648 THE PLEISTOCENE PERIOD 



ment among geologists as to the number of glacial epochs. 1 In the 

 Alps four are recognized. 2 These are designated 3 Giinz (pre- 

 Kansan?), Mindel (Kansan?), Riss (lowa-Illinoian), and Wiirm 

 (Wisconsin?). The glacial formations of other continents have 

 not been studied in detail in many places, but recent studies in 

 Turkestan indicate that there were several glacial epochs in the 

 Thian Shan Mountains. 4 



CAUSE OF GLACIAL CLIMATE 



Many hypotheses of the cause of the glacial period have been 

 offered, but none commands universal assent. Most of them appeal 

 to a combination of agencies, but each centers on some one factor 

 which gives character to the hypothesis. They fall mainly into 

 three classes: (i) those based on elevation of the land, the hypso- 

 metric hypotheses; (2) those based on phenomena and relations out- 

 side the earth itself, the astronomic hypotheses, and (3) those based 

 on changes in the constitution, movements, or cloud-content of the 

 air, the atmospheric hypotheses. 



Hypothesis of elevation. 5 Since the best-known glaciers are 

 in mountains, the suggestion was natural that elevation of the 

 glaciated regions was the cause of the great ice-sheets. The chief 

 evidence of the elevation postulated is the submerged valleys of the 

 sea-coasts, especially those of the northern latitudes. It has been 

 held by advocates of this hypothesis that 4,000 feet or more of eleva- 

 tion is indicated by the northern fiords, and that this elevation, to- 

 gether with accompanying geographic changes, was competent to 

 produce the Pleistocene glaciation. Those who question this view 

 doubt the fact of so great elevation, and doubt whether any eleva- 

 tion which there may have been was contemporaneous with the 

 ice-sheets. Further, they offer evidence that the land was lower 

 than now at certain important stages of the glacial period. The 

 elevation hypothesis also encounters grave difficulty in explaining 

 the repetition of glacial epochs and interglacial epochs, and in 

 accounting for the mild climates of interglacial times. In its simple 



1 Geikie, Jour. Geol., Vol. Ill, pp. 241-269. Keilhack, ibid., vol. Ill, 

 pp. 113-125. 



2 Penck, Die Alpen im Eiszeitalter. 



3 Penck, Science, Vol. XXIX, p. 359. 



4 Huntington, Explorations in Turkestan, Carnegie Institution. 



6 Dana, Manual of Geology, 4th ed., p. 970, and Upham, Am. Geol., Vol. VI, 

 p. 327, and Am. Jour. Sci., Vol. XII, p. 33. 



