376 STUDIES FOR STUDENTS 



which can directly influence the rate of erosion unless the land uplift 

 is sufliiciently prolonged for the streams to steepen their grades from 

 the sea backward to the headwaters, while glacial material would be 

 absent until the increasing ice sheets actually invaded the headwaters 

 of the streams building the formations. In the present connection 

 it is desired to bring into more prominence the climatic factor, but 

 without presuming to decide upon the relative importance in this 

 case of the tectonic and climatic causes, as a final decision must be 

 based upon broad field-work with both hypotheses in mind. It may 

 be pointed out, however, that this quickening in erosion and transpor- 

 tation appears to have been very extensive over the United States 

 and has been ascribed to a time of cool moist climate by a number 

 of writers from several lines of evidence, such as the included bog 

 deposits;^ but there is also a suggestion by Chamberlin and Salisbury, 

 from the oxidized character of the formation, that there were effective 

 dry seasons.^ This might simply imply, however, a more diversified 

 climate while at the same tinie more rainy. A somewhat similar 

 paradox is pointed out by Chamberlin and Salisbury in regard to 

 the relationship of the loess to the lowan ice invasion and the absence 

 of gravel trains, "both phenomena, perhaps, implying aridity, strange 

 as that may seem in a glacial epoch. "■^ 



In view of the diversity of opinion upon the origin, age, and 

 extent of the Lafayette, an opinion in the present connection should 

 be expressed with caution. It would seem, however, that the great 

 development of sand and gravel on the margins of the Gulf and to a 

 lesser extent up the Mississippi River corresponds with such an ero- 

 sion of piedmont sands and gravels as is seen to have taken place 

 from the High Plains. In each case there is the added volume of 

 water required in the one region for erosion, in the other for the trans- 

 portation of such material. Both events are known to have occurred 

 near the close of the Tertiary or early opening of the Quaternary, 

 and by correlating the two the necessity is avoided, either of assigning 

 the flood waters to the melting and retreat of the ice sheet, or to a 

 greater southward slope to the Mississippi Valley, or to local tilting 

 of the High Plains in order to rejuvenate erosion. The climatic 



1 Hilgard, op. cit., p. 401. 



2 0/>. cj^., Vol. Ill, p. 304. 3 O/). «7., Vol. Ill, p. 412. 



