LAURENTIAN (LABRADORIAN) ICE-BODY 137 



ice-sheet must incline to right angle or normal to the ice-front, the direc- 

 tion of latest movement in any district can be approximately known if 

 the ice limit is determined. The series of maps accompanying this writ- 

 ing show several stages in the waning and disappearance of the ice and 

 suggest the direction of flow at different positions. 



RECESSION 



The set of maps 2 prepared for illustration of this address give fifteen 

 positions of the ice-front during its recession across the State. A larger 

 number could be depicted, but only those have been selected which have 

 some significance in the lake and drainage history. The criteria used in 

 locating the ice-front positions are the moraines and the ice-border river 

 channels, the latter correlating with lake levels and shore features. 



The recession of the ice-front was certainly not steady or continuous, 

 but must have had considerable oscillation, readvances, and re-retreats. 

 The heavier belts of moraine and the lines of long-lived ice-border drain- 

 age probably represent readvanced positions. 



The length of time represented by the passage of the ice-front across 

 New York is unknown, but is certainly scores of thousands of years. 

 Probably 100,000 years is not too long. We may not judge the rate of 

 waning by the present behavior of the ice-fronts in Greenland and Ant- 

 arctica, as the climatic factors due to difference of latitude must have 

 been effective. If the oscillations of the ice-front were due to any ir- 

 regular or nonperiodic variations of climate, then we can have no idea 

 of the time involved either in the advance or the waning of the ice-sheet ; 

 and the only periodicity in climatic factors now recognized that seems 

 adequate is the precession of the equinoxes, having a variable but average 

 period of about 21,000 years. Taylor has studied the Cincinnati-Mack- 

 inac moraine series from this viewpoint, and concludes thai the fifteen 

 rather equally spaced moraines represent 75,000 to 150,000 years, using 

 the minimum length of the precession period. 3 The eight or ten mo- 

 rainic belts which we now recognize in western New York may correlate 

 with that many on the Cincinnati-Mackinac meridian, but the recession 

 of the ice-front on the Hudson-Champlain meridian probably represents 

 a much longer time than the Ohio-Michigan series. 



If the changes in geologic climates be due to variation in solar radia- 

 tion it is conceivable that some minor secularity might be responsible for 

 the oscillations of the ice-front. Variation in the amount o\' carbon 



3 The maps are omitted in (liis publication. 

 s Journal of Geology, vol. 5, 1807, pp. 1-1 165. 



