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.