The close of the Triassic period was marked by enough uplift to leave the whole eastern two-thirds of the continent dry land undergoing erosion. The Triassic deposits of the Atlantic Coast are much broken up into large fault blocks, and this faulting probably took place as a result of the crustal disturbances toward the end of the period. In the west the Triassic conditions seem to have continued without much change into the next (or Jurassic) period. During the Jurassic period the relations of land and water in North America were very simple. In the earlier Jurassic all was dry land except portions of the western fringe of the continent from southern California to Alaska, where marine strata 2,000 to 10,000 feet thick accumulated. Late in the period the conditions were the same, except for a long, narrow arm of the sea or mediterranean which extended from the Arctic Ocean southward across the site of the Rocky Mountains to Arizona. There is no evidence for the existence of anything like real mountains anywhere on the continent during the period. [Illustration: Fig. 42.--Structure section showing profile and underground relations of rocks across the Connecticut Valley (through Mount Tom) of Massachusetts. Js and Jl are sandstone strata, with included lava sheets (in black) resting upon Paleozoic rocks on either side. The rocks have been notably tilted and faulted. (After Emerson, U. S. Geological Survey.)] Profound crustal disturbances marked the close of the Jurassic period in the western part of the continent. Strata which had accumulated to great thickness during millions of years of time, mainly over the sites of the Sierra Nevada and Cascade Mountains, finally yielded to a tremendous force of lateral compression, especially in the Sierra region, and were folded, crumpled, and upraised. Thus the Sierra-Cascade district was originally built up into a high mountain range. Since that time the Sierras have been much cut down by erosion and they have been rejuvenated by faulting and tilting of the great earth block. The Cascade Range from northern California into British Columbia was apparently not so profoundly raised, and its present height is mainly due to subsequent volcanic activity. The rocks of the Klamath Mountains of northwestern California, and of the Humboldt Range of Nevada, were also folded at that time. During the mountain-making disturbances on the western side of the continent great quantities of molten granite were forced up into the lower portions of the folding strata. Because of profound subsequent erosion this granite is now widely exposed as, for example, in the great walls of the Yosemite Valley.