968 STUDIES FOR STUDENTS 



-all the stratified drift deposited in advance on the territory 

 which it occupied later, and that in others it may have left 

 only patches of once extensive sheets. This may help to explain 

 why it so frequently happens that a section of drift at one point 

 shows many layers of stratified drift, while another section close 

 by, of equal depth, and in similar relationships, shows no strati- 

 fied material whatsoever. It also makes it clear that the inter- 

 relations of the two types of drift are, on the whole, less com- 

 plex than they might have been had all the deposits once made 

 by the ice and its accompanying waters escaped destruction. 



After what has been said, it is hardly necessary to add that 

 two beds of till, separated by a bed of stratified drift, do not 

 necessarily represent two distinct glacial epochs. 



In any region, which has been affected successively by two 

 or more ice-sheets, the complication of stratified and unstratified 

 drift may be even greater. While the ice of one epoch is likely 

 to destroy in part the deposits of earlier epochs, it is not likely 

 to obliterate them altogether. In some regions, indeed, the full 

 series of one epoch is buried beneath the deposits of a second, 

 ,as the soil between shows. In addition therefore to the com- 

 plicated series of stratified and interstratified deposits of a first 

 >epoch, the ice of a second developed a full set of its own. A 

 prolonged series of ice epochs might bring about a most com- 

 plicated set of relations, the complete unraveling of which would 

 be a most arduous task. 



In America the exposed portion of the formation made by 

 the ice-sheet which reached the greatest extension — the Kan- 

 san' — should possess less complex combinations of stratified 

 drift than the drift of the region further north which was 

 affected by two or more ice-sheets. The drift of the region 

 where the lowan formation is exposed, should present in ver- 

 tical section, more alternations of stratified and unstratified 

 drift than the Kansan, but less than the drift of the region 

 where the Wisconsin formation occurs, since the drift of this 

 latter region is the product of at least three ice-sheets. 



'Journal of Geology, Vol. Ill, p. 270 ; The Great Ice Age, Geikie, p. 753 etseq. 



