CAPE COD GEOLOGY 



291 



THE DEFLATION OF THE GLACIAL GRAVEL 



At many places on the Cape, particularly near the brows of the cliffs, the 

 wind has swept away the soil and is moving the glacial sand across tracts of 

 lag gravel. The pebbles and the angular rock fragments in such tracts thus 

 become highly polished and carved. The "blowholes" or lag-gravel tracts and 

 the windswept face of the cliff at Highland Light afford an excellent field for 

 observing the action .of wind on glacial material. Some of the effects of this 

 action have been described by Davis, 1 who found that sand-blasted pebbles or 

 glyptoliths are widely distributed on the Cape in places where the stratified 

 glacial gravel is exposed to the action of the wind. He also described numerous 

 glyptoliths that were buried, with the carved side up, in the superficial part 

 of the glacial gravel. He writes : 2 



The highest cliff of stratified sands and gravels on the southern shore is at Succonnesset 

 Point, between Falmouth and Cotuit. The bluff here rises twenty or thirty feet above sea- 

 level. The occasional pebbles scattered through the greater part of this section were not 

 facetted, but in the upper eight or ten feet, where pebbles were common, many carved 

 pebbles were found, and, as before, they lay with the facetted side uppermost. The greatest 

 depth at which a pebble with distinct facets was found was about nine feet beneath the 

 surface. Many carved pebbles were found along the base of the cliff, sometimes, apparently, 

 in place, but then held in large masses of gravel which had slipped down from the top of 

 the cliff. 



If it be postulated that these buried facetted pebbles have been carved by wind-blown 

 sand, as they certainly seem to be, then it must be concluded that at least those beds of 

 stratified sands and gravels which contain facetted pebbles were deposited by subaerial pro- 

 cesses, and not by marine processes below sea level. 



In addition to the more or less symmetrical carved pebbles bearing facets 

 whose intersection produces edges there are pebbles in which grooves and pits 

 have been worn by the sand blast. These sand-blasted pebbles are found in 

 situations which indicate that they were shaped at many times since the glacial 

 period, or at all times, for they are obviously now being shaped. 



The following data concerning the direction of the wind at Provincetown, 

 obtained by Dr. T. W. Vaughan from the United States Weather Bureau, may 

 be serviceable in an investigation of the dunes and sand-blasted pebbles at the 

 upper end of Cape Cod. 



1 Davis, W. M., Facetted pebbles on Cape Cod, Mass., Proc, Boston Soc. Nat. Hist. 26, pp. 166- 

 175, pis. 2, 1894. 



2 Op. tit., pp. 168-169. 



