322 MYRON L. FULLER 



overlying till and nearly twice that of the tills of the drumlins, 

 which represent the most clayey tills previously known. The 

 difference in the amount and character of the included rock 

 material is also very marked. The lower till was found to con- 

 tain only about io per cent, of pebbles, mainly under an inch in 

 diameter and consisting principally of quartzite. The upper till 

 contained some 50 per cent, of pebbles and cobbles, besides a 

 large number of massive bowlders of granite and diorite varying 

 from five to ten or even twenty feet in diameter. 



The underlying rock was not exposed in the immediate vicin- 

 ity of the till here described, but it is known to be.a conglomer- 

 ate similar in character to that of the Center street locality, and 

 to be likewise considerably decomposed. 



The great dissimilarity of the lower till from the overlying 

 till, the sharp line of demarcation between the two, the evident 

 derivation of the former from deeply decomposed conglomerate, 

 and the exceptionally close resemblance of the lower till to that 

 of the Center street exposure, have led the writer to correlate it 

 with the latter and to refer it to the same early Pleistocene 

 invasion. 



PEARL STREET EXPOSURE, BROCKTON HEIGHTS 



The writer's attention was called to this exposure of what 

 may probably be considered as a representative of pre-Wisconsin 

 till by Mr. M. S. W. Jefferson, of Brockton, to whom the credit 

 of the discovery of the locality is due. The exposure was within 

 a gravel pit of some size on the south side of Pearl street, a 

 short distance north of its junction with Rockland street (Fig. 2, 

 Exposure 3). 



The height of the section was about five feet, of which the 

 upper two feet was of the ordinary type of buff till containing 

 numerous bowlders. The lower three feet was of an entirely 

 different and somewhat remarkable character, being composed 

 of an arkose-like mass of disintegrated material evidently derived 

 from the coarse porphyritic granite which is known to underlie 

 it. At first sight it bears a slight resemblance to a granite dis- 

 integrated in situ, but a closer examination reveals the presence 



