REPRESENTATIVES OF PRE-WISCONSIN TILL 325 



Pond exposure the till appears to have been originally com- 

 posed largely of bowlders, the decay of which was well under 

 way at the time of the laying down of the till, but which did 

 not completely disintegrate until some time afterwards. Some 

 of the larger bowlders still show undecomposed cores, but as a 

 rule the disintegration is 



complete. The color is '^X^kr^^-^-r \%']v^j. 

 rather a dark brown, some- 

 what similar to the reddish- 

 brown color of decomposed '"""■"' 



diabase, and serves tO Vertical and horizontal scale : 1 in. = 30 ft. 



Sharply separate the lower Fig. 5.— Section showing the relations of 



r ,, , .,, the older and younerer tills in the Pine street 



from the upper till. „. J . . ,„ , „. . 



rr exposure, Stoughton. (Exposure 5 of Pig. 2.) 



The upper till is of the 

 ordinary heterogeneous type abounding in foreign fragments, 

 many of them rather far-traveled. There is proportionally 

 little of the dark syenite in the upper till, differing in this 

 respect from the Pearl street and Ames Pond exposures in which 

 the predominating material of both tills is the same. When 

 present in the upper till the syenite is fresh. 



In the case of the Center street and Intervale Park expo- 

 sures of Brockton the reasons have been given for regarding the 

 tills as probably representing the earliest of the Pleistocene 

 advances. One of the most prominent of these reasons, namely, 

 the position of the till upon deeply decayed and unglaciated 

 rock surfaces, cannot be applied with certainty to the last three 

 tills described, since the immediately underlying rock is not 

 exposed and its condition is not known. The difference in the 

 colors is also a noticeable feature, the granite and syenite tills 

 showing nothing of the high colors which characterize the tills 

 derived from the conglomerate. A study of granites decayed 

 in situ, however, shows that high colors are not the necessary 

 accompaniment of disintegration such as the granite of the tills 

 has undergone. The same close dependence of composition 

 upon the underlying or immediately adjacent rock, the same 

 small percentage of foreign material, the same highly weathered 



