ICE-PERIOD IN AMERICA. 
99 
held its ground long enough to allow, time for 
the collection of these debris. In the same 
way we may trace the southern border of our 
ancient ice-sheet on this continent by the limit 
of the boulders ; beyond that line it evidently 
did not advance as a solid mass, since it ceased 
to transport the heavier materials. But as 
soon as the outskirts of the ice began to yield 
and to flow off as water, the lighter portions 
of the drift were swept onward; and hence we 
find a sheet of finer drift-deposit, sand and 
gravel more or less distinctly stratified, carried 
to greater or less distances, and fading into 
the Southern States, where it mingles with the 
most recent river-deposits. 
One naturally asks, What was the use of 
this great engine set at work ages ago to 
grind, furrow, and knead over, as it were, the 
surface of the earth ? We have our answer 
in the fertile soil which spreads over the tem¬ 
perate regions of the globe. The glacier was 
God’s great plough; and when the ice van¬ 
ished from the face of the land, it left it pre¬ 
pared for the hand of the husbandman. The 
hard surface of the rocks was ground to pow¬ 
der, the elements of the soil were mingled in 
