53 



The surface of the Groat Phiiiis is usually gently rolling", but in some 

 localities buttes,* headiands, and detached masses of rock vary its 

 otherwise monotonous aspect. The soil is generally alight mold, from 

 10 to 15 inches in depth, resting on a bed of sand, gravel, and bowlders. 

 In some places there is considerable clay mixed with the surface sod. 



In their natural condition the plains, mesas, and foot hills are gen- 

 erally covered with a short but succulent grass. Some portions pro- 

 duce sage brush, among which nutritious grasses are interspersed. 

 In other parts sage brush is the only growtli. A comparatively small 

 pro})ortion is wholly barren. 



Plateaus. — The great plateaus upou the western flank of the mountain 

 system have a mean elevation of about 7,000 feet above the level of the 

 sea. They extend from southern Wyoming through western Colorado 

 and eastern Utah into Arizona and l^ew Mexico, and are there lost in 

 low desert plains. 



By faults in the geologic structure, and b}" lines of clilis and deep 

 canyons, the various plateaus are separated one from another. Their 

 general level is broken b^^ mountains— single and in groups— buttes 

 and towering escarpments of rock. Some thirty mountain peaks, rang- 

 ing in altitude from 10,000 to 12,000 feet above the sea, have here been 

 noted. The few streams of the region find their way through canyons, 

 the walls of which are of immense height. Aridity is the prevailing 

 condition, and vegetation is dwarfed and scanty. 



Water-courses. — That the main crest of the Eocky Mountains consti- 

 tutes the ''Continental Divide'^ or line separating the eastern and west- 

 ern water systems is known to all; that there is also a great Transverse 

 Divide, forming northern and southern water systems, is not so gener- 

 ally understood. Commencing in the northwestern part of Nebraska, 

 the divide last named runs westward through central Wyoming to the 

 southern border of Yellowstone Park; thence southwestward to the 

 northwestern corner of IS'evada. 



Hence there are four great basins; one sloping to the northeast, 

 drained by the waters of the Upper Missouri ; one at the northwest, 

 drained by the Columbia Eiver; one at the southwest, which is double 

 and discharges its waters into the Great Salt Lake Basin and the Colo- 

 rado River; and one at the southeast, which is drained by the Arkan- 

 sas and Rio Grande Rivers and their affluents. These several systems 

 radiate from a common center, and have their sources in the high and 

 snowy mountains of Wyoming and Colorado. The fact, however, should 

 not be overlooked, that other and surrounding parts of the Rocky 

 Mountain region largely contribute to the volume of water in the prin- 

 cipal streams. 



In considering further the several systems here outlined, it appears 

 that at the northeast the South Platte rises in South Park and the 



* Butte (Fr., pronounced bute), aa isolated peak or elevation of land in the central 

 and western parts of North America (too high to be called a hill or ridge, and not 

 high enough to he called a mountain). — (Webster.) 



