Page:Popular Science Monthly Volume 52.djvu/634

614 marked contrast with the steep walled canons. This kind of topography is to be observed from a point south of Mount Whitney for more than a hundred and fifty miles northward past Mono Lake, and is undoubtedly the remnant of an old base-leveled condition reached during the early Tertiary and preceding the last important uplift. Mount Whitney even has an almost level summit, breaking away into vertical cliffs, which on its eastern side are over three thousand feet high, and form a portion of the cirque at the head of Lone Pine Creek. The plateaulike character is plainly discernible about Mono Lake, where it has been described by Russell.

At the mouth of each of the canons debouching upon the desert there is a débris cone or fan of remarkable proportions. These fans have been formed by the radial distribution of the débris over the valley bottom below the mouths of the gorges whence the streams issue. The material of which they are formed consists of gravel and bowlders, and has been spread out over many square miles of the valleys. They are among the most striking features of the desert. The long, even slopes, sometimes reaching six to eight miles into the desert, terminate abruptly against the rocky walls of the mountains. To one not familiar with the desert they do not seem of such immense size, but if a climb is attempted this illusion is soon dispelled, for the slope is long and rough, covered with bowlders and intersected with dry water courses. Lone Pine Creek, heading under the great precipices of Mount Whitney, has many of these huge bowlders strewn along its course. By some their position might be attributed to glacial action, but in reality this is not the cause. Six miles below the mouth of the canon there are some ten to twelve feet in diameter, while at various points one to three miles below occur others reaching a diameter of twenty to twenty-five feet. The swiftly flowing streams, at times augmented to torrents by sudden cloud-bursts upon the mountains, are enabled to do an almost incredible amount of work in transporting material.

At the close of the Miocene began the volcanic disturbances which modified so much of the old topography of the northern portion of the Sierra Nevadas. These flows were chiefly andesite with some rhyolite, and issued from fissures along the lines of faulting. The solid flows, breccias, conglomerates, and ash built up great ridges and mountains at many points. Between Lake Tahoe and Mono Lake much of the older surface was buried. From the latter lake southward for seventy-five miles there is a vast barren tableland, while between the North Fork of Owen's River and the San Joaquin the volcanic rocks reach up to and even form the crest of the Sierras for a number of miles. The volcanic eruptions continued through the Pliocene, and at its close occurred another