ice: one over northeastern Quebec and Labrador,one over the Keewatin 
area west of Hudson Bay, and a third over the northern Rockies of 
Canada. Within a few tens of thousands of years, ice masses that 
had developed in these areas spread out to coalesce, interfinger or 
overlap one with the other, (Fige a The southern margin of th 
compound ice sheet reached Long Island on the east, Ste Louis in the 
certral interior, and the Canadian border on’ the northwest where the 
motion was impeded somewhat by the rugged peaks of the western moun- 
tain rangese 
Motion of the Ice Sheets 
The rates at which different parts of the ice crept over the 
land varie considerably from place to place and from time- to time. 
We have no way of measuring the precise speed of advance exceot by 
comparison with the motion of modern glaciers, but it seems safe to 
conclude that it could ‘generally have been measured in yards per 
month rather than in-miles.— Obstructions, slope of the ground, 
earthquakes, weather conditions, and amount of * snow in the centers 
where its accumulation was greatest were important influences. In 
part the movement was probably like the slow,steady, viscous: flow of 
a thick syrup, and.in part a sort of interrupted, staccato skidding. 
Naturally the ice would move more rapidly along valleys and 
would be protected from melting in them longer than it was on adja- 
cent uplands and divides. Consequently the rims of the expanding 
glaciers. became very irregular in outline. Large volumes of ice 
moved into major topographic depressions like theSt. Lawrence valley 
lowland inthe form of lobes that stretched far in advance of the 
‘main body of icée Some of these lobes were large enough to spread 
laterally while they advaneed, whereas thinner “ones. were confined 
within valley walls. During later melting and recession of the ice 
borders, these lobate shapes became even more pronouncede 
Effect of Glaciation on Life 
‘As the zone of more rigorous climati¢é .conditions surrounding 
these glaciers © moved outward, animals and plarits unable to cope with 
the pee 1sing environment perished, unaware ‘that the remote glaciers 
were aling thém a lethal blow. Other animals and plants sensitive 
to Stich chanses migrated far in advance of the oncoming ice, or 
retaired their habitat for awhile by adapting themselves ‘to the new 
conditions. There was no sudden evacuation, however, because the 
chanzes were very gradual, almost imperceptible toan individual 
seneration. Nevertheless, even the hardier creatures finally yielded 
to. the relentless onslaught of savage, biting winds that blew down 
ade the ice and spread their chill influence for hundreds ofmiles 
ad-of the ice front. Some of these rorthern creatures took up 
oe in Florida. At the same time, the hairy ancestors of modm. 
man were huddling and shivering in the caves of southern France.» The 
land close to the ice was, however, not barren by any meanse Rugged 
plants and arctic animals thrived in the vicinity of a glacier or 
