The best idea of the generalized topographic form as well as of the origin of the Sierra Nevada, which the reader may be asked to hold in mind, is that the range consists of a block of the earth's crust about 500 miles long and from 70 to 100 miles broad, which has been upraised along its eastern edge so as to give its surface a westward inclination. From this vast monolith the profound cañons and multitude of sharp tapering spires which give to the range its magnificent scenery have been sculptured. Like most generalized pictures of great geographical features, however, this outline of the form and structure of the great mountain range of California has to be modified when studied in detail. From extensive and most painstaking studies by H. W. Turner, of the United States Geological Survey, the conclusion is reached that "the Sierra Nevada may be described as a monogenetic range, composed of highly compressed schists and slates with large areas of associated igneous rocks, chiefly granite and diabase, upon which lie uncomfortably a series of later Cretaceous and Tertiary sediments and volcanic rocks."
Could the profound valleys carved in the long western slope of the Sierra Nevada be filled so as to restore the conditions as they existed when the mountain block was first upraised and tilted, we would have an inclined plane in which the edges of previously folded rocks would be exposed. In other words, the western slope is a tilted peneplain, bearing on its surface remnants of older uplands. High up on the range there are detached areas of well-worn gravel, which were deposited when the slopes were less inclined than at present, and before the existing peaks and cañons came into existence. Certain of the valleys carved during a portion of the earlier stage of erosion were
subsequently filled by lava-floods, which buried gold-bearing gravel beneath thick layers of basaltic rock. Erosion has since cut away the softer beds bordering these ancient lava-sheets, and left them in bold relief as table mountains, underneath which miners have excavated tunnels in order to reach the stream-deposited gravels of the ancient cañons. In these instances valleys have been changed to uplands, owing to the resistance to erosion afforded by the volcanic rocks discharged into them.
The long gentle slope on the west side of the Sierra Nevada has been dissected by westward-flowing rivers, which have sunk their channels 2,000 or 3,000 feet or more into the rocks. Other streams having much shorter courses flow down the steep eastern slope of the range and have also excavated cañons. These two systems of drainage, one leading westward to the Great Valley of California and the other eastward to the valleys of the Great Basin, extended their head branches until they came into rivalry with each other, and cut deep notches in the crest of the range. During a late stage in its history the higher portions of the mountains were covered with a great field of perennial snow, from which glaciers flowed both eastward and westward. These ice-streams, by deepening and broadening the previously water-cut channels, still further increased the diversity of the topography and impressed upon it characteristics such as only glaciers can produce.
The highest and most typical part of the Sierra Nevada is in its south-central portion, and is known as the High Sierra. This region, although at present not accurately defined, is well worthy of recognition. Throughout its entire extent, from the neighbourhood of Lake Tahoe on the north to Tehichipe Pass at the south, a distance of about 240 miles, it is diversified by rugged serrate peaks and narrow stream-cut valleys of great depth. Many of the mountains attain elevations of from 12,000 to over 13,500 feet. The highest summit is Mount Whitney, in the southern part of the range and near its eastern border, which rises 14,522 feet above the sea and has the added distinction of being the highest mountain in the
United States, exclusive of Alaska. It is a notable fact that this great mountain-peak should be situated only about 100 miles from Death Valley, the lowest depression on the continent.
Another of the remarkable features of the Sierra Nevada is the great depth and the wonderfully precipitous walls of some of the valleys carved by the westward-flowing rivers. The most famous of these is the sublime Yosemite Valley, now world-renowned. This cleft as it appears in solid light-coloured granite, over a mile deep, is believed by Turner, the last of several geologists to discuss its origin, to be due to stream erosion. The fact seems well established, however, that glacial ice has assisted in the great task. The Yosemite is not such an unique feature as was at one time supposed, but is approached if not equalled in depth and magnificence by Hetch Hetchy Valley, through which flows the Tuolumne River, and is duplicated, in part at least, by other similar stream-cut gulfs.
Among the chief elements in the glorious scenery of the Sierra Nevada is the multitude of lakes left as a rich inheritance by the departed glaciers. These occur not only high up amid the bare peaks where their basins were excavated by the flowing ice, but also in the lower valleys where the ancient ice-streams built morainal dams.
The High Sierra was swept nearly clean of soil and débris by the ancient glaciers, and the hard rocks thus exposed were rounded and burnished by the ice that flowed over them. But little disintegration or decay of the rocks has taken place since an amelioration of climate changed the drainage from a solid to a liquid form. On account mainly of the general absence of soil the forests are less dense than might be expected from the height of the regions where they occur and its general climatic conditions. The more lofty peaks reach far above the forests and are riven and shattered by frost. The crests and cliffs at somewhat lower altitudes are also bare, but in the cañons and on the meadow-like valley bottoms smoothed by the glaciers, open park-like groves of pine and spruce grow in picturesque disorder. On the ledges of the great precipices, and on
many of the secondary summits, gardens of alpine flowers blossom in late summer, and at times impart a rich warm glow to the heights that support them. The views of nature, unmarred by the hand of man, which reward the persistent mountaineer in this silent wonderland of the upper world, are not only grand beyond all description, but beautified by a delicacy of decoration where snow-fields and alpine gardens meet, that is undreamed of by the dwellers in the denser air of the plains and seaside. Lovers of nature who are unable to climb the towering summits of the High Sierra and see for themselves the marvellous beauty there so lavishly displayed can at least find a glowing pen picture of it in John Muir's fascinating book The Mountains of California. On the lower western slope of the Sierra Nevada the forests become continuous and luxuriant, the trees are of large size, and the lovely flowers carpeting the valleys and hillsides take on a more familiar appearance than the gorgeous blossoms of the alpine meadows. It is in this region that the gigantic Sequoia still lingers as a remnant of a nearly extinct flora.