7113 (2) - Laserfiche WebLink

Home Browse Search

I i 70 FISHES AND FLSHESIFS OF NEVADA And so, not only because it is the most generally applicable designa- tion, but the most widely used, the term "Great Basin" will here mean that part of the central intermontane West lying between the Roeky and Sierra Nevada Mountains which has no drainage to the exterior. This includes large parts of southern Oregon, fragments of southern Idaho and southwestern Wyoming, most of western Utah, most of Nevada and a large section of southeastern California-literally a little bit of everything in sight. The typical physiography of the Great Basin can be better under- stood against the background of a summary of its formation. The phenomenon of "block faulting," in which portions of the earth's crust move up or down, has been largely responsible for the present appearance of the Basin. As height differentials build up between various parts of these moving blocks, the depressions begin to fill with soft, unconsolidated debris derived from erosion of higher areas; the sediments so deposited in these troughs are mostly fanglomerates and lake beds, with local development of fluviatile or river sands and, even more restrictedly, of glacial materials. All of these are easily dissectible by later flowing waters, and even subject to much deflation by winds. Interbedded with these sediments are great thicknesses, in many places, of intrusives and lava flows, basalt predominating over rhyolite in the latter case. Because of the unconsolidated nature of the sedimen- taries, and the ease with which they are moved by water, even inter- mittent streams normally have little difficulty in carving out the troughs. However, in an area of enclosed basins, the opposite effects are achieved-the basin is rapidly filled with sediments, for there is no place for them to be carried. When lakes with no outlets occupy these basins, as was usually the case, these sediments are finely sifted and become typical lake bottom silts over most of the basin. When the lakes disappear temporarily, fanglomerate materials, lipreading out from the two ranges paralleling and enclosing the basin, sweep in and cover the lake beds so that over a long period of time the sediments of such a basin become grossly layered-fanglomerates, lake silts, fanglomerates, lake silts, etc. As the mountain blocks on each side continue to move upward-or the basin sinks-sediments may accumulate to a remarkable depth. Southern Nevada 1Iioeeue beds are known to exceed 3,000 feet, or better than half a mile, in depth. As a consequence of this structure, younger stream courses parallel the mountain ranges in a systematic north-south pattern. The older streams, however, cut across the ranges; being antecedent to the n?oiui- tains, these ancient rivers have been able to downeut their channels at the same rate the land was rising. Physically, the Great Basin presents a study in extremes. Its two highest peaks-Boundary Peak on the western border and Wheeler Peak near the Utah lice-slightly exceed 1:3,000 feet in height, several thousand feet above timberline. Its lowest valleys range down to 300 feet above sea level along the Colorado River in the extreme southern tip of the State. In the intervening country in central Nevada, a prominent block of mountain ranges-Shoshone, Toiyabe, Toquima and 'Monitor Ranges- a