AS4 FRANK BURSLEY TAYLOR 
the broad Huron and later Erie-Huron ice-lobes advanced are so 
wide and smooth that the circumstances favoring simplicity were 
at a maximum. 
Suppose it took the ice-front thirty-five years to retreat from 
the Fort Wayne to the Defiance moraine, receding at a uniform 
rate. That would be a little over twenty and a half feet a day. 
But this allows no time for retreat beyond Defiance, nor for the 
halt at the warm climax of retreat and the building of a moraine 
there of equal magnitude with that now seen at Defiance, nor 
for the readvance to Defiance, nor finally for the building of 
the Defiance moraine. If the Defiance moraine was built after 
a readvance and if the nature of the oscillation was such as 
shown in Fig. 1, then the time taken for the retreat from Fort 
Wayne to Defiance must have been only a fraction of the whole 
period of oscillation. We may measure the period of oscilla- 
tion between the crests of the moraines, including always the 
destroyed moraine of the warm climax. We may assume further 
that the readvance is at least half of the previous retreat, and in 
this case the moraine of the warm climax next after the Fort 
Wayne moraine would be built at some point near Toledo and 
after it was finished the ice-front would readvance to Defiance. 
Then the first half of the period of oscillation would be measured 
between the crests of the Fort Wayne and Toledo moraines, and 
the second half between the crests of the Toledo and Defiance 
moraines. The two middle points of oscillation would then be 
half way between Fort Wayne and Toledo and half way between 
Toledo and Defiance. The first of these would be about at Defi- 
ance. On this basis the retreat from the crest of the Fort Wayne 
moraine to the middle point at Defiance would take one-fourth 
of the time of the whole oscillation and this includes half of the 
time of the building of the Fort Wayne moraine. On the basis 
of thirty-five years for the whole, this one-fourth part would 
occupy only about eight and three-fourths years, and surely half 
of this time would have to be taken to build half of the Fort 
Wayne moraine, and that would leave only four and three-eighths 
years for the ice-front to retreat fifty miles. The rate of retreat 
