I. Bowman — Atlantic Preglacial Deposits. 319 



blocks are filled in some cases by a silty deposit which can in 

 every instance be traced up to the till or sand above, from 

 which it was derived by percolating waters. The great bowld- 

 ers in the till are continually falling from the cliff face to the 

 beach below and give added protection to the basal clays 

 which determine the level of the beach. The clays, the par- 

 tially indurated sands and the till — all alike possess sufficient 

 tensile strength to stand in bold cliffs and thus yield under an 

 encroaching sea a remarkably clean and perpendicular section. 



Former Interpretations. 



Both Third and Fourth Cliffs have been described by 

 Upham.* As his interpretation differs widely from the one 

 just given, the following summary of his results seems appro- 

 priate in this place. It should be said that the section is today 

 in much better condition than when examined by Upham and 

 probably shows the relationships of the various deposits very 

 much more clearly than at any time heretofore. 



The two cliffed hills are referred to as "two extraordinary 

 drumlins .... which consist of till .... to a depth that 

 varies from 15 to 25 feet .... but below include beds of 

 modified drift that attain in Third Cliff a thickness of at least 



40 feet, reaching to the bowlder-strewn shore " 



Neither the yellow clays which, partly masked by bowlders, 

 form the shore, nor the unconformities above and in the so- 

 called modified drift were noted. That the material is not 

 modified drift is shown by a wholly dissimilar structure and 

 lithologic character ; and by the total absence of erratic mate- 

 rial, and a remarkably pronounced and persistent unconformity 

 between the erratic and non-erratic beds, with several smaller 

 unconformities within the latter (see conclusion, p. 325). 



Upham speaks further of the anticlinal structure of the 

 modified drift and the approximate coincidence of the upper 

 surface of these beds with the surface of the till at the cliff 

 top. This is offered as evidence of the manner in which drum- 

 lins are deposited underneath the ice, but in the better section 

 of today the cliffs show in the clearest possible manner that 

 the apparent anticlinal structure is in reality the effect pro- 

 duced by successive faultings of broken blocks of white and 

 red sands as shown near the left margin of fig. 1 ; and that 

 the coincidence or parallelism of the structural surfaces called 

 anticlinal is due not to similar dynamic conditions imposed by 

 the ice, but to the control exercised on ice movement and 

 deposition by the form of the subjacent terrane. It cannot, 

 therefore, be argued alone from the relations exhibited here 

 that drumlins are a subglacial deposit built up by successive 

 accretions from the debris-laden lower part of the ice. It is 

 *"The Structure of Drumlins." B. S. N. H. Proc, xxiv, 228-242, 1889. 



