CHANNELS OF BAYS. 73 



It is well to contrast the rapid and effective erosion work of the conti- 

 nental glacier with the relatively very slight action that free water or 

 atmospheric decay has had upon the rocks of this district since the ice 

 mantle passed from its surface. Since the surface entered on its present 

 state there has been but very little decay of the rocks. Even where they 

 have remained without a covering of soil, as is the case in the summit of 

 Iron Hill, the penetration of decay in most instances is inconsiderable, and 

 the actual loss of material is so slight that the lowering of the surface has 

 not on the average exceeded 2 or 3 inches. In some cases bosses of the 

 harder conglomerate which have had no other protection than a coating of 

 lichens and the thin layer of detritus which they gather on a steep slope, 

 still retain the deeper groovings which the ice impressed on them. So far 

 as the bed rocks are concerned, the removal of matter from this region since 

 the close of the last Glacial period has been entirely unimportant, and the 

 decay, such as has penetrated so deeply in the Southern States, has hardly 

 begun even in the most advantageous situations for the process. These 

 facts point to the conclusion that the period which has elapsed since the ice 

 left this district has been, in a geologic sense, very brief. 



It is to be noted that the channels in which the main arms of Narra- 

 gansett Bay lie are still rather deep, though their bottoms are probably 

 covered by a considerable thickness of drift, materials, both that which was 

 originally deposited when the ice was retreating and that which has been 

 swept to its place by tidal action. The question suggests itself as to what 

 extent these depressions are due to the direct cutting action of the ice and 

 what to the concomitant action of the subglacial streams. While it must be 

 admitted that the general distribution of the channels of the bay and their 

 relation to the river channels connected therewith favor the supposition 

 that the arrangement of the valleys is in the main the result of ordinary 

 river action,- it can not well be denied that the glacial work greatly changed 

 the forms and in a measure the distributions of these depressions. Thus 

 the several rocky islands of the bay, with deep water between them, can 

 not well be explained by the supposition that they are the remains of 

 divides which once separated adjacent parallel river valleys. The channel 

 between Bristol Neck and the north end of Aquidneck Island appears to 

 be inexplicable on the theory of a submergence of river topography, but 

 it may be accounted for on the assumption that it is due to glacial scouring. 



