The third great system of the Mesozoic includes many formations, marine and continental, which record a long and complicated history marked by great oscillations of the crust and wide changes in the outlines of sea and land.

Early Cretaceous.  In eastern North America the lowest Cretaceous series comprises fresh-water formations which are traced from Nantucket across Martha’s Vineyard and Long Island, and through New Jersey southward into Georgia.  They rest unconformably on the Triassic sandstones and the older rocks of the region.  The Atlantic shore line was still farther out than now in the northern states.  Again, as during the Triassic, a warping of the crust formed a long trough parallel to the coast and to the Appalachian ridges, but cut off from the sea; and here the continental deposits of the early Cretaceous were laid.

Along the Gulf of Mexico the same series was deposited under like conditions over the area known as the Mississippi embayment, reaching from Georgia northwestward into Tennessee and thence across into Arkansas and southward into Texas.

In the Southwest the subsidence continued until the transgressing sea covered most of Mexico and Texas and extended a gulf northward into Kansas.  In its warm and quiet waters limestones accumulated to a depth of from one thousand to five thousand feet in Texas, and of more than ten thousand feet in Mexico.  Meanwhile the lowlands, where the Great Plains are now, received continental deposits; coal swamps stretched from western Montana into British Columbia.

The middle Cretaceous.  This was a land epoch.  The early Cretaceous sea retired from Texas and Mexico, for its sediments are overlain unconformably by formations of the Upper Cretaceous.  So long was the time gap between the two series that no species found in the one occurs in the other.

The upper Cretaceous.  There now began one of the most remarkable events in all geological history,—­the great Cretaceous subsidence.  Its earlier warpings were recorded in continental deposits,—­wide sheets of sandstone, shale, and some coal,—­which were spread from Texas to British Columbia.  These continental deposits are overlain by a succession of marine formations whose vast area is shown on the map, Figure 260.  We may infer that as the depression of the continent continued the sea came in far and wide over the coast lands and the plains worn low during the previous epochs.  Upper Cretaceous formations show that south of New England the waters of the Atlantic somewhat overlapped the crystalline rocks of the Piedmont Belt and spread their waste over the submerged coastal plain.  The Gulf of Mexico again covered the Mississippi embayment, reaching as far north as southern Illinois, and extended over Texas.

A mediterranean sea now stretched from the Gulf to the arctic regions and from central Iowa to the eastern shore of the Great Basin land at about the longitude of Salt Lake City, the Colorado Mountains rising from it in a chain of islands.  Along with minor oscillations there were laid in the interior sea various formations of sandstones, shales, and limestones, and from Kansas to South Dakota beds of white chalk show that the clear, warm waters swarmed at times with foraminiferal life whose disintegrating microscopic shells accumulated in this rare deposit.