In the Triassic, the earliest period of the Mesozoic, the eastern part of the United States was almost entirely land, with wind, water and other agents of erosion wearing down the eminent ridges and peaks formed by the Appalachian revolution at the close of the Permian, with deposition of sediments in intermon tane seas and marshes such as the trough in which the Connecticut valley now lies, and with considerable volcanic activity locally; the Rocky Mountain region was covered over in part with some marine sediments from seas intruding from the west, but in much greater part with freshwater mud and sand, or wind-blown sand and loess; and the Pacific coast belt was submerged beneath a sea in which normal marine deposits were laid down. The Triassic formations of the eastern part of the United States—in the Con necticut valley, across northern New Jersey, south-eastern Penn sylvania, Maryland, and into Virginia and North Carolina, are pre vailingly red in colour, coarse, and indicate by the character of their sediments and fossils that they were worn down from a high land of crystalline rocks, and deposited, in a semi-arid climate with hot summers and possibly cold winters, upon the surface of such troughs as that of the Connecticut valley. The Triassic for mations of the Rocky Mountain cordillera comprise a series of red or variegated sandy shales am: cross-bedded sandstones, with thick beds of gypsum. The Triassic formations of the Pacific border are dominantly calcareous and rich in corals and other marine fossils; submarine or littoral volcanoes poured out thick beds of igneous material, particularly from the middle Jurassic to its close. The Jurassic closed with a significant uplift of great block mountains in the eastern part of the country, known as the Palisade disturbance.
The life of the Triassic, strikingly new and different from the Permian, has not been adequately recorded in the rocks and only fragmentary knowledge has been discovered. The floras were small; the insect life was negligible; fish were rather abundant in the freshwater lakes and lagoons and in the salt seas ; land verte brates were varied, particularly such reptiles as the great dino saurs; primitive reptilian mammals had evolved; and the seas swarmed with ammonids, squids and siphonate gastropods, and reef-building corals formed reefs hundreds of feet thick. Coal beds of limited extent were laid down in the Triassic depressions in Virginia and North Carolina, and at the close of the Triassic some copper ore was deposited in New Jersey with the copious basaltic intrusions.
The Jurassic in the eastern United States opened with a period of active erosion as a result of the great uplift of the Palisade disturbance, which continued throughout the period and reduced the high Appalachian and massive Palisade mountains to a pene plain. Along the Pacific coast the sea, after its withdrawal from the continent at the close of Triassic time, again began in the Jurassic to transgress upon the coast and into the Californian sea of Oregon, California and Nevada, where great thicknesses of sandstones and shales and some limestone were deposited. From Alaska and British Columbia a middle Jurassic sea, Logan sea, invaded Montana, Idaho, Wyoming, Colorado and Utah, where widely and locally variable deposits of sandy clays, shaly marls, impure limestones, crossbedded sandstones and universally pres ent oyster shells indicate only shallow water conditions. Great tectonic activity characterized the close of the Jurassic in the western part of North America; the Sierra Nevadas, the Coast range of California, the Humboldt range of Nevada, the Cas cades and the Klamath mountains were elevated. A great geo syncline, called the Coloradoan, east of the Sierra Nevadan up lift, and the Great valley of California were formed near the close of the Jurassic. Considerable volcanic activity continued through out the period, increasing in intensity and extent toward its close.