DAVIS. — OUTLINE OF CAPE COD. 309 



Although no close accuracy is to be expected in restoring the sea- 

 ward extension of the clifFed mainland, there are nevertheless some 

 simple principles that will at least serve to guide us towards a not al- 

 together imaginary reconstruction. First, it must be remembered that 

 general subaerial denudation has not eflfected significant changes in 

 glacial topography dui'ing postglacial time. Second, the restored out- 

 line should possess irregularities of pattern comparable to those in the 

 protected bays of to-day, advancing from the headlands and retreating 

 towards the troughs or " valleys " in the high ground. Third, the 

 amount of land restored should be much less on protected shores than 

 on exposed shores. Fourth, cliiFs that are now protected by forelands 

 of marsh and bar must not be built out so far that their recession could 

 not have been accomplished before the bars began to grow in front of 

 them. 



Possible Changes of Level. 



These four guiding principles do not include reference to the effects 

 of change of level, because, if any change has occurred since the time 

 of accumulative construction of the mainland, it has been of small 

 amount, and it has, to my mind, acted on the whole in favor of de- 

 creasing the land area by submergence, thus co-operating with the 

 destructive action of the sea. This view is in accord with that ex- 

 pressed by Upham, who thinks that, when the drift was deposited 

 hereabouts, the land stood somewhat higher than at present, and that 

 the numerous small indentations or re-entrants of the shore line, such 

 as occur along the south side of the Cape, are results of a slight sub- 

 mergence of trough-like depressions or valleys. The digitate bays of 

 Martha's Vineyard would seem to lend support to this view ; but they 

 are otherwise interpreted by my colleague. Professor Shale r, who re- 

 gards them as having been formed by subglacial streams acting on sea- 

 floor deposits that had been strewn in front of the ice margin when the 

 sea stood higher than now, although he suspects also that " at the close 

 of the glacial period this region was considerably higher than at pres- 

 ent" (Geol. Martha's Vineyard, 7th Ann. Report U. S. Geol. Survey, 

 1888, pp. 318, 319, 350). The latter view is further supported by the 

 small amount of erosion — about three miles — suffered by the low 

 sandy southern shore of Martha's Vineyard (Ibid., p. 349) since the 

 present level of the land was assumed. 



Without undertaking to determine precisely the original level of 

 the Cape mainland, the most plausible explanation of the facts seems 

 to me that the washed gravels and sands correspond to the supermarine 



