Fig. 41.—Block diagram of the region westward from New York City and vicinity, showing the main relief features, the underground relations of rocks of widely different ages, and the relation of the relief to the rock formations. (Part of larger drawing by A. K. Lobeck.)
During the time of the accumulation of the late Triassic strata in the down-warp basins there was considerable igneous activity, as proved by the occurrence of sheets of igneous rock within the body of strata. In some cases true lava flows with cindery tops were forced out on the surface and then buried under later sediments, while in other cases the sheets of molten rock were forced up either between the strata or obliquely through them, thus proving their intrusive character. As a result of subsequent erosion, these very resistant lava masses often stand out conspicuously as relief features. Perhaps the most noteworthy example is the great layer of such intrusive igneous rock, part of which outcrops for seventy miles mostly as a bold cliff forming the famous Palisades of the Hudson, near New York City. During the process of cooling and solidification of the molten mass there was contraction which expressed itself by breaking the rock mass into great, crude, nearly vertical columns, and hence the origin of the name “Palisades.” The cliff character of the outcrop is due to the fact that the lava is much more resistant to erosion than the sandstone above and below it. In the Connecticut Valley of Massachusetts a layer of lava several hundred feet thick boldly outcrops, forming the crest of the well-known Mount Tom-Mount Holyoke Range.
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.
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.