246 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1952 



is usually only a few feet in thickness, although in the plains region 

 it may be much thicker. In some areas where the till is comparatively 

 thick the flow of the overriding ice has molded it into the whaleback 

 hills known as drumlins. Commonly these occur in broad groups, the 

 individual drumlins ranging in length up to 3 miles. Thus far drum- 

 lins have been reported principally from the region west of Hudson 

 Bay, as far west as northern Saskatchewan. Undoubtedly further 

 exploration will bring to light many more than are now known. The 

 value of drumlin study lies in the fact that the long axes of these hills 

 record the general direction of flow of the glacier ice at the time when 

 they were built. 



The esker, another type of drift accumulation, is also useful in 

 reconstructing the movements of glaciers. Eskers are long and usu- 

 ally winding ridges, several tens of feet in height, and in some cases 

 more than 100 miles in length. Some of them branch like streams, 

 and all are built of stratified glacial sediments. They are believed 

 to be the deposits made by streams of meltwater that flowed in tun- 

 nels beneath the ice or in channels in the surface of thin ice, in the 

 marginal part of an ice sheet. They were built during the period 

 of decay, just before the ice melted away, and are generally aligned 

 at right angles to the trend of the glacier margin at the time they were 

 built. Thus they may record successive positions of the edge of the 

 ice sheet during its shrinkage. Eskers are numerous in Arctic North 

 America, but as yet they have been mapped in two regions only: 

 east of Great Slave and Great Bear Lakes (Wilson, 1939, 1945) and 

 in central Quebec and Labrador. 



End moraines— ridges of drift heaped along the margin of the ice 

 sheet — occur here and there. Except in a few localities, however, 

 no systematic attempt has been made to map them. When fully 

 mapped these features, too, like the drumlins and eskers, will consti- 

 tute a valuable clue to the successive positions of the retreating margin 

 of the ice sheet. 



DISTRIBUTION AND TYPES OF FORMER GLACIERS 



Plate 1, adapted from the glacial map of North America (Flint 

 et al., 1945), shows the areas at present believed to have been covered 

 by former glaciers in North America. Excepting in the region of 

 the Arctic Ocean, the limits of the highland glaciated regions are 

 fairly well known, though many details remain to be filled in. Of 

 the vast lowland area west, north, and east of Hudson Bay, our knowl- 

 edge is hardly even elementary. 



The distribution and directions of flow recorded by the former 

 glaciers show that the two great groups of highlands in Arctic North 

 America — the western or Pacific and the eastern or Atlantic! — were 



