SIGNIFICANCE OF ESKER TERRACES 



287 



These facts become important in modifying the calculations which 

 have been made concerning the age of the terraces in the valley of the 

 Somme, and in those of other streams in northern France and southern 

 England where paleolithic implements have been found. It is by no 

 means necessary to suppose, as has been generally done, that the valley 

 of the Somme, for instance, was first filled with gravel to the height of 

 the 90-foot terrace and then the material eroded by the present small 

 stream from the area occupied by the present broad floodplain. In dis- 

 cussions of the problem in the valley of the Somme, both Dr. Warren 

 Upham^ and Prof. E. B. Tylor^ have maintained that these terraces were 

 merely marginal accumulations of gravel, but neither of them has taken 



X 



Figure 5. — Section X-Y of Figure S 



Showing the depth of gravel which had accumulated In the original valley, whose bottom 



is indicated by the lower line 



advantage of the probable existence of stagnant ice in the valleys acting 

 as a temporary barrier to determine the course of the currents which 

 deposited the gravel. 



Other more general problems present themselves in the broader areas 

 of the glaciated region nearer the center of the Mississippi Valley. Over 

 much of this area, as the glacial boundary is approached, the supply of 

 drift was diminished, so that it was less and less able to fill up aud there- 

 fore to disguise the inequalities caused by preglacial erosion. As good 



* American Geologist, vol. xxii, pp. 350-362. 



' Proceedings of the Geological Society, London, vol. xxlv, pp. 103-126 ; vol. xxv. pp. 

 57-100. 



