DIVERSITY OF THE GLACIAL PERIOD 



767 



rate of about lOO feet per mile. These records are not numerous 

 enough to do more than indicate roughly the general position of 

 the rock surface (Fig. 4), and the extreme irregularity of the 



P'lG. 4. — Sketch map showing the approximate position of the surface of the "c re- 

 Pleistocene formations on Long Island and vicinity. 



rock surface in New York city, where enough records have been 

 collected by Mr. W. H. Hobbs to make its detailed mapping 



