STRUCTURE AND RELATION OF THE DEPOSITS 457 



that the beds have been involved in the Gay Head diastrophe, and such 

 disorder may as well be due to internal movements of the section as to 

 original deposition. 



Glaciated Pebbles in the Deposit 



In April, 1899, the writer began a search in the boulder bed for glacial 

 pebbles, and found three specimens which are unmistakably glaciated. 

 Two of these were hornblendic granitite ; the third was diorite. The 

 smaller of the granite pebbles is shown in plate 42, figure 1. The flat- 

 tened side or sole, the snubbed ends, and the strise are alike indications 

 of the glacial abrasion of this pebble. The stride, though faint, are better 

 defined than on many glacial pebbles of that rock in the granitic area 

 about Boston. 



It can not be successfully contended that these pebbles are due to 

 mud-flows or to talus-sliding, for their position precludes that. Neither 

 are they due to solution in the bed after deposition, for quartz and feld- 

 spar grains alike are scored and grooved. Solution would have pro- 

 ceeded at unequal rates on these minerals and would have pitted the 

 surface. The objection that the pebbles were scratched by coast ice and 

 not b}' land ice is met by the peculiar distribution of the deposits with 

 regard to the derivation of the materials from the coast plain and the 

 New England extension of the Piedmont, from localities now at sea- 

 level to those having an elevation of at least 400 feet above the present 

 sealevel, in the manner above stated. 



Relation of Deposits to the Subterrane 



Undoubtedly an ice-sheet advancing on the coast plain as it must 

 have existed along the southern shore of New England before the glacial 

 period would have produced some disturbance in the deep soft rock sec- 

 tion of which it was composed. Erosion by ice and water action would 

 have taken place here and there on the disturbed and undisturbed sec- 

 tions alike. As pointed out in my paper before the Society in 1896, 

 these boulder deposits everywhere rest unconformably by erosion on 

 the Cretaceous or Miocene beds of the Gay Head section, and in one 

 part of the cliff the Cretaceous beds stand vertically beneath the boulder 

 bed. The local folding of the soft clays and sandy clays and the ero- 

 sion, followed by the deposition of the boulders, may be considered as 

 sequential parts of the advent of the first ice-sheet along this shore. 

 The great overturning of strata at Gay Head and elsewhere on Marthas 



LXV— Bull. Geol. Soc, Am., Vol, 11, 1899 



