Intermontane Plateaus
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 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.
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.
However the most famous example is the Grand Canyon of Arizona, eroded by the Colorado river across the uplifted platform of Carboniferous limestone.
Mount Taylor in western New Mexico is of similar age, but here dissection seems to have advanced farther, probably because of the weaker nature of the underlying rocks.
The dissection has resulted in removing the smaller cones and exposing many lava conduits or pipes in the form of volcanic necks or buttes.
The Henry Mountains in southwestern Utah are peculiar in owing their relief to the doming or blistering up of the plateau strata by the underground intrusion of large bodies or cisterns (laccolites) of lava, now more or less exposed by erosion.
The province is characterized by numerous disconnected mountain ranges trending north and south, from 30–100 miles (48–161 km) in length, the higher ranges reaching altitudes of 8,000–10,000 feet (2,400–3,000 m), separated by broad, intermontane desert plains or basins at altitudes varying from sea-level (or a little less) in the southwest, to 4,000–5,000 feet (1,200–1,500 m) farther inland.
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.
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 (Jurassic) 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.
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.
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 (U.S.) and northern Sonora (Mexico) states, the ranges are well dissected and some of the intermontane depressions have rock floors with gentle, centripetal slopes.
Many streams descend from the ravines only to wither away on the desert basin floors before uniting in a trunk river along the axis of a depression.
A few of the large streams, such as the Mojave River, when in flood may spread out in a temporary shallow sheet on a dead level of clay, or playa, in a basin center, but the sheet of water vanishes in the warm season (dry lake) and the stream shrinks far up its course, the absolutely barren clay floor of the playa, impassable when wet, becomes firm enough for crossing when dry.
They cover over 210,000 square kilometres (81,000 sq mi)[2] in southeastern Washington, eastern Oregon, and southwestern Idaho, and are known to be 4,000 feet (1,200 m) deep in some river gorges.
Here, the shore-line of the lava contours evenly around the spurs and enters, bay-like, into the valleys of the enclosing mountains, occasionally isolating an outlying mass.
One of the most remarkable features of the Intermediate Province is seen in the temporary course taken by the Columbia River across the plains, while its canyon was obstructed by Pleistocene glaciers that came from the Cascade Range on the northwest.
