658 HISTORICAL GEOLOGY 



The question of the origin of this widespread deposit has given rise 

 to much discussion, but to a large extent the aqueous theory, i.e., that 

 loess is a deposit that was laid down in standing water, has been re- 

 placed by the eolian. According to the latter, glacial streams, heavily 

 loaded with rock flour, spread silt upon their flood plains, exposing 

 it to the action of the winds, which caught it up and redeposited it 

 on the adjacent uplands, where after its deposition it was held by the 

 vegetation. A similar deposit is forming to-day on the western plains, 

 where loess-like dust is held by the grasses and is slowly building up 

 portions of the surface. If the above explanation is correct, the 

 presence of such extensive areas of loess indicates aridity during some 

 of the glacial or interglacial stages, since if the climate was moist, 

 the action of the winds would be inconsiderable. 



REFERENCES ON LOESS 



Chamberlin and Salisbury, — Geology, Vol. 3 ; (for references to literature). 

 Wright, G. F., — The Ice Age in North America, pp. 359—371. 



Duration 



The difficulty of arriving at a definite conclusion as to the length 

 of the Glacial Period, as expressed in years, is seen when the ele- 

 ments upon which such estimates must be based are analyzed. 

 These are (1) the weathering and erosion of drift; (2) the time 

 necessary for the climatic changes between the glacial and interglacial 

 stages; (3) the amount of vegetable growth in interglacial stages; 

 (4) the time necessary for the immigration of plants and animals. 

 These all show that a long period of time must have elapsed, but 

 afford little basis for an estimate in terms of years. (5) The time 

 required for the advance and retreat of the ice sheets, however, 

 affords something of a clue, since the rate of the advance and retreat 

 of existing glaciers is known ; but such estimates, at best, are subject 

 to wide variations, depending upon the rate used as a basis. This is 

 well shown in the figures given by different investigators, which vary 

 from 100,000 years (Upham), to 500,000 to 1,000,000 years (Penck). 

 The former estimate, however, is evidently much too small. 



The time which has elapsed since the beginning of the retreat of 

 tin last ice sheet is better known, because of other lines of evidence. 

 These are (1) the time required for the retreat of the ice, and (2) the 

 time necessary for the excavation of the Niagara (Fig. 573) and St. 

 Anthony gorges, the present rate of the recession of these falls being 



