THE ICE AGE 
This total volume of ice corresponds to 30,800,000 cubic km. of 
water. Taking the area of the ocean as 361,000,000 square km., this 
water quantity represents a layer over that area 83 m. (272 feet) thick. 
The ice sheets and glaciers on the Northern Hemisphere are thought 
to have reached their greatest extent at practically the same time. On 
the other hand the climaxes of the glaciations were perhaps not en- 
tirely synchronous on the different hemispheres, though alternation 
is out of question. The volume of the ice on the Southern Hemisphere 
in excess of the present quantity is estimated to have been some 
4,100,000 cubic km., which corresponds to 3,760,000 cubic km. of 
water and represents a layer over the area of the oceans 10 m. (33 feet) 
thick. Therefore, if the glaciations reached their climax simultane- 
ously on both sides of the equator the sea level was lowered by some 
93 m. (305 feet). If the contemporaneity was only partial the sea level 
may at most have been lowered 88 m. (290 feet). 
This process steadily, although very slowly, caused a 
lowering of the sea level and exposed to the air thousands 
of square miles of the earth’s surface that had hitherto 
formed part of the bed of the sea. 
There is scarcely a part of the globe where the effects of 
this rising and sinking of the land are not manifest. Thus 
in many regions we find one or more “‘raised beaches,” old 
strands on which the sea once broke for long periods, but 
which are now raised far above the reach of the highest 
waves (Plate 20). 
The opposite sort of earth movement, that of slow 
subsidence, left traces in “‘sunken rivers.” The taking of 
accurate soundings has traced the former courses of 
many of the present-day rivers sometimes for long dis- 
tances along the sea bottom. The latter, that is to say, 
was once elevated above sea level, so that existing rivers 
flowed across it far beyond their present mouths. 
Thus both England and Ireland have in the past been 
joined to the mainland, as was the case even after the 
close of the Ice Age, not so very many thousand years 
ago. And Europe was connected with Africa both across 
the Strait of Gibraltar and by way of Italy and the pres- 
ent islands of Sicily and Malta, thus dividing the Medi- 
terranean Sea into two landlocked basins. The ‘“‘land- 
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