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. Both north and south of the Columbian Plateau there was also much volcanic activity in the Rocky Mountain region during Tertiary time. A single formation in Colorado consists mostly of volcanic "ash" or dust over 2,000 feet thick. There was also much volcanic activity over the Colorado Plateau area of southern Utah, New Mexico, and Arizona. The volcanoes there exhibit all stages from those which are very recent and practically unaffected by erosion to others which have been completely cut away with the exception of the cores or "volcanic necks." During the second half of the Tertiary period the whole region known as the Great Basin, between the Sierra Nevada Mountains of California and the Wasatch Mountains of Utah, began to be affected by profound faulting or fracturing and tilting of portions of the earth's crust. The two largest faults, one on the western side of the Wasatch Range and the other on the eastern side of the Sierra Range, are each hundreds of miles long. Each of these ranges owes most of its present altitude to the uptilting of great fault blocks, and most of the many nearly north-south Basin Ranges of Nevada and Utah are in reality recently tilted fault blocks.