166 bulletin: museum of comparative zoology. 



tioned sandstone comes a conglomeratic mass which differs from the 

 Roxbury in having the fragments and lenticular layers of slate, men- 

 tioned above in the description of this locality. From a study of this 

 bed I infer that the ice had come near when these fragments of clay 

 were deposited. Just above this bed come about forty-seven feet 

 of slate and sandstone layers with ripple-mark and some boulderets 

 from eight to ten inches in diameter. At this time the ice must 

 have made a temporary halt or retreat. At least deeper or slower 

 water conditions prevailed. Certain layers in this slate bed are con- 

 torted, and immediately above and below these layers there are no 

 signs of contortion. The tops of the arches are eroded, thus proving 

 that the contortions were made while the deposition of the slate 

 beds was going on. The ripple-mark suggests a stream of slow speed 

 which might very well have eroded the tops of these folds. Above 

 these slate and sandstone layers occurs another conglomeratic mass 

 with more slate fragments and probably a bed of tillite. At this place 

 there are no good exposures for fifty or sixty feet ; but tillite, which I 

 believe to be in situ outcrops, in one place. There is no tillite im- 

 mediately to the north or northwest of this spot, so it does not seem 

 probable that this outcrop is a boulder. Above this horizon comes a 

 bed of sandstone about twenty feet thick. In the midst of this sand- 

 stone are some very thin layers of conglomerate and a few slate frag- 

 ments, one of which measures eight by ten inches. These last men- 

 tioned beds indicate another advance and retreat of the ice-sheet. 

 The relatively thick sandstone bed shows that the ice retreated for 

 some distance and might or might not indicate an interglacial epoch. 



Above this sandstone comes the main body of the tillite. The differ- 

 ence between the tillite and the water-laid conglomerate which contains 

 the slate lumps is obvious. The contact between the sandstone and the 

 tillite is very ragged, showing disruption of the sandstone. The tillite 

 pierces the sandstone as if pushed into it. With the exception of very 

 thin layers of slaty material no intercalated beds are met with in this 

 exposure for about 150 feet, when a bed of conglomerate and sandstone 

 is found not over two feet thick. It is probable that this bed is the 

 equivalent of the bed of coarse gravel found at Squantum Head and 

 indicates a retreat of the ice. Above this bed the tillite is continuous 

 as far as the outcrops extend, but it is evident that not much more 

 than one half of the tillite is exposed at this locality. 



To obtain an idea of the sequence of events near the top of the 

 tillite a study of the exposure at the southeast Squantum locality is 

 best, as the section is almost all exposed to view for several hundred 



