198 



EARTH FEATURES AND THEIR MEANING 



in saline deposits, and the great burden of sediment accumulates 

 in thick stratified masses which in magnitude outstrip the largest 

 deltas in the ocean. 



The self-registering gauge of past climates. From the ini- 

 tiation of the desert in its isolation from the lands tributary to the 

 sea, its history becomes an individual and independent one. An 

 increasing quantity of rainfall will be marked by larger inflow to 

 the basin, and the lakes which form in its lowest depression will, 

 as a consequence, rise and expand over larger areas. A contrary 

 climatic change will bring about a lowering of the lakes and leave 

 behind the marks of former shorelines above the water level (Fig. 

 205). Deserts are thus in a sense self-registering climatic gauges 

 whose records go back far beyond the historic past. From them 



FIG. 205. Former shore lines on the mountain wall surrounding the desert of the 

 Great Basin. View from the temple in Salt Lake City (after Gilbert) . 



it is learned that there have been alternating periods of larger and 

 smaller precipitation, which are referred to as pluvial and inter- 

 pluvial periods. 



From such records it is learned that the Great Basin of the 

 western United States was at one time occupied by two great desert 

 lakes, the one in the eastern portion being known as Lake Bonne- 

 ville (Fig. 206). With the desiccation which followed upon the 

 series of pluvial periods, which in other latitudes resulted in great 

 continental glaciers and has become known as the Glacial Period, 

 this former desert lake dried up to the limits of Great Salt Lake and 

 a few smaller isolated basins. Between 1850 and 1869 the waters 

 of Great Salt Lake were rising, while from 1876 to 1890 their level 

 was falling, though subject to periodic fluctuations, and in recent 

 years the waters of the lake have risen so high as to pass all records 

 since the occupation of the country. As a consequence the so- 

 called Salt Lake " cut-off " of the Union Pacific Railway, con- 

 structed at great expense across a shallow portion of the lake, has 



