Western Kuen-lun and Astin-tagh) is throughout double; and this " twinning " of the mountain-ranges, as also of the intermont lake-basins among the Kuen-lun ranges, is a peculiar feature of the Tibetan plateau.
E.S.E., and separated by high intermont valleys, which are choked with disintegrated material and divided into a chequered pattern of self-contained, shallow lacustrine basins.
As a rule, the streams flow alternately east and west down the intermont latitudinal valleys, until they break through some transverse glen in the range on the northern side of the valley.
Nevertheless, owing to the fact that nearly all the longer and more important crossings of Tibet and its northern montane region have been made from north to south, or vice versa, that is, transversely across the ranges, and comparatively few from east to west along the intermont latitudinal valleys, the identifications between ranges in the east and ranges in the west are in more than one instance more or less doubtful.
When the altitudes of the intermont latitudinal valleys are compared, the significance orographically of the Chimen valley and of the Kum-kol valley is strikingly emphasized.
The Cordilleran system on the western side of the continent is lofty, broad and complicated, with heavy forests near the north-west coast, but elsewhere with trees only on the higher ranges below the Alpine region, and with treeless or desert intermont valleys, plateaus and basins, very arid in the south-west.
The~ rivers that drain the Atlantic slope of the Appalachians are comparatively short; those that drain the Pacific slope include only two, the Columbia and the Colorado, which rise far inland, near the easternmost members of the Cordilleran system, and flow through plateaus and intermont basins to the ocean.
The Cordilleran Region.From the western border of the Great Plains to the Pacific coast, there is a vast elevated area, occupied by mountains, plateaus and intermont plains.
The intermont plains are at all altitudes from sea-level to 4000 ft.; the plateaus from 5000 to 10,000 ft.; and the mountains from 8000 to 14,000 ft.
In this central region, however, it is only by way of exception that the cirques were so far enlarged by retrogressive glacial erosion as to sharpen the preglacial dome-like summits into acute peaks; and in no case did glacial action here extend down to the plains at the eastern base of the mountains; but the widened, trough-like glaciated valleys frequently descend to the level of the elevated intermont basins, where moraines were deployed forward on the basin floor.
The intermont basins which so strongly characterize the Rocky Mountain system are areas which have been less uplifted than the enclosing ranges, and have therefore usually become the depositories of waste from the surrounding mountains.
North-south, is the largest intermont basin; it is well known from being traversed through its greatest length by the Union Pacific railway.
In length, the higher V ranges reaching altitudes of 8000 or 10,000 ft., separated by broad, intermont desert plains or basins at altit,udes varying from sea-level (or a little less) in the south-west, to 4000 or 5000 ft.
Many of the intermont plainsthese chiefly in the north-appear to be heavily aggraded with mountain Waste; while others-these chiefly in the southare rock-floored and thinly veneered with alluvium.
In the southern part of the Basin Range province the ranges are well dissected and some of the intermont depressions have rock floors with gentle, centripetal slopes; hence it is suggested that the time since the last dislocation in this part of the province is relativel remote; that erosion in the current cycle has here advanced muc farther than in the central or northern parts of the province; and that, either by outwash to the sea or by exportation of wind-borne dust, the depressions-perhaps aggraded for a time in the earlier stages of the cyclehave now been so deeply worn down as to degrade the lower and weaker parts of the tilted blocks to an evenly sloping surface, leaving the higher and harder parts still in relief as residual ranges.
A few intermont areas in the north-west part of the province have outlet westward by Kla1nath river through the Cascade range and by Pitt river (upper part of the Sacramento) through the Sierra Nevada: a few basins in the south-east have outlet by the Rio Grande to the Gulf of Mexico; a much larger but still narrow medial area is drained south-westward by the Colorado to the head of the Gulf of California, where this large and very turbid river has formed an extensive 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.
More important is San Francisco Bay, situated about midway on the Pacific coast of the United States, the result of a moderate depression whereby a transverse valley, formerly followed by Sacramento river through the outermost of the Coast ranges, has been converted into a narrow straitthe Golden Gate and a wider intermont longitudinal valley has been flooded, forming the expansion of the inner bay.
South of Mt Shasta, the Valley of California is an admirable example of an aggraded intermont depression, about 400 m.
The rivers that drain the Atlantic slope of the Appalachians are comparatively short; those that drain the Pacific slope include only two, the Columbia and the Colorado, which rise far inland, near the easternmost members of the Cordilleran system, and flow through plateaus and intermont basins to the ocean.