MESOZOIC AND CENOZOIC GEOGRAPHY 479 
the central region, and partly the result of the greater 
accumulation of ice in this centre. Ice may be made 
to flow in a manner somewhat like that of wax; and 
if accumulated to a great depth in one place, this 
flowage may cause an out- 
ward movement from the 
place of accumulation. It 
is as though it were 
squeezed outward by the 
great pressure of the over- 
lying mass. 
Before* the ice came, 
the surface of the country 
was carved into hills and 
valleys, very much as it is 
now, in the region south 
of the glaciated land. 
The rocks of this pre-gla- 
cial land had been decayed, — j4ea1 map showing extension of the ice 
and upon them soil had sheets in North America (Cham- 
berlin). 
Fic. 265. 
gathered. Moving over 
this surface, and grinding it as it passed, the ice swept 
away this loose material, and even dug down into the 
rock itself. Hills were thus planed down somewhat, 
and valleys deepened; but the changes of this nature 
were not sufficient to erase, or even greatly modify, 
the larger features of the land. 
