I 



STRATIFIED DRIFT 969 



CLASSIFICATION ON THE BASIS OF POSITION. 



In general the conditions and relations which theoretically 

 should prevail are those which are actually found. 



On the basis of position stratified drift deposits may be 

 classified as follows : 



1 . Extraglacial deposits, made by the waters of any glacial epoch 

 if they flowed and deposited beyond the farthest limit of the 

 ice. 



2. Siipermorainic deposits made chiefly during the final retreat 

 of the ice from the locality where they occur, but sometimes 

 by extraglacial streams or lakes of a much later time. Locally 

 too, stratified deposits of an early stage of a glacial epoch, lying 

 on till, may have failed to be buried by the subsequent passage 

 of the ice over them, and so remain at the surface. In origin, 

 supermorainic deposits were for the most part extraglacial 

 (including marginal), so far as the ice-sheet calling them into 

 existence was concerned. Less commonly they were sub- 

 glacial, and failed to be covered, and less commonly still 

 superglacial. 



3. The submorainic {basal) deposits v^qyq made chiefly by extra- 

 glacial waters in advance of the first ice which affected the 

 region where they occur. They were subsequently overridden 

 by the ice and buried by its deposits. Submorainic deposits, 

 however, may have arisen in other ways. Subglacial waters may 

 have made deposits of stratified drift on surfaces which had been 

 covered by ice, but not by till, and such deposits may have 

 been subsequently buried. The retreat of an ice-sheet may have 

 left rock surfaces free from till covering, on which the marginal 

 waters of the ice may have made deposits of stratified drift. 

 These may have been subsequently covered by till during a 

 readvance of the ice in the same epoch or in a succeeding one. 

 Still again, till left by one ice-sheet may have been exposed to 

 erosion to such an extent as to have been completely worn away 

 before the next ice advance, so that stratified deposits connected 



