Physiographic Geography 

 the original land surface — ^less whatever material 

 the winds may export altogether from the region. 



The Arid Plateau Province is naturally sub- 

 divided into three parts, the Columbia Plateau, the 

 Basin Plateau, and the Lower Colorado Plateau. 

 The central division which most travelers enter at 

 Great Salt Lake is typical in having no drainage to 

 the sea, but is not to be thought of as a single drain- 

 age basin, for in reality there are still hundreds of 

 separate depressions notwithstanding the tendency 

 to integration through filling. The Columbia Plateau 

 is least typical of the arid or desert province because 

 of its slightlv greater rainfall and its drainage to the 

 sea by the Columbia River, and also because its to- 

 pography is so largely controlled by the nearly hori- 

 zontal beds of the great Tertiary lava flow. The 

 southern third of the Arid Belt is also traversed by 

 a large river, the Colorado, but many depressions of 

 considerable size are undrained and the tempera- 

 ture and aridity make it in many ways more repre- 

 sentative of arid provinces than is the Columbia 

 Plateau. (See Pis. IV and V, and fig. 5.) 



The Great Basin region may possibly be traversed 

 at least one way by more visitors to the Panama- 

 Pacific Exposition than any other section of the 

 Arid Province, and it offers more examples of 

 typical topography already familiar in scientific and 

 popular literature. Great Salt Lake is but the last 

 remnant of its great predecessor, and from the car 

 windows may be seen many fragments of terraces 

 marking the shore-lines of successive levels of Lake 

 Bonneville, the extensive lake of glacial times (Gil- 

 bert, 1890). In northwestern Nevada similarly are 

 found fragments of the shore-lines of the contem- 

 poraneous Lake Lahontan (Russell, 1885). These 

 shore-lines are visible from either the Southern 

 Pacific or the Western Pacific railroad and both 

 lines pass through or near several of the character- 

 istic playa lakes, sometimes partially filled with thin 

 sheets of water, but nearly always showing por- 

 tions of the mud flats of their beds incrusted with 

 the salts deposited from the evaporating water. 



The traveler by the Sunset Route at the extreme 

 south will find much typical arid province scenery 

 through Arizona. Entering California at the head 

 of the old delta of the Colorado River which makes 

 the rich lands of Imperial County, he will have the 

 exDerience of descending below sea level in passing 

 Salton Sea. Throughout the arid region the peculiar 

 vegetation, sage-brush, ?:rease-wood, or cactus, con- 

 stitutes no small part of the characteristic scenery; 

 but this is described in a later chapter. 



35 



