The Interior Plains, of which the Great Plains is the western, mostly unglaciated part (fig. 2), is the least complicated part of our continent geologically except for the Coastal Plain. For most of the half billion years from 570 million (fig. 5) until about 70 million years ago, shallow seas lay across the interior of our continent (fig. 6). A thick sequence of layered sediments, mostly between 5,000 and 10,000 feet thick, but more in places, was deposited onto the subsiding floor of the interior ocean (table 1). These sediments, now consolidated into rock, rest on a floor of very old rocks that are much like the ancient rocks of the Superior Upland.

About 70 million years ago the seas were displaced from the continental interior by slow uplift of the continent, and the landscape that appeared was simply the extensive, nearly flat floor of the former sea.