Geography ofInterior United States
| image:US_interior_physiographic_regions_map.jpg |
| Physiographic regions ofU.S. Interior See:legend |
For purposesdescription,physical geography ofUnited Statessplit into several major physiographic divisions, threewhich beingLaurentian Highlands, Interior Highlands andInterior Plains (see subdivisions 111-15) lie ininterior ofU.S. Please refer toGeography ofUnited States forother areas.
| Tablecontents |
|
2 Region ofGreat Lakes 3 The Prairie States 4 The Gulf Coastal Plain 5 The Great Plains |
The Superior Upland
An outlying upland ofLaurentian highlandsCanada projects into the United States westsouthLake Superior. This upland, part ofCanadian Shield along withAdirondacks,a greatly deformed structureis composed primarilycrystalline rocks commonly associated withrugged landscape. At some ancient period, this hadstrong relief, but todayupland aswholegently rolling withinter-streams surfaces being plateau-liketheir evenness. Here they have altitudes1,4001,600 feettheir higher areas. In this province, we findpartthose ancient mountains regions that were initated by crustal deformationthen reduced bylong continued erosion topeneplainmodern relief. A peneplain withoccasional moderately high monadnocks left behind duringpeneplanation of the rest ofsurface. The erosion ofregion must have been far advancedancient times, even practically completed, becauseeven peneplain surfaceoverlapped by fossiliferous marine strata from an early geological data ( Cambrian ). This shows thatdepression ofregion beneath an ancient sea took place afterlong existence as dry land.
The extent ofsubmergence andarea over whichPalaeozoic strata were depositedunknown. Becausethe renewed elevation without deformation, erosionlater periods has stripped off an undetermined amount ofcovering strata. The valleys by whichuplandsherethere trenchedmoderate depth appearbe,part at least,workstreams that have been superposed uponpeneplain throughnow removed coverstratified rocks. Glaciation has strongly scoured awaydeeply-weathered soils that presumably existed herepreglacial time. It left behand firmrugged ledges inlow hillsswells ofgroundspread an irregular drift cover overlower parts, wherebydrainagegenerally disordered being depositedlakesswampselsewhere rushing down rocky rapids.
Region ofGreat Lakes
The Palaeozoic strata, already mentioned as lapping onsouthern slope ofSuperior Uplandaroundwestern side ofAdirondack Mountainsbut parts ofgreat areasimilar strata hundredsfeetthickness. These strata decline gently southward from the great upland ofLaurentian highlandseastern Canada. The visible upland areatoday was butsmall part ofprimeval continent withremainderit still buried underPalaeozoic cover. The visible part waslast part ofprimeval continentsink underadvancing Palaeozoic seas. Whenuplandits overlapstratified deposits were elevated again,overlapping strata must have hadappearance ofcoastal plain. Of course that wasago, since then the strata have eroded substantiallytoday possess neitherarea nor the smooth formtheir initial extent. This district may be placed considered an ancient coastal plains. Asalwayscase inbroad denudation ofgently inclined stratasuch plains,weaker layersworn downsub-parallel beltslower land betweenupland andbeltsmore resistant strata, which riseuplands.
Few better illustrationsthis typeformsto be found than that presented indistrict ofGreat Lakes. The chief upland belt or escarpmentformed byfirm Niagara limestone, which takes its name fromgorgefalls cut throughupland by the Niagara River. Asall such forms,Niagara Escarpment has a relatively strong slope or infacing escarpment onside towards the upland, andlong gentle slope onother side. Its reliefseldom more than 200 or 300 feetis generally small. Its continuityits constrast withassociated lowlands on the underlyingoverlying weak strata sufficemakean important feature. The escarpment would lie straight east-west ifslant of the strata were uniformly tosouth. However,stratasomewhat warpedsoescarpment's coursestrongly convex tonorth in the middle, gently convex tosouth at either end.
The escarpment begins where its determining limestone begins,west-central New York. There,separateslowlands that containing Lake Ontario from Lake Erie. It curves tonorthwest throughOntario province to the island belt that dividesGeorgian Bay from Lake Huron. From there, it heads westward throughland-arm between Lake SuperiorLake Michigan and sout-westward intonarrow points dividing Green Bay from Lake Michigan. Finally,fades away withthinning out oflimestoneis hardly traceable acrossMississippi River.
The arrangement ofGreat Lakesclosely matchescourse oflowlands worn ontwo beltsweaker strata on either side ofNiagara escarpment. Lake Ontario, Georgian BayGreen Bay occupy depressions inlowland oninner side ofescarpment. Lake Erie, Lake HuronLake Michigan liedepressions inlowland onouter side. Whentwo lowlandstraced eastward,become confluent afterNiagara limestone has faded awaycentral New York andsingle lowlandcontinued undernameMohawk Valley. Thisan east-west longitudinal depression that has been eroded onbeltrelatively weak strata betweenresistant crystalline rocks ofAdirondacks onnorth andnorthern escarpment ofAppalachian plateau (Catskills-Helderbergs) onsouth. Early inU.S. history, this providedvital economic route betweenAtlantic seaports andU.S. interior.
In Wisconsin,inner lowland has an interesting feature. It isknobresistant quartzites, known as Baraboo Ridge, rising fromburied upland floor throughpartly denuded coverlower Palaeozoic strata. This knob or ridge can be thoughtas an ancient physiographic fossil asis an ancient monadnock having been preserved from destructive attacksweather by burial under sea-floor deposits. It has been recently re-exposed through the erosionits cover.
The occurrence oflake basins inlowland belts on either side ofNiagara escarpmentan abnormal feature. Ordinary erosion does not explain it. Glacial erosion has formed them throughglacial drift obstructingnormal outlet valleys andcrustal warpingconnectionor independent ofglacial sheet.
Lake Superiorunlikeother lakes. The greater partits basin occupiesdepression inupland area, independent ofoverlap of Palaeozoic strata. The western half ofbasin occupiestrough of synclinal structure. The Great Lakespeculiarreceivingdrainagebutsmall peripheral land area, enclosed by an ill-defined water-parting fromrivers that runHudson Bay orGulfSt Lawrence onnorthtoGulfMexico onsouth.
The three lakes ofmiddle group stand at practicallysame level:
- Lake Michigan
- Lake Huronconnected
- Lake Erie
The Prairie States
The originally treeless prairies ofupper Mississippi basin beganIndianaextended westwardnorth-westward untilmergedthe drier region known asGreat Plains. An eastward extensionthe same region, originally tree-covered, extendedcentral Ohio. Thusprairies generally lie betweenOhioMissouri rivers onsouth andGreat Lakes onnorth. The prairies arecontribution ofglacial period. They consist formost partglacial drift, deposited unconformably on an underlying rock surfacemoderate or small relief. Here,rocks are an extension ofsame stratified Palaeozoic formations already described as occurring inAppalachian regionaroundGreat Lakes. Theyusually fine-textured limestonesshales, lying horizontal. The moderate or small relief thatwere given by mature preglacial erosionnow buried underdrift.
The greatest area ofprairies, from IndianaNorth Dakota, consiststill plains, that is, sheetsunstratified drift. These plains30, 50 or even 100 ft. thick coveringunderlying rock surfacethousandssquare miles except where postglacial stream erosion has locally laidbare. The plains have an extraordinarily even surface. The tillpresumably madepartpreglacial soils, but itmore largely composedrock waste mechanically transported bycreeping ice sheets. Althoughcrystalline rocks from Canada and some ofmore resistant stratified rocks south ofGreat Lakes occur as bouldersstones,great part oftill has been crushedground toclayey texture. The till plains, although sweepingbroad swellsslowly changing altitude, often appear level toeye withview stretching tohorizon. Herethere, faint depressions occur, occupied by marshy sloughs, or floored withrich black soilpostglacial origin. Itthus by sub-glacial aggradation thatprairies have been levelled up tosmooth surface,contrast tohighernon-glaciated hilly country just tosouth.
The great ice sheets formed terminal moraines around their border at various end stages. However,morainic beltsof small reliefcomparison togreat area ofice. They rise gently fromtill plains toheight50, 100 or more feet. They may be one, two or three miles widetheir hilly surface, dotted overboulders, contains many small lakesbasins or hollows, insteadstreamsvalleys. The morainic beltsarrangedgroupsconcentric loops, convex southward, becauseice sheets advancedlobes alonglowlands ofGreat Lakes. Neighboring morainic loops join each otherre-entrants (north-pointing cusps), where two adjacent glacial lobes came together and formed their moraineslargest volume. The morainesof too small reliefbe shown on any maps except oflargest scale. Small asare, they arechief relief ofprairie states, and,associationthe nearly imperceptible slopes oftill plains,determinecoursemany streamsrivers, which aswholeconsequent uponsurface form ofglacial deposits.
The complexity ofglacial periodits subdivision into several glacial epochs, separated by interglacial epochsconsiderable length (certainly longer thanpostglacial epoch) hasstructural consequence insuperpositionsuccessive till sheets, alternatingnon-glacial deposits. It also hasphysiographic consequence invery different amountnormal postglacial erosion suffered bydifferent parts ofglacial deposits. The southernmost drift sheets, as in southern Iowanorthern Missouri, have lost their initially plain surface andnow maturely dissected into gracefully rolling forms. Here,valleysevensmall streamswell openedgraded,marsheslakesrare. These sheetsof early Pleistocene origin. NearerGreat Lakes,till sheetstrenched only bynarrow valleys oflarge streams. Marshy sloughs still occupyfaint depressions intill plains andassociated moraines have abundant small lakestheir undrained hollows. These drift sheetsof late Pleistocene origin.
Whenice sheets extended toland sloping southward toOhio River, Mississippi RiverMissouri River,drift-laden streams flowed freely away fromice border. Asstreams escaped from their subglacial channels,spread into broader channelsdeposited sometheir loadthus aggraded their courses. Local sheets or apronsgravel and sandspread more or less abundantly alongouter side ofmorainic belts. Long trainsgravelsands clogvalleys that lead southward fromglaciated tonon-glaciated area. Later, when the ice retreated farther andunloaded streams returnedtheir earlier degrading habit,more or less completely scoured outvalley deposits, the remainswhichnow seenterraces on either side ofpresent flood plains.
Whenice oflast glacial epoch had retreated so far that its front border lay onnorthward slope, belonging todrainage area ofGreat Lakes, bodieswater accumulatedfront ofice margin, forming glacio-marginal lakes. The lakes were small at first,each had its own outlet atlowest depressionland tosouth. Asice melted further back, neighboring lakes became confluent atlevelthe lowest outlet ofgroup. The outflowing streams grew insame proportionerodedbroad channel acrossheightlandfar down stream, whilelake waters built sand reefs or carved shore cliffs along their margin,laid down sheetsclay on their floors. Allthese featureseasily recognized inprairie region. The present siteChicago was determined by an Indian portage or carry acrosslow divide between Lake Michigan andheadwaters ofIllinois River. This divide lies onfloor offormer outlet channel ofglacial Lake Michigan. Corresponding outlets are knownLake Erie, Lake HuronLake Superior. A very large sheetwater, named Lake Agassiz, which once overspreadbroad till plainnorthern MinnesotaNorth Dakota. The outletthis glacial lake, called river Warren, erodedlarge channelwhichMinnesota River evident today. The Red River ofNorth flows northward throughplain formerly covered by Lake Agassiz.
Certain extraordinary features were produced whenretreat ofice sheet had progressed so far asopen an eastward outlet formarginal lakes. This outlet occurred alongdepression betweennorthward slope ofAppalachian plateauwest-central New Yorkthe southward slope ofmelting ice sheet. When this eastward outlet camebe lower thansouth-westward outlet acrossheightland toOhio or Mississippi river,discharge ofmarginal lakes was changed fromMississippi system toHudson system. Many well-defined channels, cutting acrossnorth-sloping spurs ofplateau inneighborhoodSyracuse, New York, marktemporary paths ofice-bordered outlet river. Successive channels are found at lowerlower levels onplateau slope, indicatingsuccessive courses taken bylake outlet asice melted farther and farther back. On somethese channels, deep gorges were eroded headingtemporary cataracts which exceeded Niagaraheight but notbreadth. The pools excavated byplunging waters athead ofgorgesnow occupied by little lakes. The most significant stagethis serieschanges occurred whenglacio-marginal lake waters were lowered so thatlong escarpmentNiagara limestone was laid barewestern New York. The previously confluent waters were then divided into two lakes. The higher one, Lake Erie, supplyingoutfiowing Niagara river, which poured its waters down the escarpment ofescarpment tolower, Lake Ontario. Lake Ontario's outlet fortime ran downMohawk Valley toHudson. This gave rise toNiagra falls.
In certain districts,subglacial till was not spread out insmooth plain, but accumulatedelliptical mounds, 100-200 feet. high and 0.5 - 1.0 mile longaxes parallel todirection ofice motion as indicated by striae onunderlying rock floor. These hillsknown byIrish name, drumlins, usedsimilar hillsnorth-western Ireland. The most remarkable groupsdrumlins occurwestern New York, where their numberestimated at over 6,000,in southern Wisconsin, where itplaced at 5,000. They completely dominatetopographytheir districts.
A curious depositan impalpably fineunstratified silt, known byGerman name bess, lies onolder drift sheets nearlarger river courses ofupper Mississippi basin. It attainsthickness20 ft. or more nearriversgradually fades away atdistanceten or more miles on either side. It contains land shells,hence cannot be attributedmarine or lacustrine submergence. The best explanation is that, during certain phases ofglacial period,was carried as dust bywinds fromflood plainsaggrading rivers,slowly deposited onneighboring grass-covered plains.
South-western Wisconsinparts ofadjacent statesIllinois, IowaMinnesotaknown asdriftless area, because, although bordered by drift sheetsmoraines, itfree from glacial deposits. It must therefore have beensortoasis, whenice sheets from the north advanced pastoneastwestjoined around its southern border. The reasonthis exemption from glaciation isconversethat forsouthward convexity ofmorainic loops. For whilemarkpathsgreatest glacial advance along lowland troughs (lake basins),driftless area isdistrict protected from ice invasion by reason ofobstruction whichhighlandsnorthern WisconsinMichigan (part ofSuperior upland) offered to glacial advance.
The course ofupper Mississippi riverlargely consequent upon glacial deposits. Its sourcesinmorainic lakesnorthern Minnesota. The drift deposits thereaboutsso heavy thatpresent divides betweendrainage basinsHudson Bay, Lake Superior andGulfMexico evidently standno very definite relation topreglacial divides. The course ofMississippi through Minnesota is largely guided byform ofdrift cover. Several rapidsthe FallsSaint Anthony (determiningsiteMinneapolis)signsimmaturity, resulting from superposition throughdrift onunder rock. Farther south, as far asentrance ofOhio, the Mississippi followsrock-walled valley 300400 ft. deep, withflood-plain 24 m. wide. This valley seemsrepresentpathan enlarged early-glacial Mississippi, when much precipitation thattoday dischargedHudson Bay andGulfSt Lawrence was delivered toGtilfMexico, forcurves ofpresent river aredistinctly smaller radii thancurves ofvalley. Lake Pepin (30 m. below St. Paul),picturesque expansion ofriver across its flood-plain,due toaggradation ofvalley floor whereChippewa River, coming fromnortheast, brought an overloadfluvio-glacial drift. Hence evenfatherwaters, like so many other rivers inNorthern states, owes manyits features more or less directlyglacial action.
The fertility ofprairies isnatural consequencetheir origin. Duringmechanical transportation oftill no vegetation was present to removeminerals essentialplant growth, as iscase insoilsnormally weathereddissected peneplains. The soil is similar toAppalachian piedmont which though not exhausted byprimeval forest cover,by no means so rich astill sheets ofprairies. Moreover, whateverrocky understructure,till soil has been averaged bythorough mechanical mixturerock grindings. Hence the prairiescontinuously fertilescoresmiles together. The true prairies were once covered withrich growthnatural grassannual flowering plants, but todaycoveredfarms.
The Gulf Coastal Plain
The westward extension ofAtlantic coastal plain aroundGulfMexico has certain features that were already describedseveral new ones. As inAtlantic coastal plain, itonlylower, seaward partthis region that deservesnameplain,there alone issurface unbroken by hills or valleys. The inner part, initiallyplain by, has been maturely dissected into an elaborate complexhillsvalleys, usuallyincreasing altituderelief as one passes inland. The Gulf Plain features not found inAtlantic coastal plain are:
- the peninsular extension ofplainFlorida.
- the belted arrangementreliefsoilsAlabamain Texas.
- the Mississippi embayment or inland extension ofplain half-way upcourse ofMississippi river, withMississippi flood plain there included.
A typical example ofbelted coastal plainfoundAlabama andadjacent partMississippi. The plainhere about 1.50 m. wide. The basal formation if chieflyweak limestone, which has been stripped from its original Alabama innermost extensionworn down toflat inner lowlandrich black soil, thus gainingname ofblack belt. The lowlandenclosed by an upland or escarpment, known as Chunnenugga Ridge, sustained by partly consolidated sandy strata. However,upland is not continuous, butmaturely dissected escarpment. It hasrelatively rapid descent towardinner lowland, andvery gradual descent tocoast prairies, which become very low, flatmarshy before dipping underGulf waters, where theygenerally fringed by off-shore reefs.
The coastal plain extends 500 miles inland onaxis ofMississippi embayment. Its inner border affords admirable examplestopographical discordance wheresweeps northwestward square acrosstrend ofpiedmont belt,ridgesvalleys, andplateau ofAppalachians. Allwhichterminated by dipping gently beneathunconformable cover ofcoastal plain strata. Insame waywestern side ofembayment, trending southsouthwest, passes alonglower southeastern side ofdissected Ozark plateausouthern Missourinorthern and central Arkansas. The southern Missourinorthern Arkansas Ozark plateau resemblesmany waysAppalachian plateau. Ascoastal plain turns westwardTexas,bordersArbuckle hillsOklahomasmall analog ofAppalachian crystalline belt. Inembayment ofcoastal plain some low escarpment-like beltshillsassociated stripslowlands suggestfeatures ofbelted coastal plain. The hilly belt or dissected escarpment determined byGrand Gulf formationwestern Mississippi ismost distinct. Important salt deposits occur incoastal plain strata nearcoast. The most striking feature ofembayment isbroad valley whichMississippi has eroded across it.
The small proportiontotal water volume supplied fromgreat Missouri basindue tolight precipitationthat region. The lower Mississippi has no large tributaries fromlower east, but two important ones come fromwest. The Mississippi Arkansas drainage area beinglittle less thanOhio River andbasin ofRed RiverLouisiana being about half as large. The Mississippi River drains an areaabout one-third ofUnited States. The head ofcoastal plain embayment is nearjunction ofOhio andMississippi. It flows southward560 miles throughsemi-consolidated strata ofplain. The river has erodedvalley about 40-50 miles wide enclosed by bluffs one or two hundred feet high innorthern part. These bluffs decrease towardssouth, but with local increaseheight associated withdecreaseflood plain breadth oneastern side whereGrand Gulf escarpmenttraversed. This valley incoastal plain, withmuch narrower rock-walled valley ofupper river inprairie states, istrue valleythe Mississippi River. Howeverpopular usage,Mississippi valley is takenincludelarge central part ofMississippi drainage basin. The valley floorcovered withflood plainfine silt, havingsouthward slopeonly halffoot tomile. The length ofriver itself, fromOhio mouth toGulfabout 1,060 miles dueits windings. Its mean fallabout 3 inchs per mile. On account ofrapid depositionsediment nearmain channel at timesoverflow,flood plain, asnormallycase on mature valley floors, haslateral slopeas much as 5, 10, or even 12 ft. infirst mile fromriver, but this soon decreases toless amount. Thus justshort distance fromriver,flood plainoften swampy, unless its surfacethere aggraded bytributary streams. For this reason Louisiana, ArkansasMississippi rank immediately after Floridaswamp area.
The great river receives an abundant loadsilt from its tributaries, and takes uplays down silt from its own bedbanksevery change of velocity. The swiftest current followsouter sideevery significant curve inchannel. Thusconcave bank onside offastest part of the riverworn away. Any chance irregularityexaggerated,in time a serieslarge serpentines or meandersdeveloped,most-symmetrical examples at present being those near Greenville, Mississippi. The growth ofmeanders tendsgiveriver continually increasing length. This tendencycounteracted bysudden occurrencecut-offs from time to time, so thatfairly constant lengthmaintained.
The floods ofMississippi usually occurspring or summer. Owingthe great size ofdrainage basin,seldom happens thatthree upper tributariessimultaneously flooded. It isserious problem for the lower river if two oflarge tributaries flood atsame time. In this case,lower river will rise30, 40 or even 50 feet. The fall ofriversignificantly steepenedits velocity is accelerated down stream frompointhighest rise. Conversely,fall andvelocityboth diminished up stream fromsame point.
The loadsilt carried down stream byriver finally, after many halts onway, reacheswaters ofGulf. There,decreasevelocity aided bysalinity ofsea water, causesformationa remarkable delta, leaving less aggraded areas as shallow lakes (Lake Pontchartrain oneast,Grand Lake onwest ofriver). The ordinary triangular formdeltas, due tosmoothing ofdelta front by sea action,here wanting, because ofweaknesssea action in comparison withstrength ofcurrenteach offour distributaries or passes into whichriver divides near its mouth.
After constriction fromMississippi embayment250 mileswestern Louisiana,coastal plain continues southwestwardthis breadth untilnarrowsabout 130 in.southern Texas nearcrossing ofColorado river (Texas Colorado River, notmajor Colorado River), butagain widens300 miles atnational boundary asjoint effectembayment upvalley ofRio Grandeofseaward advancethis rivers rounded delta front. These several changes take place indistanceabout 500 miles. It includesregionover 100,000 sq. miles less than half oflarge stateTexas. A belted arrangementreliefssoils, resulting from differential erosion on strataunlike compositionresistance, characterizes almostentire area ofcoastal plain. Most ofplaintreeless prairie, butsandier beltsforested. Twothem are known as cross timbers, because their trendtransverse togeneral course ofmain consequent rivers. An inland extension fromcoastal plainnorth-central Texas leads tolarge escarpment known as Grand Prairie (not structurally included incoastal plain), upheld at altitudes1,200 or 1300 ft. byresistant Cretaceous limestone. This dips gently seawardits scalloped inland-facing escarpment overlookingdenuded central prairie regionirregular structureform. Its gentle coastward slope (16 ft. tomile)dissected by many branching consequent streams. In its southernpart asapproachesColorado river,escarpmentdissected intobeltdiscontinuous hills. The western cross timbers followsandy belt alonginner base ofragged escarpmentGrand Prairie. The eastern cross timbers follow another sandy belt inlowland betweeneastern slopeGrand Prairie andpale western escarpment ofimmediately eastwardlower Black Prairie escarpment. This escarpmentsupported at an altitude700 ft. or less bychalk formation, which gives an infacing slope some 200 ft.height. Its gently undulating or rolling seaward slope (2 or 3 ft. inmile), coveredmarly stratarich black soil, determines an important cotton district. Then comesEast Texas timber belt, broad innortheast, narrowing topoint before reachingRio Grande, a lowthoroughly dissected escarpmentsandy Eocene strata. Thisfollowed byCoast Prairie,very young plain, withseaward slopeless than 2 ft. inmile, its smooth surface interrupted only bystill more nearly level flood plainsthe shallow, consequent river valleys. NearColorado river, the dissected escarpment ofGrand Prairie passes southward changing tomore nearly horizontal structure intodissected Edwards plateau. The Edwards plateaureferredlater as part ofGreat Plains. The plateau terminates inmaturely dissected fault scarp approximately 300 or 400 feetheight asnorthern boundary ofRio Grande embayment. FromColorado toRio Grande,Black Prairie, the timber belt andCoast Prairie merge invast plain, little differentiated, overgrownchaparral (shrub-like trees, often thorny), widening eastward inRio Grande deltaextending southward into Meçico.
AlthoughCoast Prairie issea bottomvery modern uplift,appears alreadyhave sufferedslight movementdepression. Its small rivers all enter embayments. However,larger rivers seemhave counteracted the encroachment ofsea onland bysufficiently active delta building withresulting forward growth ofland intosea. The Mississippi has already been mentioned as rapidly building forward its digitate delta. The Rio Granide, nextsize, has built its delta about 50 miles forward fromgeneral coastline. Since this rivermuch smaller thanMississippi, its delta frontrounded by seashore effects. In front ofBrazos andColorado,largest ofTexan rivers,coast-linevery gently bowed forward as if by delta growth. The sea touchesmainland innearly straight shore line. Nearly all the rest ofcoastfringed by off-shore reefs, built up by waves from the very shallow sea bottom. Due toweak tides,reefs continuelong unbroken stretches betweenfew inlets.
The Great Plains
A broad stretchcountry underlaid by nearly horizontal strata extends westward from97th meridian tobase ofRocky Mountains, a distancefrom 300500 miles. It extends northward fromMexican boundary far into Canada. This isprovince ofGreat Plains. Althoughaltitudeplains increases gradually from 6oo or 1,200 ft. oneast4,000-5,000 or 6,000 feet nearmountains,local reliefgenerally small. The sub-arid climate excludes tree growthopens far-reaching views. The plainsby no meanssimple unit. They arediverse structure andvarious stageserosional development. Theyoccasionally interrupted by buttesescarpments. Theyfrequently broken by valleys. Yet onwhole,broadly extended surface of moderate relief so often prevails thatname, Great Plains, forregion aswholewell deserved. The western boundary ofplainsusually well defined byabrupt ascent ofmountains. The eastern boundary ofplainsmore climatic than topographic. The line20 inannual rainfall trendslittle eastnorthward near97th meridian. Ifboundary must be drawn where nature presents only a gradual transition, this rainfall line may be takendividedrier plains frommoister prairies. The plains may be describednorthern, intermediate, centralsouthern sections,relationcertain peculiar features.
The northern section ofGreat Plains, northlatitude 44°, including eastern Montana, north-eastern Wyomingmost ofDakotas, ismoderately dissected peneplain. Thisone ofbest examplesits kind. The strata hereCretaceous or early Tertiary, lying nearly horizontal. The surfaceshownbeplaindegradation bygradual ascent herethere tocrest ofragged escarpment,escarpment-remnant ofresistant stratum. Therealsooccasional lava-capped mesasdike-ridges, surmountinggeneral level by 500 ft. or moremanifestly demonstratingwidespread erosion ofsurrounding plains. All these reliefsmore plentiful towardsmountainscentral Montana. The peneplainno longer incycleerosion that witnessed its production. It appearshave sufferedregional elevation, forupper Missouri Riverits branches no longer flow onsurface ofplain, butwell graded, maturely opened valleys, several hundred feet belowgeneral level. A significant exception torulemature valleys occurs, however, incase ofMissouri,largest river, whichbroken by several falls on hard sandstones about 50 miles east ofmountains. This peculiar featureexplained asresultdisplacement ofriver frombetter graded preglacial valley byPleistocene ice-sheet. Here,ice sheet overspreadplains from the moderately elevated Canadian highlands far onnorth-east, instead of frommuch higher mountains near by onwest. The present altitude ofplains nearmountain base4000 ft.
The northern plainsinterrupted by several small mountain areas. The Black Hills, chieflywestern South Dakota, arelargest group. They rise likelarge island fromsea, occupying an oval areaabout 100 miles north-south by 50 miles east-west. At Harney Peak,reach an altitude7,216 feethave an effective relief overplains2000 or 3000 ft. This mountain mass isflat-arched, dome-like structure, now well dissected by radiating consequent streams. The weaker uppermost strata have been eroded down tolevel ofplains where their upturned edgesevenly truncated. The next following harder strata have been sufficiently erodeddisclosecoreunderlying crystalline rocksabout half ofdomed area.
Inintermediate section ofplains, between latitudes 44°42°, including southern South Dakotanorthern Nebraska,erosioncertain large districtspeculiarly elaborate. Known asBad Lands, it isminutely dissected form withrelief offew hundred feet. Thisdueseveral causes:
- dry climate, which preventsgrowth ofgrassy turf
- fine texture ofTertiary strata inhad land districts
- every little nil, at timesrain, carves its own little valley.
The southern section ofGreat Plains, between latitudes 35.5°2~ ~. lieseastern Texaseastern New Mexico. Like the central section, itformost partdissected fluviatile plain. However,lower lands which surroundon all sides placein so strong relief thatstands up astable-land, known fromtimeMexican occupation asLlano Estacado. It measures roughly 150 miles east-west400 miles north-south. It isvery irregulal outline, narrowing tosouth. Its altitude5500 feet athighest western point, nearestmountains whence its gravels were supplied. From there, it slopes southeastward atdecreasing rate, first about 12 ft., then about 7 ft. per mile,its easternsouthern borders, where it2000 ft. in altitude. LikeHigh Plains farther north, itextraordinarily smooth. Itvery dry, exceptoccasional shallowtemporary water sheets after rains. The Llanoseparated fromplains onnorth bymature consequent valley ofCanadian river,frommountains onwest bybroadprobably mature valley ofPecos river. Oneast, itstrongly undercut byretrogressive erosion ofheadwatersthe Red, BrazosColorado riversTexaspresentsragged escarpment approximately 500800 ft. high, overlookingcentral denuded areathat state. There, betweenBrazosColorado rivers, occursseries of isolated outliers capped bylimestone which underlies bothLlano onwest andGrand Prairies escarpment oneast. The southernnarrow part oftable-land, calledEdwards Plateau,more dissected thanrest,falls off tosouth infrayed-out fault scarp. As already mentioned, this scrap overlookscoastal plain ofRio Grande embayment. The central denuded area, east ofLlano, resembles the east-central section ofplainsexposing older rocks. Between these two similar areas, inspace limited byCanadianRed rivers, risesubdued forms ofWichita MountaiIisOklahoma,westernmost member ofOuachita system.
