WHE! ICE (AGE 
maximum distance from the sun and therefore received 
correspondingly less of its heat. This would have made 
the Glacial Period begin 240,000 years ago and last for 
160,000 years, thus coming to an end 80,000 years ago. 
This theory at one time found wide acceptance. But 
growth of knowledge has developed serious objections to 
it, and prehistorians have come to feel convinced that 
the maximum severity of the Ice Age occurred much less 
than 80,000 years ago. 
Still others have tried to explain the great expansion of 
the ice caps as due to causes arising on our earth itself, 
such as changes in the shape of the continents, produced 
by the elevation or sinking of the land. It has also been 
claimed that an ice age has followed every great period of 
mountain upheaval. Such earth movements have no 
doubt played a part, perhaps an important one, but they 
do not explain everything. 
Another hypothesis, in some ways more promising than 
any of those outlined above, ascribes the advance and 
retreat of the great ice fields to fluctuations in solar radia- 
tion; for our sun appears to be what astronomers call a 
variable star, giving out less heat at certain times than 
at others. 
But whatever the cause or causes, they led to a lowering 
of the temperature, although not necessarily a great one. 
In fact, meteorologists believe that even under present 
atmospheric conditions a fall in the average yearly tem- 
perature of only seven to nine degrees Fahrenheit would 
bring on another glacial period in Europe. 
The formation of glaciers requires that two conditions 
be met: First, an annual heavy fall of snow, so that it lies 
in great drifts; and second, summers either too cool or too 
short to melt all the snow that falls. The snow thus keeps 
on growing deeper and deeper, until the lower and older 
layers, subjected to great pressure by the superstructures, 
gradually turn into solid ice. When this takes place on 
level ground, immense ice sheets form in time, to remain 
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