GLACIAL EROSION. 71 



contains relatively few large bowlders which have been derived from the 

 conglomerate, while the pebbles from that source are extremely abundant, 

 is worthy of note. Thus, on Marthas Vineyard, where the bowlders and 

 pebbles which lie on the Cretaceous and Tertiary rocks have all been 

 imported from the Narragansett Basin, the amount of pebbles which have 

 been separated from the matrix of conglomerate is very great, but the 

 number of bowlders of the conglomerate rock is so limited that an inspec- 

 tion of a thousand of these erratics revealed only half a dozen of this 

 nature, the others being from the granitic and other old rocks which form the 

 margins of the basin or from some of its inliers of like rocks. In its ordi- 

 nary undecayed state the conglomerate fractures in such a manner that 

 the rift intersects the embedded pebbles. We are therefore justified in 

 believing that at the +ime the glacial flow began to attack this region the 

 deposits were decayed to a considerable depth, so that the attrition broke 

 up the adhesions and separated the pebbles from the matrix. The fact that 

 the bowlders of the massive conglomerate are very rare in the moraine of 

 Marthas Vineyard, which was formed at an early stage of the ice time, 

 while they are relatively much more common in the drift lying in the basin, 

 gives support to this view. It is certain that the ddbris from this basin 

 which is found in Marthas Vineyard was derived from the area in the 

 earlier stages of the glacial excavation, while that formed on the surface of 

 the conglomerate represents, of course, the last part of the erosion which 

 was effected. 



AMOUNT OF EROSION. 



Although the amount of erosion which was accomplished by the ice 

 in the last Glacial epoch can not well be determined, the evidence goes to 

 show that it was considerable. Thus the moraines in Falmouth and on the 

 island of Marthas Vineyard and the Elizabethan group, all of which appear 

 to owe their materials in the main to the rocks of the basin and its margins, 

 contain in the aggregate a mass of matter which, evenly distributed over the 

 basin, would cover it to the depth of several feet. It is not to be believed 

 that these accumulations represent anything like half the rocky matter 

 which was worn away from the Narragansett district. Better evidence 

 as to the amount of erosion, as well as much information concerning the 

 distance to which the drift has been carried, is afforded by the bowlder 

 trains of this field. These trains are traceable from certain of the peculiar 



