MORAINES OF RECESSION 405 
ditions have their crests forward; they have relatively short, 
steep front slopes and long gentle back slopes. 
4. In attempting to deduce a period of oscillation from the 
study of existing things, the present ice-cap of Greenland was 
taken as the closest analogue for interpreting the drift of the 
Laurentide ice-sheet, and the chief reliance is placed in the 
observations of Professors Chamberlin and Salisbury. From 
these the conclusion is reached that the frontal edges of the 
great ice lobes that made the moraines of recession moved only 
very slowly, so slowly as to be in substantial accord with a 
period of oscillation equal to the minimum value of the preces- 
sional period. A period of between 5000 and 10,000 years, 
however, would seem to accord most closely with the phe- 
nomena. From Fort Wayne to Port Huron there are five 
moraines with four intervals of about fifty miles each. On this 
basis the Fort Wayne moraine with a width of five miles would 
have required something between 700 and 1600 years for its 
making. 
5. The regularity of the oscillation and also of the greater 
climatic change upon which it was superposed both point clearly 
to astronomical causes. The periodic oscillation may have been 
due to precession, but no opinion is ventured as to the cause of 
the greater change. On this basis then, it took the ice-front 
75,000 to 150,000 years to retreat from Cincinnati to Mackinac, 
and the whole glacial epoch lasted at least 150,000 years and 
possibly 300,000 or more. And if glacial epochs are periodic, 
as they may possibly be, then this period is only a fraction of 
the great cycle of climatic change. 
FRANK BurRSLEY TAYLOR. 
