THE PHYSICAL GEOGRAPHY
OF THE REGION
The Continent of North America can be divided into eight major physical regions, all of which are represented in the United States.1 These major divisions of America are the Laurentian Upland, the Coastal Plains, the Appalachian Highlands, the Interior Plains, the Interior Highlands, the Rocky Mountain system, the Intermontane Plateaux, and the Pacific Mountain system (see Fig. 7). In the area at present under consideration the first three of these regions are not represented (Fig. 7).
The Interior Plains consist of the interior of the continent and contain only small differences of local relief. The major unit can be divided into two parts, the Central Lowlands, sometimes known as the Prairies, and the Great Plains. Both regions are underlain by rocks in nearly horizontal beds. In the east the age is mainly Palaeozoic, but in the west large areas are covered by younger sediments. The northern part of these plains has been heavily glaciated while the southern part has not felt the influence of ice. The boundary between the Central Lowlands and the Great Plains is an escarpment facing east, and usually not more than a few hundred feet in height. The chief differences between the two regions are differences of rainfall and natural vegetation rather than differences of topography. The boundary between the two regions corresponds very closely to the 20-inch isohyet, which is almost the same as the hundredth____________________