WISCOKSIN CLAYS 553 



and other deposits of modified drift are due to tlie fact tliat tliey are 

 actually Montauk retreatal deposits formed in Moutauk glacial lakes. If 

 this be true it may place a new interpretation on many phenomena ob- 

 served in the study of sand plains and glacial lakes. As yet there are no 

 stated criteria by which plains of the two series can be definitely dis- 

 tinguished, except where overlying deposits of till occur. 



WISCONSIN CLAYS 



As has been said, many of the flat clay plains along the coast ("low- 

 level" clays) are certainly of Wisconsin age. As they overlie the older 

 clays in places and are similar in character, having been formed under 

 similar retreatal or interglaeial conditions, the several clay deposits can 

 not be distinguished with certainty. It is known that fossils occur in the 

 pre-Wisconsin clays, but their absence in the Wisconsin clays has not been 

 proved. Some clays are very fossiliferous, like those at Eliot, which are 

 almost certainly Wisconsin. There seems to be no reason to expect that 

 Wisconsin clays would be destitute of fossils, any more than clays form- 

 ing in the bays at the present time. As the lapse of time between the 

 two stages of glaciation was not sufficient to cause any radical changes in 

 the species, there is probably no way to differentiate the post-Gardiner 

 clays on the basis of their fossils. As they can not be differentiated by 

 lithologic character, we must depend wholly on their stratigraphic posi- 

 tion, amount of weathering, and topography, as explained on pages 

 531-544. 



POST-WISCONSIN OR LATE WISCONSIN DEPOSITS 



In many parts of Maine there ai-e evidences of local valley glaciers of 

 post- Wisconsin or late Wisconsin age. These arc most pr':>nonncfd in the 

 vicinity of mount Katahdin. That several valleys on the northern slope of 

 that mountain are shown by the position of moraines to have been occu- 

 pied by local glaciers was proved by Tarr.* In the region southeast of 

 the mountain more Avidely distributed evidences are found, which consist 

 chiefly of moraines formed entirely of large granite boulders of the type 

 found on mount Katahdin. As the strike of the glacial strise in north- 

 eastern Maine is directly south, and as the granitic moraines are found 

 both south and southeast of the mountain, it seems probable that the 

 material was deposited by glaciers of post-Wisconsin or late Wisconsin 

 age moving outward and south or southeastward from mount Katahdin 



* E. S. Tarr: Glaciation of mount Katahdin, Maine. Bull. Geol. Soc. Am., vol. 11, 

 1900, pp. 433-448. 



XLVIII — Roll. Geol. Soc. A.m., Vol. \U \'.Hn; 



