6i8 



THE PLEISTOCENE PERIOD 



depositing agent except glaciers habitually marks the stones which 

 it deposits in this way. 



3. Structure. The larger part of the drift is unstratified, but 

 a considerable part is stratified, some of it irregularly. The un- 

 stratified drift (Fig. 509) or till (for some of it the name bowlder-day 

 is appropriate) has no orderly arrangement of its parts. The 

 structure of the stratified drift (Fig. 511) shows that it was deposited 



Fig. 508. 

 Geol. Surv.) 



Stones of the drift, striated and beveled by glacial wear. (U. S. 



by water, which doubtless sprang, in large part, from the melting of 

 the ice. Either of the two great types of drift, the stratified and the 

 unstratified, may overlie the other, or the two may be interbedded. 

 The association of the two is such as to demonstrate their essential 

 contemporaneity of origin. No agents but glacial ice and glacio- 

 fluvial waters could have brought about such relations between the 

 stratified and unstratified drift over such extensive areas. 



4. Distribution. The distribution of the drift is essentially the 

 same as that of the ice-sheets and glacial waters; but apart from 

 this general fact, several special features may be noted, (a) Within 

 the area of its occurrence, the drift is measurably independent of 

 topography. That is, its vertical range is as great as the relief of 

 the surface itself. Within the state of New York, for example, it 



