During the Mesozoic era and the succeeding Tertiary period, the additions to the land areas of the east were confined to the Atlantic border and the Gulf region. From the enlarged lands of the east, following the Appalachian uplift, ma terials were available for further extension. In the west, the growth was interior as well as on the border, and these later times are marked by the filling in of partially inclosed seas. and by retreat of waters, due to massive continental uplift. Thus gradually the western interior sea of the Great Plains region disappeared (as did that of the Colorado basin), and the Pacific shore line Iva: pushed to its present position.
Some of the important episodes of Mesozoic and Tertiary continental evolution may now be noted. The eastern border region has a series of areas of red and brown shale and sandstone, of Triassic age. These formations underlie the lowlands of the Connecticut Valley in Massachu setts and Connecticut. Another belt extends from the Palisades of the Hudson into Virginia, and there are other and smaller areas. Asso elated with these rocks arc the lava sheets which form the Orange Mountains of New Jersey, the Palisades of the Hudson, and the Mount Holyoke range of Massachusetts. These rocks are known as the Newark foxmation and were not accumu lated in the open sea, but in brackish or fresh waters, in basins whose character is little known. The Cretaceous and Tertiary beds of the Atlantic border form southern New Jersey, and the outside lowlands of all the States southward to Florida, and make the coastal plain of this region. They slant gently down and become continuous with the beds that lie below the marginal waters of the Atlantic. Owing to their comparative re cency, they are often partially or wholly uncon solidated and occur as sands, gravels, clays, and marls. But they may consist also of well-in durated sandstones and limestones.
Similar statements may be made concerning the formations and the lands that border the Gulf. Here belongs the entire State of Florida, which is already given. It is low not because of denudation, but7beeause of gentle and limited uplift of the undisturbed and youthful strata which lie beneath its surface. As has been in timated, the ancient Mississippi discharged, not far from the present mouth of the Ohio, into a gulf that thus lay between south-reaching lands on the east and west. Its successive burdens of land waste served gradually to fill the embay ment, and its delta reached more and more to the south, encroaching, as it is still, upon the Gulf.
Mesozoic deposits of Triassic. Jurassic. and Cretaceous age are found along the eastern base of the Rocky Mountains, where they are up turned at various angles, as in the vertical or highly inclined strata of the Garden of the Gods.
Eastward, at a little remove from the mountains, these beds, which have yielded perhaps the most remarkable series of fossil vertebrate remains that have been obtained in any country, become hori zontal and are often covered with still younger stratified formations, which all together make the underlying masses of the Great Plains. The breaking and upturning of the strata seen in the Rocky Mountain foothills points to the main up lift of these mountains which took place at the close of the Mesozoic era.
After the Rocky Mountain revolution, the Great Plains area ceased to be a region of salt water deposition and was characterized by swamps, and great lakes of or fresh water. The Laramie formation belongs to this era of low-lying lands in that region when the sea was excluded, and some of the largest coal deposits of the West were accumulated in the marshes of the time.
Over many thousands of square miles in Colo rado, Kansas, Wyoming, and Nebraska are sheets of incoherent or partly consolidated gravels, sands, and clays, which have usually been at tributed to sedimentation in such lakes. It is probable, however, that in part, at least, these beds are due to torrents carrying down enor mous volumes of waste from the mountains and distributing it in their wanderings over the plains. Present conditions were not approached in this region until the late Tertiary. By that time the entire belt, including the plains and Rocky Mountains, had received a massive uplift, by which the lakes were drained, and the plains given an eastward slant, from altitudes of 5000 to 6000 feet at the base of the mountains, to the low prairies west of the Mississippi River.
The rocks of the Colorado plateaus consist of some thousands of feet of sedimentary beds of Mesozoic and younger rocks, overlying a Paleo zoic and Archican foundation. The Colorado River furnishes a great natural section, since for some distance it has sunk its channel through the Paleozoic strata and cut far down into the basal granites. Like the Great Plains, it was long a region of marine deposition, followed by lakes and streams as the lands emerged. Ilere, too, great and widespread uplifts took place, in which the strains were so great as to produce profound fractures and dislocations or faults, and attended at times by large outflows of lava. These upflows sometimes stopped below the sur face, and domed up the overlying strata, making a kind of mountain known as laccolithie, of which the Henry Mountains, in Utah, are the type. Largely by such faults, running in a north and south direction, the ancient strata of the Great Basin have been cut into large blocks, and so tilted that the higher edges of the blocks make the parallel ranges of Utah and Nevada.