186 
THE QUATERNARY ICE AGE IN NORTH AMERICA 
By Ernst ANTEVS 
First Glaciation 
Several hundred thousand years ago the temperature of the earth 
began to fall and the snow precipitation to increase in medium high 
latitudes, in. North America, especially in the region of Hudson 
Bay. Snow began to accumulate from year to year in mountains 
and highlands, forming glaciers such are now to be found in the 
higher regions of the Rockies, the Alps, and other mountains. 
The glaciers grew gradually. They filled the valleys and ex- 
panded on the lowlands at the foot of the mountains. Here they 
formed ice fields which, as time passed, became ever larger and 
thicker. The ice helds grew by accumulation of ice in the moun- 
tains and in the central region, and by centrifugal flow caused by 
the pressure of this ice. They grew also by accumulation of ice 
along the borders. The growth of the ice was in a sense a per- 
petinon mobile, for since the primary temperature fall had caused 
formation of glaciers, these in their turn produced a lowering of 
the temperature. Ultimately four main centers of ice dispersion 
were formed in North America, viz., in Labrador, in the region 
Hudson Bay, in the region south of Hudson Bay and 
west O 
west of James Bay, and in the Canadian Rockies. The four ice 
caps coalesced and formed an enormous ice sheet, that finally cov- 
ered approximately the northern half of the North American con- 
tinent. It extended from the Atlantic to the Pacific and from the 
Aretic ocean about to the International Boundary in the west, to 
south of the Great Lakes, and to Long Island Sound (Fig. 15). 
The length of time occupied by this cnormous growth of the 
glaciers is not known, but may have been a few tens of thousands 
of years. 
At length, however, the temperature began to rise again and the 
snowfall over the ice sheets began to decrease. The summer melt- 
ing of ice, which under the low latitude of New York must have 
been very great, now equaled the supply, and the ice sheet ceased 
to grow. Betore long ice wastage surpassed nourishment, and the 
ice border started retreating. Gradually the ice sheet shrank and 
