QUATERNARY 657 



glacial times. This is brought out by a study of the Great Basin 

 region of Utah and Nevada. Great Salt Lake is a small remnant 

 of a Pleistocene lake (Lake Bonneville), which was many times 

 larger (Fig. 572), discharging at one stage by a northern outlet to 

 the Pacific. This lake (Lake Bonneville) at its maximum was 

 1000 feet deep and covered an area of 17,000 square miles. Terraces 

 marking former levels of the lake are conspicuous features of the land- 

 scape, as seen from Salt Lake City and other portions of the basin. 

 A return to an arid climate caused the shrinkage of the lake to its 

 present area and maximum depth of only 50 feet. The salt lake 

 which doubtless formerly existed there in preglacial times was grad- 

 ually freshened as the lake level was raised, and became entirely fresh 

 when the excess water flowed through the outlet. When the climate 

 again became drier, the lake shrank, and all of the soluble salts of the 

 larger lake, as well as those brought into the basin since that time, 

 have accumulated to form the present exceedingly saline waters. 



Further west in the same basin were other lakes (Lake Lahontan) 

 which, however, were not as large as Lake Bonneville, although or 

 considerable extent. 



REFERENCES ON PLEISTOCENE LAKES 



Berlin-Greylock and Hoosick-Bennington Folios, U. S. Geol. Surv. 

 Gilbert, G. K., — Lake Bonneville: Mon. U. S. Geol. Surv., Vol. 1, 1890. 

 Textbooks of Geology, 

 Upham, W., — The Glacial Lake Agassiz: Mon. U. S. Geol. Surv., Vol. 25, 1896. 



Loess 



In the Mississippi basin, especially in Illinois, Iowa, Nebraska, 

 and states to the south, are extensive areas of a deposit (loess) in- 

 termediate between fine sand and clay (p. 52). The fact that it 

 contains angular, undecomposed particles of calcite, dolomite, feld- 

 spar, hornblende, mica, and magnetite indicates that loess was de- 

 rived from the finely ground rock flour of the glaciers. Pebbles, 

 with the exception of lime and iron concretions, are absent, except 

 at the base of the deposits. The most striking characteristic of loess 

 is its ability to form vertical cliffs, a feature which can best be seen 

 in railroad cuts and along stream courses. 



One peculiarity of its distribution is its independence of topography. 

 Its thickness seldom exceeds 50 feet, while 10 feet is more common. 

 Loess occurs on the drift, between drift sheets, and even beyond the 

 limit of the drift sheets. 



