The United States has been subdivided into physiographic regions that,
although they have great diversity within themselves, are distinctly
different from each other (fig. 2).
From the Rocky Mountains on the west to the Appalachians on the east,
the interior of our country is a vast lowland (see cover) known as the
Interior Plains. These plains are bounded on the south by a region of
Interior Highlands, consisting of the Ozark Plateaus and the Ouachita
province, and by the Coastal Plain. In the Great Lakes region, the
Interior Plains laps onto the most ancient part of the continent, the
Superior Upland. West of the Great Lakes it extends far to the north
into Canada. Certainly the Rocky Mountains are distinctly different from
the region to the east, which is the Great Plains. The Great Plains,
then, is the western part of the great Interior Plains. The Rocky
Mountains form its western margin. But what determines its eastern
margin?
During the Pleistocene Epoch or Great Ice Age, huge glaciers formed in
Canada and advanced southward into the great, central, low-lying
Interior Plains of the United States. (See figure 2.) These glaciers and
their deposits modified the surface of the land they covered, mostly
between the Missouri and the Ohio Rivers; they smoothed the contours and
gave the land a more subdued aspect than it had before they came. This
glacially smoothed and modified land is called the Central Lowland.
Although the ice sheets lapped onto the northern part, the Great Plains
is the largely unglaciated region that extends from the Gulf Coastal
Plain in Texas northward into Canada between the Central Lowland and the
foot of the Rocky Mountains. Its eastern margin in Texas and Oklahoma is
marked by a prominent escarpment, the Caprock escarpment. Its southern
margin, where it abuts the Coastal Plain in Texas, is at another abrupt
rise or scarp along the Balcones fault zone.
[Illustration: _Figure 2.—Physical divisions of the United States and
maximum extent of the continental ice sheets during the Great Ice Age._]