Page:Popular Science Monthly Volume 85.djvu/82

78 its powerful volume down the deep-gashed gorge of Tenaya Creek into the Yosemite Valley. Professor Le Conte estimated that the main Tuolumne glacier ranged in thickness from a thousand feet in the upper meadows of the modern stream to half a mile in the brimming basin of the Hetch Hetchy. Its length he measured as having been fully forty miles. In the upper canon of the main Merced, Le Conte and Muir studied the striations of grinding ice recorded on every outcropping surface, particularly in the "hanging valley" of "Little Yosemite." Together with the Tenaya glacier, the Merced River of ice mingled at the upper end of the Yosemite Valley, proper, to form a grand glacier which Le Conte believed was the most potent factor in the carving of the U-shaped trough of the Yosemite. And, despite the doubts of Whitney and Russell, Le Conte pointed as prima facie proof the remaining medial moraine at the base of Half Dome near the junction of the two reputed rivers of ice.

Several Geological Survey parties have been sent to solve the riddle of this Sphinx of the Sierras, but still no explanation of the origin of the Yosemite has won the general recognition of geologists or reconciled their differences of opinion. Mr. Francois E. Matthes, of the Survey, in his recent monograph, "The Origin of the Yosemite and Hetch Hetchy Valleys," approaches the problem along the following unbiased lines of logical observation and deduction. Like Le Conte, he lays great stress upon the phases of erosion in the more fissured zones. He accepts the general theory of the uptilting of the Sierra block during the late Tertiary period by volcanic levers, but gives little credence to the subsidence dictum of Whitney. As an ardent advocate of the glacial theory, he traces the apparent agency of ice in the evidence of over-deepened rock-basins and quarried canons like the steep gorge of Tenaya Creek. This striking instance is a graphic illustration of the excavation of a yielding mass of perpendicular cleavage set between walls of massive and adamantine granite. In brief, he argues that the Yosemite and Hetch Hetchy Valleys have been developed by an early system of rapidly-eroding streams; then "greatly deepened and enlarged by repeated ice invasions which modelledmodeled [sic] in the rough. The finer fashioned details of diversified sculpturing he attributes to more recent aqueous agencies and fracturing by frost and aerial forces. Far the most convincing evidence of the former glaciation of the Yosemite Valley he finds in the apparent scooping out of the rock-basin of the Yosemite to the depth of approximately 500 feet. This is confirmed, he contends, by the absence of sills of bed-rock, such as are usually seen outcropping among ordinary watercourses. Another important factor in the leveling of the floor of the valley he sees in the signs of several advances and recessions of terminal moraines across the bottom of this basin, in which process they were arranged in local ridges at frequent intervals. The most conspicuous terminal moraine is that which still