The Sierra Nevada Range, which originated by intense folding of rocks late in the Jurassic period, underwent profound erosion until about the middle of the Tertiary period, by which time it had been cut down to a range of hills or low mountains. Then the great fault (fracture) previously described began to develop along the eastern side. As a result of many sudden movements along this fault, which is hundreds of miles long, the vast earth block has been tilted westward with a very steep eastern face and a long, more gradual western slope, the crest of the fault block forming the summit of the range. The amount of nearly vertical displacement along this fault has been commonly from 10,000 to 20,000 feet, and, in spite of considerable erosion of the top of the fault block and accumulation of sediment at its eastern base, the modified fault face now usually stands out boldly from 2,000 to 10,000 feet high. As an evidence that this movement of faulting has not yet ceased we may cite the Inyo earthquake of 1872, when there was a sudden renewal of movement of ten to twenty-five feet along this fault for many miles. Since the great Sierra block began to tilt, the many mighty canyons, like Yosemite, Hetch-Hetchy, King’s River, and Feather River, have been carved out by the action of streams, in some cases aided by former glaciers. King’s River canyon has been sunk to a maximum depth of 6,900 feet in solid granite solely by the erosive action of the river!
The Cascade Mountains, too, were reduced to nearly a peneplain condition by late Tertiary time when they began to be rejuvenated by arching or bowing of the surface unaccompanied by great faulting or fracturing, and many canyons, like that of the Columbia River, have since been carved out.
Mention should now be made of the vigorous volcanic activity which took place in the Cascade and Sierra Nevada Ranges. Most of this activity occurred during Tertiary time (particularly in the latter part) and it has continued with diminishing force practically to the present time. In California streams of lava buried many gold-bearing river gravels which have yielded rich mines. Many well-known mountain peaks, such as Shasta, Lassen, Pitt, Hood, and Rainier, from northern California to Washington, are great volcanic cones which date from Tertiary time, and which are now mostly inactive. That this volcanic activity has not yet altogether ceased is shown by renewed eruptions of Mount Lassen (or Lassen Peak, altitude 10,437 feet) in northern California. Since the beginning of this renewed activity in 1914, several hundred outbursts have occurred. No molten rock has flowed out, but large quantities of rock fragments, dust and steam have been erupted, in many cases forming great clouds two or three miles high over the top of the mountain ([Plate 10]). At this writing (October, 1920), Mount Lassen is still showing vigorous activity. At Cinder Cone, only ten miles from Mount Lassen, there were two eruptions of cinders and a considerable outpouring of lava within the last 200 years. Still other very recent cinder cones occur in southeastern California and Arizona.
Fig. 46.—Sketch map showing the distribution of volcanic rocks of Cenozoic (mostly Tertiary) Age in western North America. Only one volcano (Mount Lassen, California) is now active in the United States proper, but a number are more or less active in Mexico and Central America. (Data from Willis, U. S. Geological Survey.)
One of the greatest lava fields in the world forms the Columbian Plateau between western Wyoming (including the Yellowstone National Park) and the Cascade Mountains from northeastern California to northern Washington. It covers fully 200,000 square miles and is really considerably larger than shown on the map because the lava in parts of the plateau region are covered by very recent sedimentary materials.
The great lava fields of the Deccan, India, and of the plateau region of western Mexico are comparable in size to the Columbian field and these lava fields are all of the same age. In the Columbian Plateau most of the lava was poured out during later Tertiary time. Sheets of molten rock, averaging fifty to one hundred feet in thickness, spread out over various parts of the region and piled up by overlapping layers one over another until the lava plateau more than a mile high was built up. Many hills and low mountains were completely buried under the molten floods, and in other places the liquid rock masses flowed against the higher mountains. “For thousands of square miles the surface is a lava plain which meets the boundary mountains as a lake or sea meets a rugged and deeply indented coast.... The plateau was long in building. Between the layers are found in places old soil beds and forest grounds and the sediments of lakes.... So ancient are the latest floods in the Columbia River Basin that they have weathered to a residual yellow clay from thirty to sixty feet in depth, and marvelously rich in the mineral substances on which plants feed. In the Snake River Valley the latest lavas are much younger (Quaternary). Their surfaces are so fresh and undecayed that here the effusive eruptions may have continued to within the period of human history.” (W. H. Norton.) Many of the lava layers are plainly visible where the Columbia River has cut its great gorge or canyon. The Snake River in places has sunk its channel several thousand feet into the lava plateau without reaching underlying rock.