OCR Text |
Show sea. Near the close of Paleozoic time the strata were folded, broken, locally uplifted, and then eroded to a surface of small relief, upon which were accumulated sediments of Mesozoic age to an estimated thickness of about 6,000 feet. Near the close of the Cretaceous period, when much of the region was below sea level, areal disturbances in the earth's crust, accompanied by volcanic activity, pushed the strata upward as a broad dome- the ancestral San Juan- far higher than the existing mountains. The uplift continued into early Tertiary time, attaining altitudes that permitted glaciation- a very rare episode in Pre- Pleistocene geologic history. Erosion also continued until the highland mass was reduced to a roughly level surface. Then followed the deposition of great beds of tuff and lava, for which the region is famous. With the cessation of volcanic activity, stream erosion once more became the dominant geologic force. On the widely spread volcanic rocks, streams tributary to the Colorado established their courses and renewed uplifts so increased their power that by the end of the Tertiary era the irregular mountain mass had been reduced to the San Juan peneplain, a generally even, regional surface above which rose residual peaks. Later uplifts introduced the present cycle of erosion, during which the extensive peneplain has been trenched by great canyons and its summit and edges modified in form. Even during Quaternary time the San Juan region has been subjected to almost continuous crustal warping, and sculpturing by vigorous streams has been interrupted only when the region was mantled by ice. During the Pleistocene epoch, except for the high peaks which doubtless were covered by snow, the summit lands were buried by ice to depths of 500 to 1,000 feet, and more than 30 ice streams 5 to 50 miles Ion? flowed down the valleys to the mountain base. The effects of glaciation, repeated three times, are everywhere plainly recorded in ice- scoured surfaces, cirques, widened valleys, moraines, outwash plains, and scores of lakes. Durango, La Plata, Silverton, Ouray, Ridgeway, and several other villages are situated on glacial debris. Furthermore, the steepening of valley walls by glacial scour in conjunction with heavy rainfall has provided conditions exceptionally favorable for the transport of material en masse. In the San Juan Mountains more than 300 landslides, 100 rock streams, and many mud flows and soil creeps have been mapped; masses of moving debris that cover l/ 2 to 2 square miles, extend up canyon walls to heights of 1,000 to 2,600 feet above the stream beds and some distance down valley- the Slumgullion flow for 6 miles. Though the picturesque erosion forms characteristic of the plateau province are lacking, the San Juan landscape is truly magnificent. The grandeur of the desolate summit lands- bare rock flats, huge domes, lofty pyramids and needles- is matched by the majesty of the gigantic canyons. For the scientist the region has special interest. It reveals chapters in geological history and displays physiographic features not duplicated elsewhere. Here the whole story of glaciation is easy to read, and such features as the great rock streams and mud flows are fully displayed. WESTERN BORDER LANDS OF COLORADO PLATEAU In topographic expression, the western border of the Colorado drainage basin is radically unlike the eastern border. In contrast with large eastern tributaries to the Colorado River, which generally rise along mountain crests at the Continental Divide and flow across Pre- Cambrian granite and Paleozoic rock, most of the western tributaries are short, carry little water, and occupy channels cut in sedimentary rocks of Mesozoic and Tertiary age. Along the Wyoming- Idaho boundary and in the Uinta Mountains some streams rise in highland valleys, but in Utah most of them originate on plateau escarpments, and in Nevada on flat lands or the slopes of scattered mountains. Across the tops of the high plateaus the drainage divide between the Colorado Basin and the Great Interior Basin is difficult to trace, and even more difficult where it lies on the slopes of Pine Valley, Snake, and Sheep Mountains, and crosses Charleston and New York Mountains. Southeastern Nevada is part of the Colorado drainage system with topographic features characteristic of the basin and range province. In the area drained by the Virgin River, some of the little- known faulted mountainous masses are composed of Paleozoic rock; some, like Beaverdam and Virgin Mountains, of Pre- Cambrian schists overlaid by Paleozoic strata; and others, like the Muddy and Spring Mountains, of thick Paleozoic 46 |