WISCONSIN TILL 545 



except on top of the Wisconsin sand, gravel, and clay plains. In thickness 

 it ranges from a few inches to 40 feet, but is commonly only 3 to 6 feet. 

 This surficial type of till is believed to be Wisconsin in age. 



Evidence of Wisconsm age. — The' evidence that this till is of Wiscon- 

 sin age is fivefold, namely : 



1. It is the uppermost till in the region. 



2. It is different in character from underlying tills. 



3. It is underlain in many places by stratified deposits of sand, gravel, 

 and clay. 



4. As a rule, it is but slightly oxidized at the surface. 



5. In many exposures there are veneers of this till over Montauk type 

 of till. 



1. Uppermost till in the region. — This statement explains itself, and 

 has heretofore led to the belief that all the New England till was Wis- 

 consin. 



2. Difference in character from lower tills. — The upper till in most 

 places is very different in character from the Montauk type. As stated 

 above, it is loose and more gravelly and its boulders are seldom well 

 striated. Its character is to a greater extent dependent on the character 

 of the underlying drift formations. For example, over a large part of the 

 town of Standish, south of Sebago lake, Maine, the surface till is very 

 sandy and gravelly, appearing more like what has been generally classed 

 as morainal material. In other places, as over some of the high-level clay 

 plains in southern Maine, the upper few feet consist of an unstratified 

 and frequently rather sandy type of clay containing more numerous 

 boulders than does the lower part. This is believed to be clay which has 

 been reworked by the overriding ice and is in reality till. Generally no 

 distinct line of demarkation from the underlying clay can be seen, but 

 where the upper zone is readily recognizable as till it is in most places 

 very clayey. Few boulders are seen over the broad expanses of clay plains, 

 but a significant fact is that in the lee of some rock hills boulders are 

 numerous. Moreover, by far the greater proportion of material in all till 

 is of local origin. 



These facts can be explained on the assumption that the Montauk 

 sheet, like the Wisconsin ice-sheet, left in its retreat glacial lakes in which 

 sand plains, moraines, and other deposits were formed. When, after the 

 elapse of some thousands of years, the Wisconsin ice-sheet extended south- 

 Ward over these retreatal deposits it must have incorporated large 

 amounts of the interglacial sands and water-worn gravels. 



A characteristic of till, in New England and elsewhere, is its depend- 

 ence in composition to a certain extent on the composition of the under- 



