cooler. The slopes and summits were encased in thick ice and snow, which kept them captive. The glacial periods of the Pleistocene were relieved by intervals during which the ice fields retreated toward the crests, yielding to climates even milder than that of California today. But when the ice of the last glacial age had finally retreated (traces of this epoch still linger in various glaciers such as those on Shasta), the Sierra crest stood stripped of vegetation and soil, exposing those bare expanses of whitish granites and schists that now give it its dazzling beauty. Yosemite and other extraordinary Sierra valleys and canyons are also glacial legacies, as are the numerous lakes in the high Sierra. Tahoe, lovely lake and the deepest in the United States, was made partially by glaciation and partly by faulting, erosion, and volcanic damming.
The volcanic activity of Miocene times was especially great in the Cascade Range, where a number of volcanic peaks rose in a comparatively short time. Mount Shasta was one; the still active Mount Lassen was another, and the volcanic range extends north into Oregon and Washington. Eastward from the range extends one of the largest lava fields in the world, covering 200,000 square miles to depths of from 200 to 2,000 feet. This lava plateau, generally decomposed on the surface, which stretches beyond California into Oregon and across into Idaho and Wyoming, did not for the most part erupt through typical volcanic vents, but flooded up through great cracks or fissures. The Pit River, flowing through the Cascades, has cut deep into the series of volcanic rocks (andesites) some 7,500 feet in thickness, and the thin but widespread basalts. Because of the depth of this covering, the pre-Miocene history of the region is uncertain.
The oldest of the accessible formations of the Klamath Mountains are pre-Cambrian metamorphic rocks including schists, quartzites, and crystalline limestones the last named consisting partly of sedimentary, partly of igneous rocks, both metamorphosed. The first two periods of the Mesozoic are represented by smaller proportions of sedimentary rocks which are covered by remnants of once extensive beds of sandstones, shales, and conglomerates of the Cretaceous period. There were also periods when volcanoes were active, especially the early Devonian period and the greater part of the Mesozoic era. The mass had been uplifted during the Jurassic period, but erosion and subsidence brought the ancestral Klamath mountains to below sea level in the Cretaceous period. This oscillation continued more or less quietly, except for an outburst of great volcanic activity in the middle of the Miocene. The most recent re-elevation, like that of the Sierra, was at the beginning of the Quaternary period. At approximately the same time, gold-bearing gravels were carried down along the sides of many canyons by erosion.
