DISCRIMINATION OF AGE OF GLACIAL DRIFT SHEETS 709 
unquestionably a single drift sheet. As has been shown in this dis- 
cussion, careful scrutiny of the topography, the drainage, and other 
relations which may affect the apparent amount of surficial alteration 
by weathering and erosion in the area under investigation may yield 
an adequate explanation of the differing phenomena without the 
postulation of distinct stages of glaciation. 
