ABRASION BY GLACIERS; RIVERS, AND WAVES 119g 
by the waves, the roll of the gravel had smoothed the solid rock; but 
elsewhere the waves are breaking either on surfaces which have not been 
appreciably eroded, or on a rocky shore composed of angular masses 
of rock of all sizes, which have been loosened by weathering. Wherever 
the shore is being worn, it is by combined weathering and plucking, 
and not by abrasion. Easton’s Point is not yet marked by a wave- 
cut cliff, and so is not the best place to show the process of erosion; 
but along the Cliff Walk, at the west end of Easton’s Beach, where 
the rock is prevailingly Carboniferous schists, a distinct cliff faces the 
ocean; the base of the cliff, however, is not rounded and smoothed 
as would be the case were it being worn back by abrasion. A very 
suggestive photograph of a raised wave-cut bench on Prudence 
Island has been published," in which both cliff and bench are rough 
and angular, the detail determined by the jointing of the shales. 
It will be easy, of course, for anyone to test the matter for himself. 
Detritus protects rather than endangers the cliff. Except at times 
of high storm, the beach material protects the cliff and acts on itself. 
It does not seem probable that the bombardment of the cliff at times 
of heavy storm would seriously affect the cliff. Certainly there is 
little evidence of such action in the detail of the cliff base. Waves 
acting by hydrostatic pressure along joint planes may be effective; 
apart from that action their work would appear to consist in reducing 
and removing materials supplied them by the processes of weather- 
ing. ‘They, like streams, are transporting and not abrading agents. 
RELATIVE IMPORTANCE OF PLUCKING AND WEATHERING 
The mechanical work of corrasion has been divided between 
abrasion and plucking. Abrasion does not appear to be an important 
factor. It is further a delicate question as to how far plucking can 
be considered a separate process, to be distinguished from weathering 
on the one hand and transportation on the other. In the case of 
glacial erosion it is easy to believe that the pressure of the ice may 
dislodge blocks from a jointed floor which has not been affected by 
weathering, though it would be difficult to show that frost-weathering 
had not had a share in loosening these blocks. It is possible that a 
t Geology of the Narragansett Basin, Monograph 33, U. S. Geological Survey, 
Plate XXIII. 
