WILLIS] GENERAL GEOLOGIC NOTES 21 



Students of Alpine glaciation recognize the Gunz, Mindel, Riss, and 

 Wurm stages, the Gunz being the earhest and the Wurm the latest 

 glaciation, which spread from the Alps upon the plains of southern 

 Germany. In the United States the deposits which were laid down 

 by successive ice sheets that flowed from the great center in Labrador 

 are known as the Wisconsin, Illinoian, Kansan, and Jerseyan. Those 

 which spread from the other center in Keewatin, west of Hudson Bay, 

 are similarly known as the Wisconsin, Illinoian (or lowan) , Kansan, 

 and Nebraskan (or Pre-Kansan) . 



The deposits which have received these names have been traced 

 over large areas in the respective regions in which they occur, and 

 have been identified in each separate field as constituting in each case 

 a sequence of formations due to recurrent glaciation, while between 

 the deposits which indicate the former presence of ice there are found 

 others whose character and included fossils demonstrate the existence 

 of an intervening epoch of milder climate. Thus it is seen that the 

 Quaternary period corresponds in duration with the development and 

 retreat of at least four continental ice sheets, and that its time scale 

 is marked off into eight epochs, namely, four which were characterized 

 by glaciation and four which were marked by milder climate. We 

 live in the latest of the milder epochs. 



Having in mind the alternation of glacial and interglacial climatic 

 epochs which have been distinguished in the Northern Hemisphere, it 

 is reasonable to inquire whether the Pampean yields any evidence of 

 similar climatic variations. Its general aspect is monotonous and 

 readily suggests an initial inference that the general conditions of 

 deposition were similarly uniform. But there are many local details 

 which demonstrate the alternate action of wind and water, and hence 

 in each such locality the alternation of climatic conditions favorable 

 to one or the other agency. In so far as we may be justified in corre- 

 lating the sequence of conditions in one locality with those in another, 

 we may establish a presumption of general climatic epochs and of 

 changes somewhat similar to those of the Northern Hemisphere. Let 

 us hasten to say that this statement is not meant to imply that the 

 Pampean formation contains a record of glacial and interglacial con- 

 ditions. The writer has not observed the slightest evidence of glacial 

 deposits in any part of the Pampean. Glacial deposits are entirely 

 wanting in the delta deposits of the Hwang River, which the Pampean 

 formation most closely resembles, and the origin of the Pampean 

 material is to be sought rather in the region of the deserts, as has 

 already been explained, than in one of the glacier-covered mountains. 

 The fact that the loess deposits of the Mississippi Valley and of central 

 Europe owe their origin to glaciers does not affect this statement, for 

 the loess deposits of China, which are far more extensive, are inde- 

 pendent of glacial origin. But the alternation of climate, of which 



