430 ' FRANK BURSLEY TAYLOR 
forward climax, where it would halt and build a terminal moraine. 
Then as climate ameliorated the ice-front would retreat in the 
same fashion and at its warm or backward climax it would again 
halt and build a moraine. The moraines of the cold climaxes 
would always be built after an advance movement, and would 
therefore be left standing. But the moraines of the warm cli- 
maxes would always be built after retreats and just before 
advances, and would therefore be overridden and destroyed. 
From these considerations it is plain that the time during which 
the ice-front would stand at or near its extreme forward position 
while building its terminal moraine at the cold climax would be 
only a fraction of the whole precessional period. The precise 
value of this fraction depends upon three factors: (1) on the 
period or duration of the precessional oscillation of climate; (2) 
on the amplitude of the oscillation of the ice-front, and (3) on 
the width of the drift belt which takes the form of a terminal 
moraine at the cold climax. The character of the moraine built 
would vary considerably according to the manner of combina- 
tion of long or short periods with small or great amplitudes. 
Other factors, such as the quality and quantity of the drift, the 
land relief, the situation with reference to the margin of the 
lobe (frontal or interlobate), latitude and local climatic influ- 
ences modify the character of the moraines more or less, but 
need not be discussed further here. 
By way of illustration let us consider a hypothetical case 
Suppose the amplitude of oscillation to be thirty miles, which is 
probably not far wrong for certain localities, and the period to 
be 10,000 years, which is in round numbers the supposed mini- 
mum value of the precessional period. The moraines are from 
two to ten miles wide, the average being not far from five. 
Their width varies considerably in different regions and different 
situations, but the figures given are approximately true for north- 
western Ohio, northeastern Indiana, and southeastern Michigan 
Figure I represents a simple harmonic motion in which 
ABCD is the circle of reference and represents the circle which 
the pole of the earth describes on the celestial sphere in 10,000 
