MAN AND THE GLACIAL PERIOD. 



631 



Fig. 163. — Typical section of the Trenton gravel in which the implements described in 

 the text are found. Note the distinct stratification and the large angular bowlder 

 near the surface, showing the presence of floating ice, since by no other means 

 could such a bowlder get into such a position as here found. The elevation of the 

 surface is here 50 feet and is about half a mile back from the river edge of the bluff. 

 Perpendicular exposure is here between 30 and 40 feet. 



(Photograph by Abbott. ) 



es. There is little trace of true clay in the deposit ; there is 

 rarely enough to give the least trace of cementation to the 

 masses. The various elements are rather confusedly arranged ; 

 the large bowlders not being grouped on any particular level, 

 and their major axes not always distinctly coinciding with the 

 horizon. All the pebbles and bowlders, so far as observed, are 

 smooth and water-worn, a careful search having failed to show 

 evidence of distinct glacial scratching or polishing on their 

 surfaces. The type of pebble is the subovate or discoidal, 

 and though many depart from this form, yet nearly all ob- 

 served by me had been worn so as to show that their shape 

 had been determined by running water. The materials com- 

 prising the deposit are very varied, but all I observed could 

 apparently with reason be supposed to have come from the 



