The Rocky Mountain region is one of the most topographically distinct and impressive parts of North America. The Rocky Mountains rise abruptly above the bordering regions, particularly on the east and northeast where they are flanked by plains, less so on the west and southwest where they are bounded by high plateaus. The Rocky Mountains comprise more than 100 individually named ranges that form a belt extending for slightly more than 5,000 km, from near Santa Fe, New Mexico, on the south to the Bering Sea on the north (Fig. 1). The belt varies in width from less than 100 km in the Canadian Rockies to nearly 600 km in the Middle Rockies of Wyoming and northeast Utah. The summits of the ranges rise 1,500 to 2,100 m above adjacent lowlands, to heights 1,800 to 4,400 m above sea level. The Southern Rockies of Colorado have the greatest amount of area, between 3,300 and 4,400 m, and the highest peak, Mount Elbert (4,400 m). The largest area of low mountains is in the Northern Rockies of Idaho and Montana, where summits are commonly only 2,100 to 2,400 m above sea level.

A substantial part of the Rocky Mountain region consists of lowlands, in the form of basins and fault-bounded troughs and trenches that lie between ranges. The Rocky Mountain Trench is perhaps the most spectacular fault-bounded lowland, even if it is not the most representative. It extends north from Flathead Lake, Montana, more than 1,500 km, and forms