GEOLOGY : NORTH SHORE. 17 



COREY HILL, BRIGHTON. 



Route. — By electrics, Allston and Newton cars from Park street by 

 Suljway, alighting at Allston street, Allston. Turn to left up Allston 

 street to Commonwealth avenue extension at foot of Corey hill; to right, 

 up the avenue. The rocks in question extend from the foot of the hill on 

 the north, to the road quarry on the summit. 



By wheel, along Commonwealth avenue and its "extension," from 

 Boston to the point mentioned above. 



Corey hill is a drumlin, the outline of which has been changed 

 somewhat through artificial terracing. The main hill is sheltered, 

 as it were, on the east of a large projection of sediments over 

 which the extension of Commonwealth avenue runs. The appear- 

 ance is as though the drift had migrated eastward under the influ- 

 ence of the ice-movement, coming to rest finally in the lea of the 

 great roche moutonnee. 



The structure of the bed-rock is that of a crushed and faulted 

 anticline, whose axis crosses the road northwestward on the north 

 side of the crest of the hill. The strikes and dips of various out- 

 crops are somewhat obscure. Sandstone, conglomerate, and sandy 

 shale compose the mass, apparently with several repetitions. This 

 appearance may be due, however, to isoclinal folding, and this is 

 known to account for one case. On the east side of the road, the 

 last ledge before the open field on the summit shows sandstone, 

 conglomerate and shale, the whole apparently at least fifty feet 

 thick. Examination of contacts will show that pinched folds are 

 present, giving probably less than six feet total thickness for the 

 mass. In many places cleavage obscures the bedding, lying at a 

 small angle with it. On the west side of the road for some distance 

 it is almost parallel with the stratification, and the two together 

 form the slab-shaped outcrops of the cliff. 



Other secondary structures are abundant, and the joint systems 

 are worthy of special notice. The east side of the road, opposite 

 the quarry, gives a face of conglomerate with six or seven systems 

 finely developed, the fracture passing through pebbles and cement 

 with equal ease. .On the west of the road and north of the quarry, 

 opposite the large open field, sandstone whose bedding and cleav- 

 age coincide shows several good systems, and occasional curious 

 local interruptions to them. 

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