R. M. Brown — Clays of the Boston Basin. 449 



]^an tucket, in which is shown a till deposit over clay. Pro- 

 fessor Shaler at another time,^ in summing up the facts of the 

 clajs, suggests that they are reconcilable with the supposition 

 that there were several clay-making periods, each marking a 

 retreat of the ice, followed by a readvance. The clays about 

 Boston were studied by Marbut and Wood worth. f Sections of 

 drumlins in Somerville have been published by them showing 

 the clay beneath the till. Professor Woodworth,:}: in the corre- 

 lation of the deposits along the coast of ]N"ew England, places 

 the Mystic clays in the second of three glacial epochs. 



Discussion of Relation of Clays to the Glacial Period. — - 

 The points that must be explained are as follows : 



1. The position of the clays beneath the drumlins in Chelsea 

 and Pevere. 2. The considerable amount of clay that makes 

 up a constituent part of the til] of the drumlins. 3. The dis- 

 tortion of the clay-beds. 



Only one conclusion can be reached in regard to the first of 

 these. The clays must have been deposited before the advance 

 of the ice which formed the drumlins. The clayey composition 

 of the drumlins in the neighborhood points to the same con- 

 clusion. In a pit recently opened in the flank of a drumlin 

 situated near Hyde Park, Massachusetts, enough clay was found 

 on the floor to give an aspect of a clay bed. This clay had 

 washed out of the exposed till during the storms, and collected 

 at the bottom of the pit. A little digging exposed a few 

 inches of clay only, and further examination explained the 

 origin of the clay. The per cent of clayey material appears to 

 be too large to be produced by the natural process of grinding, 

 during or just previous to the formation of the drumlins. It 

 seems just, under the circumstances, to presuppose a reservoir 

 of clay from which the drumlins drew their supply. Local 

 advances of the ice have been offered as an explanation for the 

 position of the clay under the drumlins as well as the large 

 percentage of clay in their composition. Emerson considered 

 such an event in the "Northampton Lake" area. While a 

 local advance of the ice after the last general retreat has been 

 proven along the line of the chain of lakes — Fresh Pond, Spy 

 Pond, the Mystic Lakes, etc., — it is not consistent to extend this 

 local advance over an area large enough to account for the 

 drumlins of Revere and Chelsea. The local advance would 

 then grow to a general invasion. 



The distortion of the clay-beds offers some additional testi- 

 mony. ISTo explanation has been given for the folding of the 

 clays that has been more satisfactory than the thrust of the 



*17th Ann. Eep. U. S. a. S., p. 969. 

 tl7th Ann. Eep. U. S. G. S., p. 989. 

 X ITtli Ann. Rep. U. S. G. S., p. 987. 



