286 PHYSICAL GEOLOGY 



rolling plains as well as level ones, and high plains (Fig. 301) 

 as well as low ones. Great plains are commonly terminated 

 upon one or more sides by an abrupt ascent to table-lands 

 or mountains. 



Origin and classes of plains. Various types of plains 

 have been discussed in previous pages. Rivers make flood 

 plains (p. 176), delta plains (p. 185), and peneplains (p. 147). 

 The ancient ice sheets covered the more or less rough pre- 

 glacial topography of extensive areas of northern United 



FIG. 301. A typical view on the high plains of western Kansas. (Gilbert, 

 U.S. Geol. Sun.) 



States with drift, forming drift plains (p. 451). The floors 

 of extinct lakes form many flat lake plains, especially in 

 glaciated regions (p. 453). Most lake plains are small. 



Great plains, like the Atlantic and Gulf coastal plains of 

 the United States and the vast interior plain which stretches 

 from the Appalachians to the Rockies (Figs. 43 and 44, pp. 

 62 and 63), cannot usually be put in any of the above 

 classes. Rather, they commonly contain many smaller plains 

 of several or all of the types enumerated. In general, exten- 

 sive coastal plains are former marginal sea bottoms, exposed 

 either by elevation of the land or by lowering of the surface 

 of the sea. Coastal plains may also be peneplains, or the 



