ICE AGE EN NORTH AMERICAN ARCTIC — FLINT 245 



On the lands of Arctic North America the glacial ages left a more 

 evident impress. In the alpine mountains of British Columbia and 

 Alaska in the west, and of Labrador, Baffin and Ellesmere Islands, and 

 Greenland in the east, the shattering action of frost, accompanying 

 the glacial climates, sculptured typical jagged peaks and serrate ridges 

 and, at slightly lower altitudes, excavated the capacious, half-bowllike 

 corries that characterize the heads of most glaciated valleys. The 

 larger valleys were converted into fiords and, in Alaska and British 

 Columbia, where the rate of flow of the glaciers was rapid owing to 

 abundant snowfall, some valleys were deepened at least 2,000 feet by 

 glacial erosion. Now partly filled with sea water, these troughlike 

 valleys are the familiar fiords characteristic of the mountainous coastal 

 regions of high latitudes. 



On the lowlands the slow grinding action of thick glaciers almost 

 entirely removed the mantle of preglacial soil and subsoil, and 

 grooved, scratched, polished, and generally smoothed the surface 

 bedrock underneath. In places where the rock is cut by closely 

 spaced fractures and other planes of weakness, the glaciers quarried 

 out blocks of many sizes, creating roughened surfaces, as well as many 

 rock basins that now contain lakes. 



The depth of glacial erosion of the lowlands was small, as is indi- 

 cated by the preservation of preglacial topographic features and 

 of chemically altered bedrock that could only have been formed close 

 to the surface during preglacial time. Most estimates place the layer 

 of soil and rock removed by glacial action at a few tens of feet at 

 most. The small figure is the result chiefly of the lowland character 

 of the country ; there were no deep valleys to channel and concentrate 

 the flow of the ice, nor were there mountains to provide steep gradients. 



Here and there throughout the lowland region, irregular deposits 

 of earth and stones (the glacial drift) left on the surface, chiefly 

 during the melting of the glaciers, created obstructions to the natural 

 drainage, forming many lakes in addition to those occupying bedrock 

 basins. 



The glacial drift is generally thin and is distributed in patches. 

 In the regions surrounding Hudson Bay and lying immediately east of 

 the Mackenzie Great Lakes, where the ancient bedrock is strong and 

 not easily eroded, bare, ice-smoothed outcrops are much in evidence 

 and drift is scanty. Farther west, in the plains region, where the 

 weaker bedrock yields more readily to erosion the covering of drift is 

 more general. It is also thick in the central region between Hudson 

 Bay and the Mackenzie River. 



Much of the drift is a variety of till — a tough, compact nonstratified 

 stony clay plastered bit by bit onto the ground from the load of rock 

 fragments carried in the base of the slowly flowing glacier. The till 



