960 STUDIES FOR STUDENTS 



ginal lakes were extinguished by having their basins filled with 

 the advancing ice, which displaced the water. But new ones 

 were formed, on the whole, as rapidly as their predecessors 

 became extinct, so that lacustrine deposits were making at inter- 

 vals along the margin of the advancing ice. 



Deposits made in advance of a growing ice-sheet, by waters 

 issuing from it, were subsequently overridden by the ice, to the 

 limit of its advance, and in the process, suffered destruction, 

 modification, or burial, in whole or in part, so that now they 

 rarely appear at the surface. 



DEPOSITS MADE BY SUBGLACIAL STREAMS. 



Before their issuance from beneath the ice, subglacial waters 

 were not idle. Their activity was sometimes erosive, and at such 

 times stratified deposits were not made. But where the sub- 

 glacial streams found themselves overloaded, as seems frequently 

 to have been the case, they made deposits along their lines of 

 flow. Where such waters were not confined to definite channels, 

 their deposits probably took on the form of irregular patches of 

 silt, sand, or gravel ; but where depositing streams were con- 

 fined to definite channels, their deposits were correspondingly 

 concentrated. When subglacial streams were confined to definite 

 channels, the same may have been constant in position, or may 

 have shifted more or less from side to side. Where the latter 

 happened there was a tendency to the development of a belt or 

 strip of stratified drift having a width equal to the extent of the 

 lateral migrations of the under-ice stream. Where the channel 

 of the subglacial stream remained fixed in position, the deposi- 

 tion was more concentrated, and the bed was built up. If the 

 stream held its course for a long period of time, the measure of 

 building may have been considerable. In so far as these chan- 

 nel deposits were made near the edge of the ice, during the 

 time of its maximum extension or retreat they were likely to 

 remain undisturbed during its melting. The aggraded channels 

 then came to stand out as ridges. These ridges of gravel and 

 sand are known as osars or eskers. It is not to be inferred that 



