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Show area continuously covering about 400 square miles - and extensive sheets of rhyolitic and andesitic lavas. Into the regional surface the Gunnison and its tributaries have sunk deep runways by cutting through sedimentary, igneous, and metamorphic rocks down to porphyritic granite. The present scenic features of the Gunnison Valley record merely the last event in the long geologic history of the Rocky Mountains and the Colorado plateaus. In Cretaceous times the region was covered by the sea, in which sediments of great thickness accumulated, then near the beginning of Tertiary time was uplifted as a great arch- the ancestral Rocky Mountains. During the long period of erosion that followed, the sedimentary rocks that once were continuous across its surface were stripped away, down to the core of the schist and granite, leaving an erosion surface of moderate relief; a broad belt that extends from the Sawatch Range westward about 70 miles, and southward merges with the foothill slopes of the San Juan Mountains. The latest major movement of the earth's crust was upward and brought the Sawatch Range and its bordering lands along the Gunnison to their present lofty position, and perhaps coincident with the uplift the newly exposed surface was flooded with lavas or buried beneath volcanic tuffs. Then followed a period of erosion whose length is the life span of the present- day streams. The progress of the streams in developing valleys and cutting channels in rocks of various types is faithfully recorded by the Gunnison River. For about 100 miles of its upper course ( Tomichi Creek) the river occupies alternately shallow canyons in intrusive granite and open valleys in less- resistant Cretaceous and Jurassic rocks. Farther down it flows between vertical walls of dark- colored contorted schists and gneisses through the spectacular Black Canyon, 1,700 to 2,400 feet deep and in places not more than 50 feet wide at the bottom. The Pre- Cambrian rock in the canyon walls terminates abruptly at an eroded surface above which stratified rocks of Upper Cretaceous age form slopes. The division line between these unlike rock masses represents a hiatus in the geologic time scale of an estimated 400,000,000 years. That the Gunnison is a superposed stream that has persisted in the course established on a surface high above its present walls is shown by its entrenched meanders and its topographic position. It has continued to cut its path across highlands through hard Archean schists, seemingly ignoring the soft rocks in the adjacent, downfaulted lowlands. San Juan Mountains.- The dominating topographic feature of southwestern Colorado is the San Juan Mountains, which lie athwart the Continental Divide and cover an area of about 6,500 square miles. ( PI. 13, Sec. 8, in pocket.) The western part of this lofty- peak- studded mass lies within the Colorado drainage basin and is the gathering ground for the waters carried northward directly to the Colorado by the Uncompahgre, San Miguel, and Dolores Rivers, and southward by the La Plata, Animas, Los Pinos, and Piedra Rivers which reach the Colorado River by way of the San Juan. Through the escarpment that walls in the mountains, these streams descend precipitously to the adjacent plateau lands. The average altitude of the main mountain mass including the western San Juan Mountains, the north- trending Cimarron and Mount Sneffels spurs, the Needles Mountains, and the somewhat isolated San Miguel Mountains is about 11,500 feet. Scores of peaks rise to heights of 13,000 feet, 13 exceed 14,000 feet, and the canyon floors lie 4,000 to 6,000 feet below the adjacent summits. The mountains consist almost entirely of Tertiary volcanic rocks; lavas, stratified tuffs, agglomerates, and breccias, 5,000 to 9,000 feet in thickness, with which are associated igneous in- trusives. Mesozoic and Paleozoic formations are exposed in the deeper canyons, and along the edges of the mountain mass they appear in sequence beneath the volcanic cap, and dip north, west and south, and within short distances from the mountain center assume a horizontal position. Compared with adjacent regions, the San Juan Mountains are a deeply, though not intricately, dissected rock- surfaced plateau in which the forms of many peaks, ridges, and canyons have been modified by glacial erosion. Their present features record merely the latest major event in a long, complex geological history. As revealed by the composition and arrangement of the exposed rocks, an enormous basal mass of Pre- Cambrian and Cambrian schists, porphyries, and quartzite was worn down, then overlaid by Paleozoic sediments 2,000 to 4,500 feet thick- most of them deposited in a 45 |