Physical geography.  The Cambrian system of North America comprises three distinct series, the lower Cambrian, the middle Cambrian, and the upper Cambrian, each of which is characterized by its own peculiar fauna.  In sketching the outlines of the continent as it was at the beginning of the Paleozoic, it must be remembered that wherever the Lower Cambrian formations now are found was certainly then sea bottom, and wherever the Lower Cambrian are wanting, and the next formations rest directly on pre-Cambrian rocks, was probably then land.

Early Cambrian geography.  In this way we know that at the opening of the Cambrian two long, narrow mediterranean seas stretched from north to south across the continent.  The eastern sea extended from the Gulf of St. Lawrence down the Champlain-Hudson valley and thence along the western base of the Blue Ridge south at least to Alabama.  The western sea stretched from the Canadian Rockies over the Great Basin and at least as far south as the Grand Canyon of the Colorado in Arizona.

Between these mediterraneans lay a great central land which included the pre-Cambrian U-shaped area of the Laurentian peneplain, and probably extended southward to the latitude of New Orleans.  To the east lay a land which we may designate as Appalachia, whose western shore line was drawn along the site of the present Blue Ridge, but whose other limits are quite unknown.  The land of Appalachia must have been large, for it furnished a great amount of waste during the entire Paleozoic era, and its eastern coast may possibly have lain even beyond the edge of the present continental shelf.  On the western side of the continent a narrow land occupied the site of the Sierra Nevada Mountains.

Thus, even at the beginning of the Paleozoic, the continental plateau of North America had already been left by crustal movements in relief above the abysses of the great oceans on either side.  The mediterraneans which lay upon it were shallow, as their sediments prove.  They were epicontinental seas; that is, they rested upon (Greek, EPI) the submerged portion of the continental plateau.  We have no proof that the deep ocean ever occupied any part of where North America now is.

The Middle and Upper Cambrian strata are found together with the Lower Cambrian over the area of both the eastern and the western mediterraneans, so that here the sea continued during the entire period.  The sediments throughout are those of shoal water.  Coarse cross-bedded sandstones record the action of strong shifting currents which spread coarse waste near shore and winnowed it of finer stuff.  Frequent ripple marks on the bedding planes of the strata prove that the loose sands of the sea floor were near enough to the surface to be agitated by waves and tidal currents.  Sun cracks show that often the outgoing tide exposed large muddy flats to the drying action of the sun.  The fossils, also, of the strata are of kinds related to those which now live in shallow waters near the shore.