|Physiographic regions of the U.S. Interior|
For purposes of description, the physical geography of the United States is split into several major physiographic divisions, three of which being the Laurentian Highlands, Interior Highlands and the Interior Plains (see subdivisions 1 and 11-15) lie in the interior of the U.S. Please refer to the Geography of the United States for the other areas.
An outlying upland of the Laurentian Upland of Canada projects the United States west and south of Lake Superior. This upland, part of the Canadian Shield along with the Adirondacks, is a greatly deformed structure and is composed primarily of igneous and metamorphic crystalline rocks commonly associated with a rugged landscape. At some ancient period, this had a strong relief, but today the upland as a whole is gently rolling with the inter-streams surfaces being plateau-like in their evenness. Here they have altitudes of 1,400 to 2,300 feet in their higher areas, such as the Misquah Hills and Huron Mountains. In this province, we find a part of those ancient mountains regions that were initiated by crustal deformation and then reduced by a long continued erosion to a peneplain of modern relief. A peneplain with the occasional moderately high monadnocks left behind during the peneplanation of the rest of the surface. The erosion of the region must have been far advanced in ancient times, even practically completed, because the even peneplain surface is overlapped by fossiliferous marine strata from an early geological date, Cambrian. This shows that the depression of the region beneath an ancient sea took place after a long existence as dry land.
The extent of the submergence and the area over which the Palaeozoic strata were deposited are unknown. Because of the renewed elevation without deformation, erosion in later periods has stripped off an undetermined amount of the covering strata. The valleys by which the uplands are here and there trenched to moderate depth appear to be, in part at least, the work of streams that have been superposed upon the peneplain through the now removed cover of stratified rocks.
Glaciation has strongly scoured away the deeply-weathered soils that presumably existed here in preglacial time. It left behind firm and rugged ledges in the low hills and swells of the ground and spread an irregular drift cover over the lower parts, whereby the drainage is generally disordered being deposited in lakes and swamps and elsewhere rushing down rocky rapids.
The Palaeozoic strata, already mentioned as lapping on the southern slope of the Superior Upland and around the western side of the Adirondack Mountains are but parts of a great area of similar strata hundreds of feet in thickness. These strata decline gently southward from the great upland of the Laurentian highlands of eastern Canada. The visible upland area of today was but a small part of the primeval continent with the remainder of it still buried under a Palaeozoic cover. The visible part was the last part of the primeval continent to sink under the advancing Palaeozoic seas. When the upland and its overlap of stratified deposits were elevated again, the overlapping strata must have had the appearance of a coastal plain. Of course that was long ago, since then the strata have eroded substantially and today possess neither the area nor the smooth form of their initial extent. This district may be considered an ancient coastal plain. As is always the case in the broad denudation of the gently inclined strata of such plains, the weaker layers are worn down in sub-parallel belts of lower land between the upland and the belts of more resistant strata, which rise in uplands.
Few better illustrations of this type of forms are to be found than that presented in the district of the Great Lakes. The chief upland belt or escarpment is formed by the firm Niagara limestone/dolostone, which takes its name from the gorge and falls cut through the upland by the Niagara River. As in all such forms, the Niagara Escarpment has a relatively strong slope or infacing escarpment on the side towards the upland, and a long gentle slope on the other side. Its relief is seldom more than 200 or and is generally small. Its continuity and its contrast with the associated lowlands on the underlying and overlying weak strata suffice to make it an important feature. The escarpment would lie straight east-west if the slant of the strata were uniformly to the south. However, the strata are somewhat warped and so the escarpment's course is strongly convex to the north in the middle, gently convex to the south at either end.
The escarpment begins where its determining limestone/dolostone begins, in west-central New York. There, it separates the lowlands that containing Lake Ontario from Lake Erie. It curves to the northwest through the Ontario province to the island belt that divides the Georgian Bay from Lake Huron. From there, it heads westward through the land-arm between Lake Superior and Lake Michigan and southwestward into the narrow points dividing Green Bay from Lake Michigan. Finally, it fades away with the thinning out of the limestone and is hardly traceable across the Mississippi River.
The arrangement of the Great Lakes is closely matches the course of the lowlands worn on the two belts of weaker strata on either side of the Niagara escarpment. Lake Ontario, Georgian Bay and Green Bay occupy depressions in the lowland on the inner side of the escarpment. Lake Erie, Lake Huron and Lake Michigan lie in depressions in the lowland on the outer side. When the two lowlands are traced eastward, they become confluent after the Niagara limestone has faded away in central New York and the single lowland is continued under the name of Mohawk Valley. This is an east-west longitudinal depression that has been eroded on a belt of relatively weak strata between the resistant crystalline rocks of the Adirondacks on the north and the northern escarpment of the Appalachian plateau (Catskills-Helderbergs) on the south. Early in the U.S. history, this provided a vital economic route between the Atlantic seaports and the U.S. interior.
In Wisconsin, the inner lowland has an interesting feature. It is a knob of resistant quartzites, known as Baraboo Ridge, rising from the buried upland floor through the partly denuded cover of lower Palaeozoic strata. This knob or ridge can be thought of as an ancient physiographic fossil as it is an ancient monadnock having been preserved from destructive attacks of weather by burial under sea-floor deposits. It has been recently re-exposed through the erosion of its cover.
The occurrence of the lake basins in the lowland belts on either side of the Niagara escarpment is an abnormal feature. Ordinary erosion does not explain it. Glacial erosion has formed them through the glacial drift obstructing the normal outlet valleys and to crustal warping in connection with or independent of the glacial sheet.
Lake Superior is unlike the other lakes. The greater part of its basin occupies a depression in the upland area, independent of the overlap of Palaeozoic strata. The western half of the basin occupies a trough of synclinal structure. The Great Lakes are peculiar in receiving the drainage of but a small peripheral land area, enclosed by an ill-defined water-parting from the rivers that run to Hudson Bay or the Gulf of St Lawrence on the north and to the Gulf of Mexico on the south.
The three lakes of the middle group stand at practically the same level:
Lake Michigan and Lake Huron are connected by the Mackinac Straits with the Mackinac Bridge spanning the straits. Lake Huron and Lake Erie are connected by the St. Clair River and Detroit River with the small Lake St. Clair between them. The land northeast of the rivers is undergoing a slow elevation. The Niagara River connecting Lake Erie and Lake Ontario, with a fall of 326 ft (160 ft. at the cataract) in 30 miles (50 km), is of very recent origin as an older river would have a mature valley. The original valley that is thought to have connected the two depressions through the Niagary Escarpment is thought to have been at the present route of the Welland Canal, and to have been completely filled with glacial drift. The same is true for the St. Lawrence, where there may not have been an original valley. The Ontarian River that was a precursor to Lake Ontario is thought to have drained westward, and the St. Lawrence drainage to have been created by subsidence due to the weight of the ice sheet.
The originally treeless prairies of the upper Mississippi basin began in Indiana and extended westward and north-westward until they merged with the drier region known as the Great Plains. An eastward extension of the same region, originally tree-covered, extended to central Ohio. Thus the prairies generally lie between the Ohio and Missouri rivers on the south and the Great Lakes on the north. The prairies are a contribution of the glacial period. They consist for the most part of glacial drift, deposited unconformably on an underlying rock surface of moderate or small relief. Here, the rocks are an extension of the same stratified Palaeozoic formations already described as occurring in the Appalachian region and around the Great Lakes. They are usually fine-textured limestones and shales, lying horizontal. The moderate or small relief that they were given by mature preglacial erosion is now buried under the drift.
The greatest area of the prairies, from Indiana to North Dakota, consists of till plains, that is, sheets of unstratified drift. These plains are 30, 50 or even 100 ft (up to 30 m) thick covering the underlying rock surface for thousands of square miles except where postglacial stream erosion has locally laid it bare. The plains have an extraordinarily even surface. The till is presumably made in part of preglacial soils, but it is more largely composed of rock waste mechanically transported by the creeping ice sheets. Although the crystalline rocks from Canada and some of the more resistant stratified rocks south of the Great Lakes occur as boulders and stones, a great part of the till has been crushed and ground to a clayey texture. The till plains, although sweeping in broad swells of slowly changing altitude, often appear level to the eye with a view stretching to the horizon. Here and there, faint depressions occur, occupied by marshy sloughs, or floored with a rich black soil of postglacial origin. It is thus by sub-glacial aggradation that the prairies have been levelled up to a smooth surface, in contrast to the higher and non-glaciated hilly country just to the south.
The great ice sheets formed terminal moraines around their border at various end stages. However, the morainic belts are of small relief in comparison to the great area of the ice. They rise gently from the till plains to a height of 50, 100 or more feet. They may be one, two or three miles (5 km) wide and their hilly surface, dotted over with boulders, contains many small lakes in basins or hollows, instead of streams in valleys. The morainic belts are arranged in groups of concentric loops, convex southward, because the ice sheets advanced in lobes along the lowlands of the Great Lakes. Neighboring morainic loops join each other in re-entrants (north-pointing cusps), where two adjacent glacial lobes came together and formed their moraines in largest volume. The moraines are of too small relief to be shown on any maps except of the largest scale. Small as they are, they are the chief relief of the prairie states, and, in association with the nearly imperceptible slopes of the till plains, they determine the course of many streams and rivers, which as a whole are consequent upon the surface form of the glacial deposits.
The complexity of the glacial period and its subdivision into several glacial epochs, separated by interglacial epochs of considerable length (certainly longer than the postglacial epoch) has a structural consequence in the superposition of successive till sheets, alternating with non-glacial deposits. It also has a physiographic consequence in the very different amount of normal postglacial erosion suffered by the different parts of the glacial deposits. The southernmost drift sheets, as in southern Iowa and northern Missouri, have lost their initially plain surface and are now maturely dissected into gracefully rolling forms. Here, the valleys of even the small streams are well opened and graded, and marshes and lakes are rare. These sheets are of early Pleistocene origin. Nearer the Great Lakes, the till sheets are trenched only by the narrow valleys of the large streams. Marshy sloughs still occupy the faint depressions in the till plains and the associated moraines have abundant small lakes in their undrained hollows. These drift sheets are of late Pleistocene origin.
When the ice sheets extended to the land sloping southward to the Ohio River, Mississippi River and Missouri River, the drift-laden streams flowed freely away from the ice border. As the streams escaped from their subglacial channels, they spread into broader channels and deposited some of their load and thus aggraded their courses. Local sheets or aprons of gravel and sand are spread more or less abundantly along the outer side of the morainic belts. Long trains of gravel and sands clog the valleys that lead southward from the glaciated to the non-glaciated area. Later, when the ice retreated farther and the unloaded streams returned to their earlier degrading habit, they more or less completely scoured out the valley deposits, the remains of which are now seen in terraces on either side of the present flood plains.
When the ice of the last glacial epoch had retreated so far that its front border lay on a northward slope, belonging to the drainage area of the Great Lakes, bodies of water accumulated in front of the ice margin, forming glacio-marginal lakes. The lakes were small at first, and each had its own outlet at the lowest depression of land to the south. As the ice melted further back, neighboring lakes became confluent at the level of the lowest outlet of the group. The outflowing streams grew in the same proportion and eroded a broad channel across the height of land and far down stream, while the lake waters built sand reefs or carved shore cliffs along their margin, and laid down sheets of clay on their floors. All of these features are easily recognized in the prairie region. The present site of Chicago, Illinois was determined by an Indian portage or carry across the low divide between Lake Michigan and the headwaters of the Illinois River. This divide lies on the floor of the former outlet channel of the glacial Lake Michigan. Corresponding outlets are known for Lake Erie, Lake Huron and Lake Superior. A very large sheet of water, named Lake Agassiz, which once overspread a broad till plain in northern Minnesota and North Dakota. The outlet of this glacial lake, called river Warren, eroded a large channel in which the Minnesota River evident today. The Red River of the North flows northward through a plain formerly covered by Lake Agassiz.
Certain extraordinary features were produced when the retreat of the ice sheet had progressed so far as to open an eastward outlet for the marginal lakes. This outlet occurred along the depression between the northward slope of the Appalachian plateau in west-central New York and the southward slope of the melting ice sheet. When this eastward outlet came to be lower than the south-westward outlet across the height of land to the Ohio or Mississippi river, the discharge of the marginal lakes was changed from the Mississippi system to the Hudson system. Many well-defined channels, cutting across the north-sloping spurs of the plateau in the neighborhood of Syracuse, New York, mark the temporary paths of the ice-bordered outlet river. Successive channels are found at lower and lower levels on the plateau slope, indicating the successive courses taken by the lake outlet as the ice melted farther and farther back. On some of these channels, deep gorges were eroded heading in temporary cataracts which exceeded Niagara in height but not in breadth. The pools excavated by the plunging waters at the head of the gorges are now occupied by little lakes. The most significant stage in this series of changes occurred when the glacio-marginal lake waters were lowered so that the long escarpment of Niagara limestone was laid bare in western New York. The previously confluent waters were then divided into two lakes. The higher one, Lake Erie, supplied the outflowing Niagara River, which poured its waters down the escarpment to the lower, Lake Ontario. This gave rise to Niagara Falls. Lake Ontario's outlet for a time ran down the Mohawk Valley to the Hudson River. At this higher elevation it was known as Lake Iroquois. When the ice melted from the northeastern end of the lake, it dropped to a lower level, and drained through the St. Lawrence area. This created a lower base level for the Niagara River, increasing its erosive capacity.
In certain districts, the subglacial till was not spread out in a smooth plain, but accumulated in elliptical mounds, 100-200 feet. high and 0.5 to 1 mile (0.8 to 1.6 km) long with axes parallel to the direction of the ice motion as indicated by striae on the underlying rock floor. These hills are known by the Irish name, drumlins, used for similar hills in north-western Ireland. The most remarkable groups of drumlins occur in western New York, where their number is estimated at over 6,000, and in southern Wisconsin, where it is placed at 5,000. They completely dominate the topography of their districts.
A curious deposit of an impalpably fine and unstratified silt, known by the German name bess (or loess), lies on the older drift sheets near the larger river courses of the upper Mississippi basin. It attains a thickness of 20 ft (6 m) or more near the rivers and gradually fades away at a distance of ten or more miles (16 or more km) on either side. It contains land shells, and hence cannot be attributed to marine or lacustrine submergence. The best explanation is that, during certain phases of the glacial period, it was carried as dust by the winds from the flood plains of aggrading rivers, and slowly deposited on the neighboring grass-covered plains. The glacial and eolian origin of this sediment is evidenced by the angularity of its grains (a bank of it will stand without slumping for years), whereas, if it had been transported significantly by water, the grains would have been rounded and polished. Loess is parent material for an extremely fertile, but droughty soil.
South-western Wisconsin and parts of the adjacent states of Illinois, Iowa and Minnesota are known as the driftless zone, because, although bordered by drift sheets and moraines, it is free from glacial deposits. It must therefore have been a sort of oasis, when the ice sheets from the north advanced past it on the east and west and joined around its southern border. The reason for this exemption from glaciation is the converse of that for the southward convexity of the morainic loops. For while they mark the paths of greatest glacial advance along lowland troughs (lake basins), the driftless zone is a district protected from ice invasion by reason of the obstruction which the highlands of northern Wisconsin and Michigan (part of the Superior upland) offered to glacial advance.
The course of the upper Mississippi River is largely consequent upon glacial deposits. Its sources are in the morainic lakes in northern Minnesota. The drift deposits thereabouts are so heavy that the present divides between the drainage basins of Hudson Bay, Lake Superior and the Gulf of Mexico evidently stand in no very definite relation to the preglacial divides. The course of the Mississippi through Minnesota is largely guided by the form of the drift cover. Several rapids and the Saint Anthony Falls (determining the site of Minneapolis) are signs of immaturity, resulting from superposition through the drift on the under rock. Farther south, as far as the entrance of the Ohio River, the Mississippi follows a rock-walled valley 300 to 400 ft (90 to 120 m) deep, with a flood-plain 2 to 4 mi (3 to 7 km) wide. This valley seems to represent the path of an enlarged early-glacial Mississippi, when much precipitation that is today discharged to Hudson Bay and the Gulf of St Lawrence was delivered to the Gulf of Mexico, for the curves of the present river are of distinctly smaller radii than the curves of the valley. Lake Pepin (30 mi [50 km] below St. Paul), a picturesque expansion of the river across its flood-plain, is due to the aggradation of the valley floor where the Chippewa River, coming from the northeast, brought an overload of fluvio-glacial drift. Hence even the father of waters, like so many other rivers in the Northern states, owes many of its features more or less directly to glacial action.
The fertility of the prairies is a natural consequence of their origin. During the mechanical transportation of the till no vegetation was present to remove the minerals essential to plant growth, as is the case in the soils of normally weathered and dissected peneplains. The soil is similar to the Appalachian piedmont which though not exhausted by the primeval forest cover, are by no means so rich as the till sheets of the prairies. Moreover, whatever the rocky understructure, the till soil has been averaged by a thorough mechanical mixture of rock grindings. Hence the prairies are continuously fertile for scores of miles together. The true prairies were once covered with a rich growth of natural grass and annual flowering plants, but today are covered with farms.
The westward extension of the Atlantic Coastal Plain around the Gulf of Mexico has certain features that were already described and several new ones. As in the Atlantic coastal plain, it is only the lower, seaward part of this region that deserves the name of plain, for there alone is the surface unbroken by hills or valleys. The inner part, initially a plain, has been maturely dissected into an elaborate complex of hills and valleys, usually of increasing altitude and relief as one passes inland. The Gulf Plain features not found in the Atlantic coastal plain are:
Here, much of the lowland drainage is underground forming many sinkholes (swallowholes). Many small lakes in the lowland appear to owe their basins to the solution of the limestones. Valuable phosphate deposits occur in certain districts. The southern part of the state includes the Everglades, a large area of low, flat, marshy land, overgrown with tall reedy grass.
The eastern coast is fringed by long-stretching sand reefs, enclosing lagoons so narrow and continuous that they are popularly called rivers. At the southern end of the peninsula is a series of coral islands, known as the Florida Keys. They appear to be due to the forward growth of corals and other lime-secreting organisms towards the strong current of the Gulf Stream from which they obtain their food. The western coast has fewer, shorter off-shore reefs. Much of it is of minutely irregular outline.
The lowland is enclosed by an upland or escarpment, known as Chunnenugga Ridge, sustained by partly consolidated sandy strata. However, the upland is not continuous, but a maturely dissected escarpment. It has a relatively rapid descent toward the inner lowland, and a very gradual descent to the coast prairies, which become very low, flat and marshy before dipping under the Gulf waters, where they are generally fringed by off-shore reefs.
As the coastal plain turns westward toward Texas, it borders the Ouachita Mountains of Arkansas and Oklahoma and the Arbuckle Mountains in southern Oklahoma. The Ouachitas and the Arbuckles may be considered an analog of and possible extension of the Appalachian fold and crystalline belts.
The small proportion of total water volume supplied from the great Missouri River basin is due to the light precipitation in that region. The lower Mississippi has no large tributaries from the lower east, but two important ones come from the west. The Mississippi Arkansas drainage area being a little less than the Ohio River and the basin of the Red River of Louisiana being about half as large. The Mississippi River drains an area of about one-third of the United States. The head of the coastal plain embayment is near the junction of the Ohio and the Mississippi. It flows southward for 560 miles (901 km) through the semi-consolidated strata of the plain. The river has eroded a valley about 40 to 50 miles (60 to 80 km) wide enclosed by bluffs one or two hundred feet high in the northern part. These bluffs decrease towards the south, but with local increase of height associated with a decrease in flood plain breadth on the eastern side where the Grand Gulf escarpment is traversed.
This valley in the coastal plain, with the much narrower rock-walled valley of the upper river in the prairie states, is the true valley of the Mississippi River. However in popular usage, the Mississippi valley is taken to include a large central part of the Mississippi drainage basin.
The valley floor is covered with a flood plain of fine silt, having a southward slope of only half a foot to a mile (100 mm/km). The length of the river itself, from the Ohio mouth to the Gulf is about 1,060 miles (1,706 km) due to its windings. Its mean fall is about 3 inches per mile (50 mm/km). On account of the rapid deposition of sediment near the main channel at times of overflow, the flood plain, as is normally the case on mature valley floors, has a lateral slope of as much as 5, 10, or even in the first mile from the river, but this soon decreases to a less amount. Thus just a short distance from the river, the flood plain is often swampy, unless its surface is there aggraded by the tributary streams. For this reason Louisiana, Arkansas and Mississippi rank immediately after Florida in swamp area.
The great river receives an abundant load of silt from its tributaries, and takes up and lays down silt from its own bed and banks with every change of velocity. The swiftest current follows the outer side of every significant curve in the channel. Thus the concave bank on the side of the fastest part of the river is worn away. Any chance irregularity is exaggerated, and in time a series of large serpentines or meanders is developed, the most-symmetrical examples at present being those near Greenville, Mississippi. The growth of the meanders tends to give the river continually increasing length. This tendency is counteracted by the sudden occurrence of cut-offs from time to time, so that a fairly constant length is maintained.
The floods of the Mississippi usually occur in spring or summer. Owing to the great size of the drainage basin, it seldom happens that the three upper tributaries are simultaneously flooded. It is a serious problem for the lower river if two of the large tributaries flood at the same time. In this case, the lower river will rise to 30, 40 or even . The fall of the river is significantly steepened and its velocity is accelerated down stream from the point of highest rise. Conversely, the fall and the velocity are both diminished up stream from the same point.
The load of silt carried down stream by the river finally, after many halts on the way, reaches the waters of the Gulf. There, the decrease of velocity aided by the salinity of the sea water, causes the formation of a remarkable delta, leaving less aggraded areas as shallow lakes (Lake Pontchartrain on the east, and Grand Lake on the west of the river). The ordinary triangular form of deltas, due to the smoothing of the delta front by sea action, is here wanting, because of the weakness of sea action in comparison with the strength of the current in each of the four distributaries or passes into which the river divides near its mouth.
Its gently undulating or rolling seaward slope of 2 or 3 ft per mile (500 mm/km), covered with marly strata and rich black soil, determines an important cotton district. Then comes the East Texas timber belt, broad in the northeast, narrowing to a point before reaching the Rio Grande, a low and thoroughly dissected escarpment of sandy Eocene strata. This is followed by the Coast Prairie, a very young plain, with a seaward slope of less than 2 ft per mile (400 mm/km), its smooth surface interrupted only by the still more nearly level flood plains of the shallow, consequent river valleys. Near the Colorado river, the dissected escarpment of the Grand Prairie passes southward changing to a more nearly horizontal structure into the dissected Edwards plateau. The Edwards plateau is referred to later as part of the Great Plains. The plateau terminates in a maturely dissected fault scarp approximately 300 or 400 feet in height as the northern boundary of the Rio Grande embayment. From the Colorado to the Rio Grande, the Black Prairie, the timber belt and the Coast Prairie merge in a vast plain, little differentiated, overgrown with chaparral (shrub-like trees, often thorny), widening eastward in the Rio Grande delta and extending southward into Mexico.
Although the Coast Prairie is a sea bottom of very modern uplift, it appears already to have suffered a slight movement of depression. Its small rivers all enter embayments. However, the larger rivers seem to have counteracted the encroachment of the sea on the land by a sufficiently active delta building with a resulting forward growth of the land into the sea. The Mississippi has already been mentioned as rapidly building forward its digitate delta. The Rio Grande, next in size, has built its delta about 50 miles (80 km) forward from the general coastline. Since this river is much smaller than the Mississippi, its delta front is rounded by seashore effects. In front of the Brazos and the Colorado, the largest of the Texan rivers, the coast-line is very gently bowed forward as if by delta growth. The sea touches the mainland in a nearly straight shore line. Nearly all the rest of the coast is fringed by off-shore reefs or barrier islands, built up by waves from the very shallow sea bottom. Due to the weak tides, the barrier islands continue in long unbroken stretches between the few inlets.
A broad stretch of country underlaid by nearly horizontal strata extends westward from the 97th meridian to the base of the Rocky Mountains, a distance of from 300 to 500 miles (500 to 800 km). It extends northward from the Mexican boundary far into Canada. This is the province of the Great Plains. Although the altitude of the plains increases gradually from 6oo or on the east to 4,000-5,000 or near the mountains, the local relief is generally small. The sub-arid climate excludes tree growth and opens far-reaching views. The plains are by no means a simple unit. They are of diverse structure and of various stages of erosional development. They are occasionally interrupted by buttes and escarpments. They are frequently broken by valleys. Yet on the whole, a broadly extended surface of moderate relief so often prevails that the name, Great Plains, for the region as a whole is well deserved. The western boundary of the plains is usually well defined by the abrupt ascent of the mountains. The eastern boundary of the plains is more climatic than topographic. The line of 20 in. of annual rainfall trends a little east of northward near the 97th meridian. If a boundary must be drawn where nature presents only a gradual transition, this rainfall line may be taken to divide the drier plains from the moister prairies. The plains may be described in northern, intermediate, central and southern sections, in relation to certain peculiar features.
The northern plains are interrupted by several small mountain areas. The Black Hills, chiefly in western South Dakota, are the largest group. They rise like a large island from the sea, occupying an oval area of about 100 miles (160 km) north-south by 50 miles (80 km) east-west. At Harney Peak, they reach an altitude of and have an effective relief over the plains of 2000 or This mountain mass is of flat-arched, dome-like structure, now well dissected by radiating consequent streams. The weaker uppermost strata have been eroded down to the level of the plains where their upturned edges are evenly truncated. The next following harder strata have been sufficiently eroded to disclose the core of underlying igneous and metamorphic crystalline rocks in about half of the domed area.
It is very dry, except for occasional shallow and temporary water sheets after rains. The Llano is separated from the plains on the north by the mature consequent valley of the Canadian River, and from the mountains on the west by the broad and probably mature valley of the Pecos River. On the east, it is strongly undercut by the retrogressive erosion of the headwaters of the Red, Brazos and Colorado rivers of Texas and presents a ragged escarpment approximately 500 to high, overlooking the central denuded area of that state. There, between the Brazos and Colorado rivers, occurs a series of isolated outliers capped by a limestone which underlies both the Llano Uplift on the west and the Grand Prairies escarpment on the east. The southern and narrow part of the table-land, called the Edwards Plateau, is more dissected than the rest, and falls off to the south in a frayed-out fault scarp. As already mentioned, this scarp overlooks the coastal plain of the Rio Grande embayment. The central denuded area, east of the Llano, resembles the east-central section of the plains in exposing older rocks. Between these two similar areas, in the space limited by the Canadian and Red rivers, rise the subdued forms of the Wichita Mountains in Oklahoma, the westernmost member of the Ouachita system.