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North and west of Yellowstone park the mountains (with few exceptions) are not in "ranges" in the usual sense of that word. Linear arrangement in agreement with structure is found to a limited extent, e.g., in the Lewis range of Glacier National park, but in general the present relief does not suggest corrugation. Much the larger part of the area is a vast mountain upland divided only by stream valleys or by "Tertiary basins" in which the mountain surface has sagged. Generally the dominant ridges within a single view are of sub-equal height so that the horizon is not far from level. Altitudes between 6,000 and 9,00o ft. are prevalent. The Bitterroot mountains on the Montana-Idaho boundary are the upturned edge of a fault block tilted westward. Except for this deformation the surface is as described above. The history of the Northern Rocky mountains is much like that of those farther south. At similar altitudes glacial features are more pronounced. They are specially marked and picturesque in Glacier National park in the Front range near the Canadian boundary. The Selkirk and Bitterroot ranges are likewise note worthy in this respect.
North of the southern Rocky mountain province is the Wyoming basin whose floor, almost 40,000 sq.m. in extent, is a plateau 6,500 to 7,500 ft. high and largely free from mountains. Beneath its horizontal Tertiary rocks, and ris ing through them here and there, are the older folded sedimen taries and granite, showing that the Rocky mountain system is structurally continuous. Within and around this plateau the drainage is strikingly out of harmony with structure and to pography. Green, Yampa, Bighorn, North Platte and Laramie rivers all leave the basin by canyons i,000 to 3,00o f t. deep which might be avoided by going around the mountains. Green river, especially, was once a favourite illustration of antecedent river. This assumes that the river was in its place before the mountains rose across its course. It is now believed that these streams are superposed, i.e., they chose their courses when the mountains were buried by weak Tertiary rocks (since eroded away) lying on top of those that now make the basin floor.
South and west of the Rocky mountains is a region of 125,000 sq.m. whose underlying rocks are in the i main horizontal and whose surface is tabular. In part it con sists of. wide-spreading plains at high altitude; elsewhere the plateau consists of tabular remnants between deep and branch ing canyons. In general the plateau abuts against the mountains on the north and east and ends in an escarpment overlooking lowlands on the west and south. The best known part of the province is the south-western section (21e) traversed by the Grand Canyon of the Colorado. This has a maximum depth of 6,000 ft. and a minimum width of less than 5 m. from rim to rim. The conditions favouring canyons are all present in this province. Great altitude above base level, strong through-flowing streams and geologically recent uplift favour downcutting, while strong rocks and arid climate favour preservation of steepness. In this section the surface is on Permian limestone not deeply eroded. These rocks dip north under younger strata. At the same time the surface rises terrace-like over system after system until Eocene (Tertiary) beds and lava flows are found capping the high plateaux of Utah
9,00o to 1 i,000 ft. high, bounded, both east and west, by escarpments. Most of the strata under lying the high plateaux formerly extended southward over the plateau in which the Grand Canyon is cut. A mass of rock equal
in thickness to the entire depth of the canyon was stripped from this plateau leaving a relatively low surface which was later up lifted. In other words, if the present canyon broadens and similar canyons develop and broaden until the entire plateau has been carried away to the depth of the present canyon bottom, then the streams of the present, newly inaugurated erosion cycle will have accomplished what was done once before. The same Eocene beds which appear in the high plateaux form a wide band on the northern margin, dipping northward until they turn up in the foothills of the Uinta range, and ending at the south in an escarpment. The general surface slopes agree with the structure. This is the Uinta basin (2i b). South of it, and east of the high plateaux is the area of arid Canyon Lands (21c) deeply trenched by Colorado river and its tributaries, though its eastern margin in Colorado contains some broad fertile valleys. Farther south is the Navajo section
like the Canyon lands but more arid and less canyoned because there are no mountains on the border to supply through-flowing streams. The Datil section (21f) on the south-east is largely lava-covered as is a very considerable area farther west (San Francisco mountain and vicinity) in the Grand Canyon section. Here also is the Zuni uplift, a dome like the Black hills.
West and south of the Colorado plateau is a region marked by numerous small, roughly parallel mountain ranges separated by nearly flat detrital plains generally 4,000 to 5,00o ft. above the sea, but declining in altitude toward the Gulf of California. The northern half has long been known as the Great Basin and is the largest area of internal drainage in North America. The loo or more mountain ranges divide it into an almost equal number of undrained basins. The hydrographic centres of some of these are marked by salt lakes ; some others have playas, mud flats covered occasionally by a few inches of water. All are filled or veneered by detritus eroded from the mountains and deposited as alluvial fans. Such surfaces are nearly level and tend to rise until the basin is full. A few basins along the course of Humboldt river have been so aggraded with detritus as to establish through drainage which may reach Carson Sink or stop at Humboldt lake, according to season and weather. Many of the mountain ranges are so to 75 m. long and 3,00o to 5,00o ft. above the basins and trend approximately north–south. Trees appear only in exceptional spots. Some of the ranges are known to be upraised or up-tilted fault blocks. As there is a strong family resemblance among ranges, it is considered probable that faulting is at least the master process in the making of these ranges. The internal structures are complex showing that the rocks were strongly folded and (in the best known cases) pene planed and lava-covered before faulting made the present moun tains. In northern Nevada and southern Oregon the mountains were uplifted so recently that their origin by faulting is beyond doubt. Farther south some ranges are so old and eroded that their origin is not obvious. Some of the ranges bear evidence that the master faults were made before the peneplain, the present moun tains consisting of the more resistant fault-blocks exhumed by selective erosion. Both of these types of mountains are present but the relative number of each, or even which is more numerous, is not yet known.