37S 



GEOLOGY. 



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, on which the marginal 

 or extra-marginal waters of the retreating ice, or of the next advancing 

 ice, may have made deposits of stratified drift. These may have been 

 subsequently covered by till during a re-advance of the ice in the same 

 epoch, or in a succeeding one. Still again, till left by one ice-sheet 

 may have been completely worn away locally before the next ice advance, 



Fig. 506. — Diagram showing the intimate association of stratified and 

 unstratified drift. 



so that stratified deposits connected with a second or later advance 

 may have been made on a driftless surface, and subsequently buried. 



4. Intermorainic stratified drift may have originated at the outset 

 in all the ways in which supermorainic drift may originate. It be- 

 came intermorainic by being buried in some one of the various ways 

 in which stratified drift may become submorainic. 



Topographic distribution of stratified drift. — Though stratified 

 drift is most abundant in valleys and lowlands, rt is not confined to 

 these positions. Karnes are measurably independent of valleys and 

 lowlands, and though eskers often show a tendency to follow valleys, 



