Intermontane Plateaus
In the context of physical geography, the Intermontane Plateaus is one of eight physiographic regions of the contiguous United States. The region consists mostly of plateaus and mountain ranges lying between the Rocky Mountains on the east and the Cascade and Sierra Nevada Mountains on the west. It is subdivided into three physiographic provinces: the Columbia Plateau in the north, the Basin and Range Province in the central and southwestern portions, and the Colorado Plateau in the southeast. In turn, each of these provinces are each subdivided into a number of physiographic sections.
Physiographic provinces
Columbia Plateau Province
The Columbia Plateau Province is a large igneous province of flood basalts erupted in Miocene and early Pliocence epochs across the states of Washington, Oregon, Idaho, Nevada, and California.Colorado Plattiary erosion landforms
The province was uplifted and divided into great blocks by faults or monoclinal flexures which were exposed to long-lasting denudation in a mid-Tertiary cycle of erosion. They were then broadly elevated again with renewed movement on some of the faults. The current erosion cycle started in the late Tertiary, when the deep canyons of the region were trenched. The results of the first cycle of erosion are seen in the widespread exposure of the resistant Carboniferous limestone as a broad platform in the southwestern area of greater uplift through central Arizona where the less resistant overlying formations have been eroded away. They are also seen in the development of a series of cuestas: huge, south-facing, retreating escarpments of irregular outline on the edges of the higher formations farther north. Each escarpment stands forth where a resistant formation overlies a weaker, less resistant one. Each escarpment is separated from the next higher one by a broad step of weaker strata.A series of these geologic formations occurs in southern Utah, where in passing northward from the Carboniferous platform one ascends in succession the Chocolate Cliffs, Vermilion and White Cliffs, the Gray Cliffs, and finally the Pink Cliffs of the high, forested plateaus. Associated with these irregular escarpments are occasional rectilinear ridges, the work of extensive erosion on monoclinal structures. A good example of this is Echo Cliffs lying east of the Painted Desert. The Mogollon Rim escarpment is part of the transition zone between the Mogollon Plateau of the Colorado Plateau Province and the Sonoran Desert of the Basin and Range Province.
With the renewal of uplift by which the earlier cycle of erosion was interrupted and the present cycle introduced, inequalities of surface due to renewed faulting were again introduced. These still appear as cliffs, of more nearly rectilinear front than the retreating escarpments formed in the previous cycle. These cliffs are peculiar in gradually passing from one formation to another, and in having a height dependent on the displacement of the fault rather than on the structures in the fault face. They are already somewhat battered and dissected by erosion. The most important line of cliffs of this class is associated with the western and southern boundary of the Plateau Province where it was uplifted from the lower ground. The few rivers of the region must have reached the quiescence of old age in the earlier cycle, but were revived by uplift to a vigorous youth in the current cycle. It is to this newly introduced cycle of physiographic evolution that the deep canyons of the Plateau province are due. Thus the Virgin River, a northern tributary of the Colorado River, has cut a vertical slit, 1000 ft. deep, hardly wider at the top than at the bottom, in the heavy Triassic sandstones of southern Utah. However the most famous example is the Grand Canyon of Arizona, eroded by the Colorado river across the uplifted platform of Carboniferous limestone.
During the current cycle of erosion, several of the faults, whose scarps had been worn away in the previous cycle, have been brought to light again as topographic features by the removal of the weak strata along one side of the fault line, leaving the harder strata on the other side in relief. Such scarps are known as fault-line scarps, in distinction from the original fault scarps. They are peculiar in having their altitude dependent on the depth of revived erosion, instead of the amount of faulting, and they are sometimes topographically reversed, in that the revived scarp overlooks a lowland worn on a weak formation in the upheaved fault-block. Another consequence of revived erosion is seen in the occurrence of great landslides, where the removal of weak clays has sapped the face of the Vermilion Cliffs, so that huge slices of the cliff face have slid down and forward, all shattered into a confused tumult of forms for a or more along the cliff base.
Volcanic features
Volcanic features occur in abundance in the Plateau province. Some of the high plateaus in the north are capped with remnants of heavy lava flows of early eruption. A group of large volcanoes occurs on the limestone platform south of the Grand Canyon, culminating in Mount San Francisco of lava, now more or less exposed by erosion.Basin and Range Province
The large Basin and Range Province is a basin and range topography resulting from crustal extension. It is located in all or parts of Oregon, Idaho, Nevada, California, Utah, New Mexico, Arizona in the Western United States; and Sonora, Chihuahua, Sinaloa, and other states southwards to the Trans-Mexican Volcanic Belt in Mexico. It involves some novel problems in its description, especially in Southern California and central Mexico.The province is characterized by numerous disconnected mountain ranges trending north and south, from in length, the higher ranges reaching altitudes of, separated by broad, intermontane desert plains or basins at altitudes varying from sea-level in the southwest, to farther inland. It is an arid region.
Many of the intermontane plains, occurring mostly in the north, appear to be heavily aggraded with mountain waste. Others, mostly in the south, are valleys also heavily aggraded by mountain erosion. The structure of the region previous to faulting was dependent on long antecedent processes of accumulation and deformation and the surface of the region then was dependent on the amount of erosion suffered in the prefaulting cycle.
When the region was broken into fault blocks and the blocks were uplifted and tilted, the back slope of each block was a part of the previously eroded surface and the face of the block was a surface of fracture. The present form of the higher blocks is more or less affected by erosion since faulting, while many of the lower blocks have been buried under the waste of the higher ones.
Northern areas
In the north, where dislocations have invaded the field of the horizontal Columbian lavas, as in southeastern Oregon and northeastern California, the blocks are monoclinal in structure as well as in attitude. Here, the amount of dissection is relatively moderate, for some of the fault faces are described as ravined but not yet deeply dissected. Hence these dislocations appear to be of recent date.Central areas
In the Great Basin of western Utah and through most of Nevada, many of the blocks exhibit deformed structures involving folds and faults of relatively ancient date. In fact so ancient that the mountains formed by the folding were worn down to the lowland stage of old age before the block-faulting occurred. When this old-mountain lowland was broken into blocks and the blocks were tilted, their attitude, but not their structure, was monoclinal.In this new attitude, they have been so maturely re-dissected in the current new cycle of erosion as to have gained elaborately carved forms in which the initial form of the uplifted blocks can hardly be perceived. Some of them still retain along one side the highly significant feature of a relatively simple base-line, transecting hard and soft structures alike indicating the faulted margin of a tilted block.
Here the less uplifted blocks are now heavily aggraded with waste from the dissected ranges. The waste takes the form of huge alluvial fans, formed chiefly by occasional boulder-bearing floods from the mountains. Each fan heads in a ravine at the mountain base and becomes laterally confluent with adjacent fans as it stretches several miles forward with decreasing slope and increasing fineness of material.
Southern areas
In the upper southern part of the Basin and Range Province, in the Mojave Desert of California, and Sonoran Desert of southern California and Arizona and northern Sonora states, the ranges are well dissected and some of the intermontane depressions have rock floors with gentle, centripetal slopes. This area also has huge alluvial fans, with heads at canyons exiting the mountains, and laterally confluent with adjacent fans as they extend for miles with rather consistent slopes.The Basin and Range Province extends southeast down into Chihuahua state in northeast Mexico, and far south along the Mexican Plateau to central Mexico at the Trans-Mexican Volcanic Belt.
Drainage basins
Only a small part of the Basin and Range province is drained to the sea. A few intermont areas in the north-west part of the province have outlets westward via the Klamath River through the Cascade range. The Sacramento and San Joaquin Rivers from the Sierra Nevada pass through the Central Valley and California Coast Ranges to San Francisco Bay. A few basins in the southeast have outlet by the Rio Grande to the Gulf of Mexico.Colorado River watershed
A much larger but still narrow medial area is drained southwestward by the Colorado River to the head of the Gulf of California, where this large and very turbid river has formed the extensive Colorado River Delta, north of which the former head of the gulf is now cut off from the sea and laid bare by evaporation as a plain below sea-level.It is in the Lower Colorado River Valley that an irrigation project, involving the diversion of some of the river water to the low plain of the Imperial Valley, led to disaster in 1904. The river flooded through a deliberately engineered breach of the Alamo Canal and flowed across the Imperial Valley plain into the Salton Sink, forming the Salton Sea. The endorheic Salton Sea remains, now supplied by subterranean flow from over-irrigated fields draining into the aquifer.