At the opening of the Cretaceous, the Atlantic coast of North America appears to have been farther to the eastward than it is at present; but just as had happened in the Triassic period, a long, narrow depression was formed, running roughly parallel with the coast, and in this depression for a long period of time, sediments in the form of gravels, sands, and clays were deposited. This is the Potomac series, which is divisible into several stages. There are unconformities within the series, which contains driftwood, some lignite, and iron ore. The beds are of continental origin and probably differed locally in their circumstances of deposition, flood-plain, delta, and marsh, being apparently all represented. The Potomac has been traced through the islands of Martha's Vineyard, Nantucket, Block Island, Long Island, Staten Island, across New Jersey, and thence southward to Georgia, where it turns northwestward, following the Mississippi embay-ment into Tennessee, and from there turning southwestward through Arkansas. In the northern part of this region, from Nantucket to the Delaware River, only the upper part of the Potomac has been found, and the same appears to be true of the Tuscaloosa, as the extension around the Mississippi embayment is called.

The Potomac is nowhere marine, and everywhere rests unconformably upon the underlying Triassic and older rocks. As the thickness of sediment is not great (not exceeding 600 feet), the process of deposition must have been very slow, or broken by long interruptions with intervals of erosion.

Fig. 298. - Map of North America in the Cretaceous period. Black areas = known exposures; white = land; dotted areas = continental formations; lined areas = sea. Vertical lines indicate Lower, and horizontal lines Upper Cretaceous seas.

Cretaceous Formations Of The United States

While along the Atlantic border the land was more extended than at present, in the southern part of the continent a different order of events was brought about. Nearly the whole of Mexico had been submerged by the great Upper Jurassic transgression, and in the Lower Cretaceous the sea extended over Texas and New Mexico into Arizona, and gradually expanded northward in the successive stages of the Comanche epoch, or Lower Cretaceous. At the base of the Lower Cretaceous strata in Texas is found a deposit of continental sands, the Trinity stage, which is the recognized equivalent of the basal Potomac. The advancing sea covered these sands, and the continued depression soon established a clear and quite deep sea, in which were formed the great masses of the Comanche limestones, that are the surface rocks of large areas in Mexico, and cover much of Texas. The Ouachita Mountains of Arkansas stood out as a promontory in the Lower Cretaceous sea, and the ancient shore-line has been traced around their foot. Over a great part of Texas the Comanche limestones are soft, and beds of chalk occur among them; while in Mexico, where they have been folded into mountain ranges, they have become much harder and more compact.

The thickness of the limestones increases southward; from 1000 feet in northern central Texas, it rises to 5000 feet on the Rio Grande, and on the Mexican plateau to an even greater amount. No less than six distinct, successive marine faunas are found in the Comanche limestones of Texas, and the faunal relationships of this region are closest with the Mediterranean province of Europe, and especially with the Lower Cretaceous of Portugal. The greatest expansion of the Comanche sea northward took place in the last of its stages, the Washita, when Oklahoma, southern Kansas, and eastern Colorado were covered by it.

Just how far north this sea reached has not yet been determined, but there is reason to think that it extended into central Wyoming. This Lower Cretaceous marine invasion of the northern interior lasted but a relatively short time, and, until quite recently, its thin deposits had escaped detection.

In the northern interior region the Lower Cretaceous beds, except those of the Washita, are of continental origin, and it is not practicable to correlate those of different areas where the strati-graphic connections cannot be traced. Part of the Morrison is probably Lower Cretaceous, though we cannot yet say how much of it, nor what particular areas. Another non-marine formation found east of the Gold Range of British Columbia, extending southward into Montana, is the Kootanie stage, the plant remains of which correlate it with the lower Potomac, and it certainly is not the oldest Cretaceous, for in British Columbia it has been found lying unconformably upon marine Lower Cretaceous. In part, the Kootanie was formed in tracts of low-lying, swampy lands, on which a luxuriant vegetation produced valuable deposits of coal. Lower Cretaceous beds have been found surrounding the Black Hills where they have been divided into the Lakota and Fuson stages, of continental origin, with abundant remains of land plants.

Along the Pacific coast Lower Cretaceous rocks are displayed on a great scale. The Great Basin land then extended from southern Nevada to 540 N. lat. in British Columbia, with the Sierra Nevada rising along part of its western border, to which the Pacific extended. North of the Gold Range in British Columbia, the ocean spread eastward, though no doubt broken by many islands, to the eastern base of the Rocky Mountains. The Coast Range of California formed a chain of islands and reefs. In the Sierra Nevada occurs an unconformity between the Lower Cretaceous and the uppermost Jurassic, but it does not imply the lapse of a very long period of time.

The older division of the Californian Lower Cretaceous is called the Knoocville, and has an estimated maximum thickness of 20,000 feet, laid down upon a slowly subsiding sea-bottom. This enormous thickness is no doubt due to an extremely rapid deposition of the debris abundantly supplied from the waste of the newly upheaved Sierras. At the end of the Knoxville age, the subsidence became more rapid and the sea began to encroach upon the land, for the Horsetown beds, which have a thickness of 6000 feet, overlap the Knoxville shoreward and extend over upon the underlying Jurassic and other pre-Cretaceous systems. Although the two stages of the Californian Lower Cretaceous are entirely conformable throughout, and appear to have been formed by a continuous process of sedimentation, yet there is a very marked faunal change between them. The Knoxville beds have a northern fauna, allied to that of Russia, showing that the connection with Russian seas, which had been established in late Jurassic times, was still kept up.