New Mexico - Arizona border,are a tableland with prominent bounding escarpments.Prominent volcanic necks, such as Shiprock, in the Four

Physiographic ProvincesJohn W. Hawley


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Corners area north of the Chuskas result from exhu-mation of feeder conduits at middle Cenozoic volcaniccenters.

Aside from narrow hogback belts eroded on steeplydipping strata of monoclines flanking major structuralupwarps, the Navajo Section is characterized by broadrolling plains carved on easily eroded rocks, and cues-tas and tablelands capped by gently dipping resistantsandstone beds. Canyonlands and escarpments ofmoderate local relief occur mainly in the eastern partof the San Juan Basin. However, most stream valleysare broad with relatively short canyon reaches; areasof high cliffs and escarpments are of limited extent.

The highest point in the New Mexico part of the NavajoSection is at Chromo Mountain (elevation 9,916 ft) onthe Continental Divide near Chama. The lowest point isthe San Juan River channel near the Four Corners atthe boundary between the Navajo and Canyonlandssections of the Colorado Plateau (elevation about 4,700ft.).

The only major perennial streams in the Navajo Sec-tion are the San Juan River, the Animas and La Platarivers (which join the San Juan near Farmington), andthe upper Rio Chama. The floodplains of these riversare flanked by stepped sequences of fluvial terraces ofPleistocene age.

In the eastern part of the San Juan Basin, badlandsare locally well developed on steep slopes carved onshaly sequences of late Cretaceous and early Ceno-zoic age. Partly vegetated aeolian sand sheets, withlow eastnortheast-trending dune ridges, and small ac-tive dune fields form extensive caps on upland surfacesin the central part of the Navajo Section, particularlybetween the San Juan and Chaco Valleys.

The Acoma-Zuni Section, the southeastern subdivisionof the Colorado Plateau, is a newly defined physi-ographic unit that includes the northern part of the areapreviously designated the Datil Section. The unit isbounded on the east by the Albuquerque Basin, a RioGrande rift basin in the northern part of the Basin andRange Province. The Datil-Mogollon (transitional) Sec-tion lies to the south.

The Acoma-Zuni Section is characterized by extensiveupper Cenozoic volcanics that form a discontinuouscover on erosional and constructional landforms typi-cal of the neighboring Navajo Section. The northeast-ern Acoma-Zuni area is dominated by Mount Taylor, acomposite stratovolcano of Pliocene age, and nearby

basalt-capped mesas. Cabezon Peak at the northeastedge of the section is a particularly prominent plug-typevolcanic neck. To the west, the elongate upwarp of theZuni Mountains (maximum elevation 9,265 ft), with acore of Precambrian crystalline rocks, is flanked byhogback and cuesta belts that have dipslopes andscarps capped by Permian and Triassic limestone andsandstone.

Broad plains south and east of the Zuni uplift are cov-ered with Quaternary basalt flows and dotted with nu-merous cinder and lava cones. The Malpais Lava Fieldsouth of Grants contains the McCarty’s basalt flow,which is about 1,000 years old and is the youngest vol-canic unit in the state.

The Acoma structural sag and adjacent Lucero Upliftin the southeast part of the section include prominentsandstonecapped mesas and buttes of the Acoma andLaguna reservations. Limestone- andsandstone-capped cuestas and benches of the SierraLucero area overlook the lower Rio Puerco Valley inthe western Albuquerque Basin. As in the northern partof the section, many tablelands are capped withPliocene basalt flows.

Much of the Acoma-Zuni Section is drained by the RioSan Jose, the major tributary to the Rio Puerco. Down-stream from its headwaters in the Zuni Mountains andMalpais Lava Field the river has few perennial reaches.Most narrow, deeply entrenched valleys of the San Josesystem are south and east of the Mount Taylor volca-nic field; elsewhere major valleys of the San Jose sys-tem are commonly broad, but still well entrenched be-low upland areas. Pleistocene basalt flows cap riverterraces in the lower San Jose Valley downstream fromLaguna.

The Datil-Mogollon Section is part of a physiographicsubdivision that is transitional between the Basin andRange Province and the Colorado Plateau. It is also anewly defined unit that includes the southern part ofthe area previously designated the Datil Section. Thisregion of volcanic highlands extends into east-centralArizona and contains several large structural basinsand block-faulted ranges. It is bounded on the east bybasins of the Rio Grande rift, which are part of theMexican Highland Section of the Basin and Range Prov-ince.

The Datil-Mogollon volcanic field is the dominant geo-logic feature in this section and lavas and tuffs are themain rock types. Major landscape units are erosionalremnants of huge cauldron structures with

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volcano-tectonic depressions and resurgent domes.There are also remnants of large stratovolcanoes,mainly composed of basaltic andesite. High tablelandsare capped with tuffs, andesite, and basalt lavas, andvolcanic-derived conglomeratic sandstones and mud-stones.

The Datil-Mogollon Section also straddles the Conti-nental Divide (6,650 to 10,000 ft elevation range). Thearea west of the divide is drained by perennial headwa-ters of the upper Gila system, including the San Fran-cisco and Tularosa rivers. Canyon incision by thesestreams has resulted in the deeply dissected tablelandtopography of the upper Gila River basin.

Internally drained structural basins (bolsons) and val-leys of major ephemeral streams (arroyos) draining tothe Rio Grande are dominant features east of the Con-tinental Divide. San Augustin Plains, the floor of thelargest closed basin (minimum elevation 6,780 ft), wasthe site of pluvial Lake San Augustin. Maximum eleva-tion of shoreline features is slightly higher than 7,000ft.

The Basin and Range Province includes much of cen-tral and southwestern New Mexico, and it extends intoadjacent areas of Arizona, Chihuahua, and Texas.

The Mexican Highland Section includes two large ar-eas of basin-and range structure and topography sepa-rated by the valley of the Rio Grande. The Rio Granderift depression coincides with the northern and easternparts of the section. The Continental Divide crossesthe bolson area west of the Rio Grande and is arbi-trarily placed along the highest local drainage dividesin the complex of internally drained basins between theRio Grande and the Gila valleys.

Block-faulted mountains commonly have Precambriancores overlain by Paleozoic sedimentary sequences.Sandia Peak (elevation 10,682 ft), the highest point inthe section, is at the crest of tilted-fault block of thistype. Organ Needle (elevation 9,012 ft) is the highestpeak in the southern Mexican Highlands. Basin depos-its which locally exceed 5,000 ft in thickness have sev-eral distinct basin-fill facies, including piedmont allu-vium, fine-grained lake and playa sediments,coarse-grained river deposits, and aeolian(wind-carried) materials. Basalt fields are also locallyextensive on basin plains; the youngest is the Carrizozoflow in the northern Tularosa Basin.

A number of the basin-floor depressions were occu-pied by perennial lakes during Pleistocene glacial-pluvial

intervals. The largest late Pleistocene lakes, each about200 square miles in area, were Lake Animas west ofLordsburg and Lake Otero in the Tularosa Basin westof Alamagordo. Large dune fields, including the White(gypsum) Sands of Tularosa Basin, are downwind fromthe pluvial-lake plains and younger playa depressions.

The Rio Grande flows through an alternative series ofbroad and narrow valley segments that coincide withmajor structural basins of the Rio Grande rift, eachseparated by bedrock uplifts. The valley for the mostpart is incised from 300 to 600 ft into upper basin filland associated volcanics. The only major canyon reachis Whiterock Canyon, east of the Pajarito Plateau, wherethe river cuts through the thick basalt sequence of theCerros del Rio volcanic field.

The Gila River crosses the northwestern Mexican High-lands (north of Lordsburg) in a valley setting very simi-lar to that along the Rio Grande. The low point of theMexican Highland Section (about 3,700 ft) is at theArizona border near Virden.

The Sacramento Section is characterized by high table-lands with broad, rolling summit plains; cuesta-formmountains, with eastward dipslopes and west-facingescarpments; and widely separated structural basins.Highlands, such as the Sacramento and Guadalupemountains and Chupadera and Glorieta mesas, arecapped with gently dipping limestone and sandstoneof Permian age. Gypsum is commonly interbedded withlimestone and sandstone sequences, and summit plainsof tablelands and cuestas include extensive areas ofkarst depressions where dissolution of calcium sulfateand carbonate has taken place. Large limestone cavesystems, such as Carlsbad and Fort Stanton caverns,are present in the Guadalupe and Sacramento high-lands.

The Sierra Blanca igneous-intrusive and volcanic ter-rain is the highest part of the province. Sierra BlancaPeak (elevation 12,003) is the southernmost glaciatedpeak in the continental United States. Topographic re-lief between Sierra Blanca and the adjacent TularosaBasin (twenty miles to the west) is about 7,600 ft, mak-ing this the area of greatest local relief in New Mexico.Other highlands formed by igneous-intrusive massesare the Capitan, Carrizo, Jicarilla, and Gallinas moun-tains.

The Estancia Basin, in the northwest part of the sec-tion (minimum elevation 6,050 ft), is a shallow struc-tural basin, extensively modified by dissolution of gyp-sum, and by water and wind erosion. Basin fill is less

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than 500 ft thick and contains lacustrine and aeoliandeposits. Lake-shoreline elevations range up to about6,350 ft. Holocene deflation has excavated numerousplaya depressions, including Laguna del Perro in thebasin floor. Small Pleistocene-lake basins and Holoceneplayas near Pinos Wells and Encino are located alongan ancient spillway route from the Estancia Basin to-ward the ancestral Pecos Valley.

The Great Plains Province occupies the eastern thirdof New Mexico and includes parts of three sections.The Pecos Valley Section includes the terraced valleysof the Pecos and Canadian rivers and flanking pied-mont plains and tablelands. Inner river valleys rangefrom reaches with broad floors, occupied by floodplainsand low terraces, to relatively shallow canyons. Eleva-tion of the Pecos Valley floor ranges from about 5,300ft near Anton Chico at the section’s northwest cornerto 2,840 ft at Red Bluff Reservoir on the Texas border.The latter site is the lowest point in New Mexico.

Higher piedmont erosion surfaces west of the Pecosare cut on eastward-dipping Permian rocks and are tran-sitional to surfaces in the Sacramento Section, high-lands. Broad upland plains and tableland summits havecaliche caprocks developed on veneers of upper Ter-tiary piedmont deposits.

The Pecos Valley Section extends southward and east-ward into western Texas along the lower Pecos,Portales, and lower Canadian valleys. The PortalesValley east of Fort Sumner is a segment of the ancientupper PecosBrazos river valley that was abandonedby middle Pleistocene time. The High Plains bordersare commonly marked by escarpments cut in sedimen-tary sequences with caliche caprocks. These featuresinclude Mescalero “Ridge” east of Roswell and “TheCaprock” escarpment south of Tucumcari. The boldCanadian Escarpment forms the boundary with theRaton Section to the north.

Much of the Pecos Valley Section is underlain by Per-mian bedrock units composed of gypsiferous and sa-line evaporites, limestone and dolomite, mudstone andshale, and sandstone. Dissolution of evaporite and car-bonate units is an active geomorphic process affectinglandscape evolution in much of the region, andsolution-subsidence depressions at a wide range ofscales are common landforms. Severalsolutionsubsidence basins southeast of Roswell haveareas of many square miles, A stepped sequence ofvalley-border surfaces also flanks the inner valleys andcanyons of the Pecos and Canadian rivers. These sur-faces are inset below the high-level piedmont plains

and reflect alternating intervals of valley incision andrelative base-level stability during Pleistoceneglacial-interglacial cycles. Quaternary wind-formed de-posits mantle large areas of older valley-border sur-faces east of the Pecos and north of the lower Cana-dian rivers.

Thick alluvial fills of late Tertiary and Quaternary ageare present in broader central valley areas along thePecos south of Santa Rosa, in the Canadian Valleynortheast of Tucumcari, and in the Portales Valley. TheLlano Estacado Section in New Mexico occurs as threeseparate areas that are western extensions of two majorpiedmont plateaus in the Panhandle region of Texasand Oklahoma. The plateau south of the Canadian Val-ley and the Caprock escarpment are designated theLlano Estacado or Staked Plains. In east-central NewMexico this area is bisected by the Portales Valley. TheStaked Plains north of the Canadian Valley are alsocalled the Panhandle subsection and are transitionalwestward to basalt-capped piedmont plains of the RatonSection. The Llano Estacado is characterized by anearly flat to undulating surface with a slight southeast-ward gradient. Elevations range from about 5,000 ft atthe northwest edge to 3,500 ft near Hobbs.

Alluvial and aeolian deposits of the Ogallala Forma-tion, with a resistant caliche caprock, underlie much ofthe High Plains surface. The caprock zone of second-ary carbonate accumulation is at or near the surfacealong plateau edges and in most of the Llano area southof the Portales Valley. Elsewhere the Ogallala is usu-ally buried by sandy to clayey deposits. Surface drain-age throughout the area is poorly developed; shallowvalleys of ephemeral streams cross the High Plains onlyat wide intervals. The broad uplands between streamvalleys are dotted with thousands of small, shallowdepressions, many of which contain playas and havelow arcuate dune ridges on their eastern (downward)margins. The dominant process involved in basin for-mation appears to be deflation. However, water ero-sion of basin sideslopes and dissolution of soluble con-stituents of underlying deposits may also play an im-portant role in depression formation and enlargement.

The Raton Section is characterized by high piedmontplains, of both erosional and constructional origin. Ex-tensive basalt flows protect many of the highlevel sur-faces from erosion. Deep canyons of the Canadian andCimarron river systems are cut below thesepiedmont-surface remnants in the southcentral andnortheastern parts of the section. To the south, strippedstructural plains are abruptly terminated by the Cana-

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dian Escarpment, which borders the Pecos Valley Sec-tion.

Some of the most striking features of the Raton Sec-tion are the numerous late Cenozoic volcanic centers,such as the andesitic Sierra Grande stratovolcano(8,720 ft) southeast of Raton, and the extensive plainsand tablelands capped by basalt flows. High mesas nearRaton, such as Johnson and Bartlett mesas, are cappedby basalts that were emplaced during the period of HighPlains surface development. Extensive younger basaltswest of Clayton and Wagon Mound cap remnants ofthe High Plains surface and associated piedmont allu-vial deposits. Basalt flows of Pleistocene age, such asthe Maxon Crater, and older Capulin flows cap riverterraces within the Mora, Canadian, and Dry Cimarroncanyons. The youngest Capulin flow locally covers thefloor of Dry Cimarron Canyon north of Folsom.

Sedimentary rocks, for the most part broadly foldedand gently dipping, underlie much of the Raton Sec-tion. Erosion surfaces on these units include the highlydissected Park Plateau west of Raton and the Trinidadescarpment which overlooks broad central plains nearSpringer. Extensive surfaces, such as Rayado andCharette mesas, are capped by upper Tertiary basalts.

Much of the southeastern third of the Raton Section ischaracterized by broad tablelands capped by an exten-sive sandstone unit (Dakota-Mesa Rica). Tablelandsummits include the broad rolling plains of the Las Ve-gas Plateau west of the Canadian Canyon and moredissected mesa areas to the northeast between UteCreek and Dry Cimarron valleys. A large High Plainsoutlier extending north from Mosquero is located be-tween the Canadian Canyon and Ute Creek Valley. Thisconstructional surface is mantled with piedmont allu-vial deposits and has a caliche caprock.


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Factors of Soil Formation

According to the Soils of New Mexico report (1978),the kind of soil that develops in any area is the result ofthe interaction of five soil-forming factors: climate, veg-etation, parent material, topography, and time. The firsttwo are called “active” factors because they act on thesoil parent material as conditioned by topography overvarying periods of time.

Climate and vegetation frequently are considered to-gether because climate is the major determinant ofvegetation. Soils of the high mountains of northern NewMexico are commonly leached, well developed, andacidic because precipitation is relatively high, tempera-tures are low, and the dominant vegetation is conifer-ous trees, which are best suited to the climatic and soilconditions. On the other hand, grasses and desertshrubs are dominant in the hot, dry desertic region.Here, the soils are not leached, are less developed,and are neutral or alkaline. Whether soil determinesthe kind of vegetation or whether vegetation determinesthe kind of soil is a much-debated point, but climatecertainly is the deciding factor for both.

In New Mexico, temperature and precipitation are re-lated principally to land elevation. For example, in therelatively short distance between the Tularosa Basinand the ski run on Sierra Blanca, temperatures drop,precipitation increases, the vegetation changes fromgrasses to trees, and the soils change from calcareousand weakly developed to acidic and strongly developed.The growing (frost-free) season varies from about 210days at the lowest part of the state near Carlsbad tofewer than 100 days in the Sangre de Cristo Mountainsof northern New Mexico. Desert grasses and shrubsare the dominant vegetation in the arid plains of thesouth, whereas alpine vegetation occurs above the tim-berline on high mountain peaks. Precipitation distribu-tion patterns differ from eastern New Mexico to west-ern New Mexico. In the east, precipitation is lowest inwinter and highest in summer. In the west, precipita-tion is at a minimum in April and May and a maximumin July and August.

Parent material consists of the geologic material fromwhich soils are developed. Soils on very young allu-vium, such as in the valley of the Rio Grande or on the

sand dunes of southern New Mexico, are essentiallyundeveloped, so their characteristics are similar to thoseof the parent materials. On the other hand, soils of thehigh mountains of northern New Mexico have charac-teristics that bear little relation to the parent materialfrom which they developed. Climate and vegetation arethe dominant soil-forming factors in humid areas.

Rocks of Cretaceous, Tertiary, and Quaternary agesdominate the surficial geology, but geologic formationsdating as far back as Precambrian occur, mostly in thenorth-central part of the state. Evidence of volcanicactivity can be seen throughout the state except in thesoutheastern quarter, where only sedimentary forma-tions are found. Lava flows occurred as recently asabout 1,000 years ago south of Grants. Limestone andsandstone are the principal sedimentary rocks for thestate as a whole, but shale is locally important northand west of Tucumcari and in the northwest corner ofthe state.

Topography affects soils greatly. Thin, eroded soils arecommonplace on steep slopes. In depressions, fine-textured, saline, poorly drained soils are a logical con-sequence of the topographical conditions. Soils on thesouth-facing slopes are subject to higher temperaturesthan their counterparts on the north sides of hills. Thetopography of the state is highly varied. The high plainsof eastern New Mexico are relatively flat. The remain-der of the state includes basins, plains, plateaus, me-sas, mountains with their valleys, and floodplains.

The importance of time to soil formation arises fromthe fact that natural processes of soil development tendto reach an equilibrium which depends upon local envi-ronmental conditions. It takes thousands of years for amature soil to develop from raw rock materials. Thelandscape of New Mexico is young, geologically speak-ing. Nearly all surficial deposits from which soils havedeveloped have been affected by climatic changes oc-curring in the last million years. Many of them owe theircharacteristics to the soil-forming processes operatingduring and since the last glacial period.

SoilsHarry J. Maker and Leroy A. Saugherty


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Soil Classification

Soil scientists use several systems to classify soils.These deal with the soil as a natural body and considerthe volume of soil affected by biological activity, whichusually extends to a depth of several feet. One suchclassification lists the five soil orders in New Mexico asAridisols, Mollisols, Entisols, Inceptisols, and Affisols.Aridisols are extensive in lower elevations over thesouthern two thirds of the state but are replaced in thecooler and moister higher elevations by Mollisols.Entisols occupy the Rio Grande Valley from Santa Fesouth to the Texas Mexico border. The northern third ofthe state and the far eastern counties are dominatedby Mollisols, Entisols, and Alfisols. Inceptisols are foundin the highest elevations of the San Juan and Sangrede Cristo Mountains. Other materials of note includethe gypsum sands of White Sands National Monumentand the lavas of the Carrizozo, Grants, and othermalpais (lava rockland).

Aridisols dominate the lower elevations of New Mexico.Aridisols lack necessary moisture for mesophytic plantgrowth for long periods. Thus, Aridisols are not suitablefor dryland agriculture. During most of the year the soilwater is held at tensions above the wilting point for mostplants. Generally, the soil horizons (distinct layers ofsoil) were formed under a more moist regime, as dur-ing former pluvial periods. The surface horizon (layer)is usually low in organic matter content and is thus lightin color. The Aridisols are often calcareous from thesurface downward and have a secondary accumula-tion of calcium carbonate (lime) and/or gypsum in thesubsoil. Soil textures range from loamy sands to claysand consistency ranges from soft to extremely hard.Most of the surface is bare much of the time and inmany instances a surface gravel pavement has formedby deflation of the finer windblown particles. TheAridisols are important resources but are easily mis-used. Both wind and water erosion are a constant haz-ard. Under agriculture, special fertility problems can existbecause of unavailable micronutrients resulting from ahigh pH. In general, however, the Aridisols have a highcontent of bases needed for plant growth.

Entisols can occur in any climate; however, most ofthese soils in New Mexico occur in an arid climate inassociation with Aridisols. Entisols have been exposedto the soil-forming processes for such a short time thatno major soil horizons have formed. Examples ofEntisols are soils on floodplains or soils frequentlymoved by wind erosion. They also occur on moderateto steep slopes where bedrock is shallow. In general,the Entisols express the properties of the parent mate-

rial with little change. Their nutrient supplying capacityis generally high. Salinity and sodicity may be limited.Erosion hazard can be high, especially on the soilsderived from wind-blown sediments. Most of the soilsof the Rio Grande Valley in agriculture are Entisols.

Inceptisols exhibit the initial sign of soil development: acolor change in the subsoil. They occur in areas of morerainfall than those yielding Aridisols. In New Mexico thisis mainly in mountains which receive more than 12-14inches of rainfall. The Inceptisols are young, occurringmainly on steep slopes where erosion removes weath-ered sediments. They also occur in areas dominatedby volcanic pumice where insufficient time has passedto allow the formation of a more weathered soil. In NewMexico, their base-supplying capacity is generally high.

Affisols also occur in climates more moist than thoseyielding Aridisols. Alfisols occur in the mountains ofnorthern New Mexico and on the plains in eastern NewMexico. Organic matter accumulation in the surfacehorizon is greater than in Aridisols but is still low enoughthat the color is either light or dark to only shallowdepths. Alfisols have been subjected to soil formationprocesses for long enough that clay has translocatedand accumulated in the subsoil. Many of the Alfisolshave sufficient moisture and nutrient supply to supportdryland agriculture.

Mollisols are characterized by deep, dark surface hori-zons of high organic matter content. They occur in ar-eas of New Mexico with more than 12-14 inches ofrainfall (similar in rainfall to areas with Inceptisols andAlfisols). Mollisols are dominantly grassland soils butdo occur in the forests of southern New Mexico wherethe base status is high and grass is the dominant un-derstory. Mollisols are very fertile soils with a high sup-ply of nutrients. Lime often accumulates in the subsoil.Mollisols, like most soils in New Mexico, are fragile whenmisused. Water erosion hazard is high in some areas.Most of the Mollisols are used to support grazing somein eastern New Mexico are used for crop production.


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The complex vegetation composition of New Mexico isthe consequence of many habitats resulting from abroad range of elevations from about 2,800 ft in thesoutheastern corner of the state to more than 13,150 ftat the summit of Mt. Wheeler in the Sangre de CristoMountains of the north-central part. The flora variety isalso related to the great geological diversity and com-plex topography, and to the differences in precipitationthroughout the state. The rainfall amounts range fromas much as 35-40 inches in the northern mountains toless than 8 inches in the southern desert areas. Thenatural vegetation of New Mexico has also become lo-cally modified by the effects of disturbance, includinglong-term human habitation, extensive overgrazing,surface and subsurface mining, and the introduction ofspecies from other areas.

The vegetational assemblages of New Mexico are di-vided into two groups for discussion and mapping pur-poses. Group I represents associations of the Conifer-ous Woodland, Montane Shrub, Montane Grassland,Mixed Conifer, Subalpine, Alpine-Tundra, and Riparianvegetation. Group 11 includes associations of theChilhuahuan Shrub, Chihuahuan Grassland, GreatBasin Shrub, Great Basin Grassland, and Plains andPrairie Grassland.

Riparian vegetation is primarily confined to a relativelynarrow band adjacent to water courses and is mostapparent along major streams. Plants common to thisassemblage include willows of several species, cotton-woods, ash-leaved maple, alder, and birch. In southernareas the sycamore, New Mexico olive, and walnut arefound along with many other shrubs and forbs. Manynaturalized taxa tend to be very common in riparianareas, especially in floodplain zones along majorstreams. Plants of this zone gradually merge with thoseof the adjacent vegetation zones.

Montane habitats are characteristic of large areas ofNew Mexico, typically at elevations from 6,000 to morethan 13,000 feet. The vegetation represented here isfound in several recognizable floristic zones, includingconiferous woodland, montane shrubland, montanegrassland, mixed conifer, spruce-fir, and alpine-tundrazones.

The Coniferous Woodland is a characteristic feature ofthe lower slopes of montane areas throughout NewMexico, mostly at elevations from 5,500 to 7,000 ft. Thiszone merges with grassland at its lower elevation limitsand with the mixed conifer association at the higherlimits, where pinyons and junipers are gradually replacedby ponderosa pine, which becomes the dominant treespecies. In some places the coniferous woodland givesway to a montane shrub association, where some juni-pers may occur but where the dominant vegetation iscomposed mostly of deciduous shrubs. Sometimesdense thickets are formed. The general aspect of theconiferous woodland is that of an open stand of thedominant conifers interspersed with species of grassesand a few forbs in the open areas. Because of the rela-tively dry site conditions, forbs are never abundant inthis zone. When montane brushland appears at thiselevation, it often represents a natural replacement forconiferous areas which have been destroyed by fire.

Coniferous woodland vegetation with pinyon and juni-per dominant is an assemblage characterized by largepopulations of either the pinyon or juniper or both. Wherepinyon is the primary dominant, juniper is a secondarydominant, sometimes without additional woody speciespresent. Herbaceous plants may be numerous, depend-ing on edaphic factors and elevation. On some sites,Gambel oak may be dispersed throughout the asso-ciation; on other sites, scattered big sagebrush may bemore common. Where juniper is the primary dominant,pinyon (often referred to as Mexican pinyon in the south-western part of the state) is usually the secondary domi-nant. Oaks, either the wavyleaf or other species, maybe common. Sometimes big sagebrush may be scat-tered throughout the association. At some locations inthe Great Basin region the primary dominant, juniper,has big sagebrush as a secondary dominant; here, pin-yon tends to be absent.

At various elevations in both the mixed conifer andspruce-fir zones there are often extensive open grass-lands or meadows. This Montane Grassland vegeta-tion is represented by an extensive herbaceous under-story, including many species of grasses as well as afew species of shrubs.

Montane grassland vegetation with fescue dominantforms open grasslands at elevations of approximately

Vegetation: Riparian and Montane; Plateau, Basin, andPlains

William C. MartinNew Mexico in Maps, Second Edition, Edited by Jerry L. Williams

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8,000-10,500 ft and is dominated by Thurber fescue athigher elevations and by Arizona fescue at lower el-evations. Where dominant, Thurber fescue is associ-ated with Arizona fescue as a secondary dominant;when Arizona fescue is dominant, it tends to be asso-ciated with mountain muhly. Bluegrass, various sedges,brome, and some herbs, such as cinquefoil, occur atthe higher elevations in this zone. On lower, warmersites, sideoats grama may be found. These areas oc-cur mostly in Mora, Taos, Sandoval, and Rio Arribacounties.

Montane grassland vegetation with mountain muhly andpine dropseed dominant appears as an open, more orless parklike grassland at intermediate elevations. Inaddition to the dominant taxa, other grasses such asjunegrass, squirreltail, and sideoats grama, among oth-ers, occur. This assemblage is found primarily in RioArriba County. At higher elevations, especially in thesubalpine region, sedges, bluegrass, tufted hairgrass,and several forbs occur in varying amounts.

The easily discernible Mixed Conifer zone begins atapproximately 7,000 feet and extends upward to about9,000 feet in elevation. Dominants in this zone includeboth ponderosa pine and Douglas fir. Other conifers,such as white fir and Rocky Mountain juniper, are alsofound here. The lower elevations in this zone are markedby a conspicuous abundance of ponderosa pine whichgradually, with increasing elevation, begins to share itsdominant status with Douglas fir until, at the upper lim-its of this zone, Douglas fir may become dominant.

Mixed conifer vegetation with ponderosa pine dominantincludes several recognizable associations. Amongthese are the ponderosa pine associations with RockyMountain juniper in northern New Mexico (especiallycharacteristic of areas of Rio Arriba County), with ashrubby understory of wavyleaf oak, sumac, or moun-tain mahogany; ponderosa pine in association withGambel oak at the lower elevations, with a characteris-tic understory similar to that of the ponderosa pine/Rocky Mountain juniper combination; and ponderosapine associated with Douglas fir at about the 8,000-footlevel, along with an understory of shrubs such as snow-berry, mountain spiraea, currants, and a few grassesincluding species of fescue and brome. In some areasponderosa pine may exist in nearly pure stands.

Mixed conifer vegetation with Douglas fir dominant iswidespread in the higher mountains of New Mexico.The upper part of this zone is represented by Douglasfir in association with Engelmann spruce. At lower el-evations, Engelmann spruce is replaced by ponderosa

pine. Between the mixed conifer zone and thealpine-tundra zone, there is an extensive, heavily for-ested region dominated by Engelmann spruce and sub-alpine fir. Frequently the conifer stands are so densethat understory vegetation is relatively sparse.

Spruce-fir vegetation with Engelmann spruce dominantis typically found between 9,000 and 12,000 feet in el-evation and is often referred to as the subalpine region.The largest extent of this association is located innorth-central New Mexico and is dominated by Engel-mann spruce with secondary dominants representedby subalpine fir and corkbark fir and scattered standsof Douglas fir.

Grassland glades are scattered throughout thespruce-fir zone. Numerous forbs are found here, alongwith several species of grasses and, in this area, sedgesbegin to become an important feature of the groundcover. Sedges become increasingly important at higherelevations, both in numbers of species and density andin value as a grazing and forage resource. In manyplaces the original native species in these forest gladeshave been largely replaced by species other than thosetypical of these areas.

Throughout this zone large stands, often thousands ofacres, of aspen form a major broadleaf deciduous treecommunity. This community, usually of relatively shortduration, is in many cases an example of a stage insecondary succession following a major disturbancesuch as fire.

Above the upper limits of sustained tree growth lies theAlpine-Tundra zone, represented by a relatively smallfraction of the montane areas of New Mexico. This zonenormally begins at approximately 12,000 feet and ex-tends upward to the summit of the high peaks. Theboundaries between this zone and the usually heavilyforested spruce-fir zone are marked by a scattering oflowgrowing, dwarfish, gnarled, usually windtrainedspruce trees with the open areas populated by numer-ous species of sedges, species of grasses includingtufted hairgrass, members of the pink family (Arenariaand Silene), caespitose phlox, saxifrages, and alpinecover. Alpine willows and several species of compos-ites are also present in varying amounts. Although sev-eral recognizable communities occur in this zone, de-pending on the degree of slope, exposure, and depthof soil, the floristic composition of the alpine-tundra zoneis relatively uniform.

The second grouping of New Mexican vegetational as-semblages, extending over the desert, basins, and

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plains (and therefore the larger part) of the state, in-cludes associations of the Chihuahuan Shrub,Chihuahuan Grassland, Great Basin Shrub, Great Ba-sin Grassland, and Plains and Prairie Grassland.

Chihuahuan Shrub vegetation with creosote bush domi-nant is characteristic of desert regions of southern NewMexico and extends northward along the valley of theRio Grande to Bernalillo County. In some areas creo-sote bush shares dominance with tarbush to form adispersed overstory. On other sites tarbush is essen-tially absent and bush muhly becomes more prominent.Other grasses, including three-awn, fluffgrass, tobosa,and black grama, may be associated, and subshrubssuch as yucca and snakeweed may occur in varyingamounts. Among forbs, probably the most prominentspecies is desert marigold.

Chihuahuan shrub vegetation with acacia dominantassociation occurs mostly in southeastern and south-western New Mexico and is usually characterized bycatclaw acacia along with mesquite. Most of these sitesare scattered and are often too small to be definitivelymapped. Probably the largest continuous associationof this kind occupies the area directly west and south-west of Carlsbad and extends into Mexico. Variousgrasses are also common here.

Chihuahuan shrub vegetation with fourwing saltbushdominant is usually associated with a scattering ofmesquite, creosotebush, tarbush, coldenia, andiodinebush. Dominant grasses of the understory includeeither alkali sacaton or tobosa; these taxa are com-monly associated with vine-mesquite, burrograss,saitgrass, sand dropseed, and gyp grama. This asso-ciation is chiefly confined to desert areas of southernNew Mexico, areas bordering the northern, eastern,and southern borders of White Sands, the Playas Lakearea of Grant County, north-central Hidalgo County, andsoutheastern Socorro and southwestern Lincoln coun-ties.

Great Basin Shrub vegetation with big sagebrush domi-nant association is characterized by a scattering ofColorado pinyon and juniper with the big sagebrush. Inthe open spaces, common grasses are sand dropseed,blue grama, alkali sacaton, galleta, and western wheat-grass. This association is located mostly between 6,000and 6,500 ft in western Taos, southeastern Rio Arriba,and northeastern Sandoval counties.

Great Basin shrub vegetation with saltbush dominantis an assemblage where dominance is usually sharedby several species of saltbush, including four-wing,

shadscale, and Nuttall saltbushes. On open slopes andmesas, pinyon, juniper, antelope brush, and service-berry may be scattered and in flats greasewood maybe locally abundant. Principal grasses include Indianricegrass, galleta, blue grama, sideoats grama, alkalisacaton, and three-awn. This association is character-istic of sites throughout much of San Juan County be-tween 6,000 and 6,500 ft, in western Sandoval, north-ern and eastern McKinley, and northwestern Bernalillocounties, and also an area of central Catron County.

Chihuahuan Grassland vegetation with burrograssdominant is an open grassland dominated byburrograss, usually containing significant amounts oftobosa, and to various degrees scatterings of fluffgrass,gyp dropseed, and gyp grama. Occasionally theboraginaceous subshrub, coldenia, will occur. This as-sociation extends from central Chaves County to cen-tral Eddy County and occurs in south-central EddyCounty but does not extend into Mexico.

Chihuahuan grassland vegetation with grama grassdominant is composed of several dominant gramagrasses, including black, blue, hairy, or sometimessideoats grama in association with threeawn or tobosaat some localities and curly mesquite or bush muhly atothers. Sometimes sand dropseed or burrograss occuras well as a scattering of cacti, juniper, or creosotebush.

Chihuahuan grassland vegetation with black gramadominant is dominated by black grama in conjunctionwith several species of dropseed and an occasionalyucca on some sites, but in conjunction with bush muhlyand a scattering of creosote bush on other sites. Spe-cies of cacti also occur at many of these sites and, insandy areas, sand sagebrush appears. This associa-tion is scattered throughout much of the ChihuahuanDesert region.

Chihuahuan grassland vegetation with dropseed andIndian ricegrass dominant is typically found on sandysites and dominated by several species of dropseed,such as sand, mesa, and spike dropseed, in conjunc-tion with ricegrass. Scattered shrubs include yucca andsand sagebrush. This association is relatively uncom-mon, occupying several sites in the central Rio GrandeValley including parts of Bernalillo, Valencia, andSocorro counties.

Chihuahuan grassland vegetation is also with sacatonand tobosa dominant. Giant and alkali sacaton, alongwith tobosa, are the important representatives here.

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This association is found in a few localities in Sierra,Lincoln, and Otero counties.

Great Basin Grassland vegetation with Indian ricegrassand galleta dominant is associated with various othergrasses, including blue grama, sand dropseed, variousthree-awns, and sometimes sideoats grama. There maybe a scattering of big sagebrush, Mormon teasnakeweed, and juniper on some sites. This associa-tion is scattered throughout the northwestern andwest-central counties.

Great Basin grassland vegetation with big sagebrushdominant is found in conjunction with Indian ricegrass,sand dropseed, galleta, three-awn, blue grama, orsometimes western wheatgrass. Rabbitbrush may belocally abundant and populations of snakeweed occa-sionally occur. This association is mostly found in SanJuan, Sandoval, and Taos counties.

Great Basin grassland vegetation also occurs with salt-bush and alkali sacaton dominant. At least two speciesof saltbush are found here, usually fourwing andshadscale, along with alkali sacaton and lesser amountsof other grasses such as blue grama and westernwheatgrass. Populations of rabbitbrush may occur spo-radically. This association is found throughout the GreatBasin region.

There is a large area of Great Basin grassland vegeta-tion with grama grasses and western wheatgrass domi-nant. Several species of grama grasses are commonhere and include sideoats, blue, black, and hairy gramain conjunction with western wheatgrass. Other grassesfound in various but lesser amounts are muhly,vine-mesquite, or three-awns and sometimes smallamounts of Indian ricegrass or junegrass appear. Thisassociation is most common in Valencia, Cibola, andCatron counties.

Plains Grassland vegetation with blue grama dominantis, depending on the site, mostly associated with west-ern wheatgrass, buffalograss, galleta, black grama, orneedlegrass. Other grasses, such as ring muhly,three-awn, galleta, sidecats grama, little bluestem, orsand dropseed, may also occur as well as shrubby taxaincluding snakeweed, winterfat, sumac, and variouscacti and yuccas. Scattered junipers may also appearhere but are not usually considered major componentsof the assemblage. This group of associations is situ-ated in eastern New Mexico and constitutes a signifi-cant part of the Plains flora.

Plains grassland vegetation with sidecats grama domi-nant is another complex assemblage of associationswith sideoats; grama typically dominant and associated,depending upon the association, with blue grama, blackgrama, curly mesquite, Metcalf muhly, and feathergrass.Less common grass species are galleta, sanddropseed, buffalograss, three-awn, plains bristlegrass,tridens, and western wheatgrass. Shrubby plants suchas snakeweed, yucca, sotol, winterfat, cacti, and some-times scrub oaks may occur in varying amounts. Also,junipers may be locally common. This assemblage isfound throughout the Great Plains region of NewMexico.

There is a small area in the state of Plains grasslandvegetation with buffalograss dominant. This taxon usu-ally shares dominance with blue grama in this reason-ably well-marked short-grass association, with the domi-nants interspersed with varying amounts ofvine-mesquite, tobosa, galleta, and alkali sacaton.Mesquite sometimes also occurs in limited amounts,usually in localized populations. This association is mostcommon in extreme eastern New Mexico, primarily ineastern Quay County.

Plains grassland vegetation with four wing saltbush andsacaton dominant usually includes the dominants withvarying numbers of other taxa interspersed. Sometimesalkali sacaton occurs in nearly pure stands. At othersites, four-wing saltbush is a strong primary dominantwith minor grass components interspersed. This assem-blage occurs mostly in relatively small areas of Catron,Socorro, Torrance, and Quay counties.

Prairie Grassland vegetation with bluestern dominantis primarily a tallgrass assemblage with little bluesternand often sand bluestern as dominants. On some sitessand sagebrush is a codominant and silver bluestem,dropseed, sideoats and blue gramas, Indian ricegrass,switchgrass, and yuccas may occur in varying amounts.On other sites shinnery oak is a secondary dominantand is usually associated with giant and sand dropseed,sideoats and black grama, bristlegrass, and yucca. Onstill other sites, sideoats grama will be a secondarydominant and associated species may include hairygrama and a scattering of juniper. This assemblage isfound in several of the eastern counties, from De BacaCounty eastward beyond the border.


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Documents

Geologic History - SMRNA · The geologic history of New Mexico is wonderfully di-verse. Exposed within the state’s boundaries are Pre-cambrian igneous and metamorphic rocks more