SOME INSTANCES OF MODERATE GLACIAL EROSION 171 



Conclusion. — That glacial erosion failed to remove the products 

 of preglacial decay on Cape. Ann is evident from the facts stated 

 above; and that the marked accumulation of bowlders on the Cape 

 is due to this lack of erosive power is advanced as a theory toward 

 which all facts point, while no facts at present known oppose it. 

 Whether there are reasons why 

 the ice was locally thus ineffect- 

 ive, or whether it is a common 

 condition along the New England 

 coast, I cannot say. The Cape 

 is a moderate salient against 

 which ice-currents may have 

 divided, leaving this region only 

 slightly affected by its erosive 

 action. Facts from a wider area 

 must be obtained to settle the 

 question whether this condition 

 is local or general along the New 

 England coast. 



Fig. 8. — To illustrate the probable 

 changes at Cape Ann. A jointed granite 

 with joint planes increasing in number 

 toward the surface, changing to a con- 

 dition of bowlders with gravel between, 

 and capped by residual soil, has, by ice 

 erosion, been lowered to the level of the 

 heavy line. 



TURNAVIK ISLAND, LABRADOR 



This island, which is approximately in latitude 55 N., is one of a 

 number of islands in an indentation on the Labrador coast. It rises 

 from 300 to 400 feet above the water and is a barren mass of rock, 

 mainly porphyritic gneiss, so far as examined. Its surface is strongly 

 glaciated, with numerous very perfect roches moutonnees forms, and 

 many perfectly preserved striae and deep grooves, but with very little 

 till and few foreign bowlders. 



Crossing the gneiss are a number of diabase dikes, one of which 

 is 60 feet wide, and where these dikes occur their sites are quite 

 uniformly indicated by chasms. The large 60-foot dike, whose site 

 is marked by a long, deep valley (Fig. 9), extends at right angles to 

 the direction of ice-motion, so that the valley cannot be explained 

 by ice-erosion. That the chasm has not been formed since the ice 

 left is proved by its rounded, ice-worn edges, and by the presence 

 of fresh striae on its walls. The valley is evidently the result of 

 earlier decay; but that the ice removed the entirely disintegrated 



