50 



CAPE COD GEOLOGY 



list the following species are essentially identical with those now living in the 

 sounds along the same coast : 



Iv-- ' 



Gastropods: 



Nassa obsoleta 

 A T assa triviltata 

 Crepidula fomicata 

 Purpura lapillus 



Pelecypods : 



Ostrea virginica 



My a arenaria 

 Venus mercenaria 

 Mytilus edulis 

 Anomia ephippium 

 Cyprina islandica 

 Area cenella 

 Solen ensis 



To this list should be added Modiomorpha nigra Gray ? of Dall's list of fossils 

 found in Miocene beds on the south side of Marthas Vineyard. 1 This species 

 was found in the gray clay now known as the Gardiners clay, of Pleistocene age. 

 This is the only marine fossil found in the Gardiners clay east of Long Island. 



This assemblage of forms indicates the marine origin of the Gardiners clay 

 under conditions not unlike those which prevail in the shallow water of sounds 

 or interisland passages off a continental coast that is being rapidly attacked by 

 waves carrying particles of boulder clay, which is being laid down in successive 

 offshore zones in the form of sheets of gravel, sand, and clay. Although isolated 

 cobbles and thin lenses of gravel are found on Block Island and elsewhere in beds 

 of clay that appear to be members of the Gardiners clay, probably as deposits 

 from floating ice in winter, that clay was doubtless laid down during an inter- 

 glacial stage, when glacial action was reduced to a minimum. 



Until the Gardiners clay has been definitely correlated with the older glacial 

 clay in the lower Hudson valley and in the New Haven region, as well as with the 

 clay in the valleys of the Charles and Mystic Rivers about Boston, the attempt 

 to determine the depth and extent of the submergence in Gardiners time must 

 remain uncertain. The folding and the deformation of the beds of clay in the 

 islands off the south coast preclude the use of the ordinary stratigraphic methods 

 of estimating the initial dip of the formation and projecting the position of the 

 shore line. At Highland Light, on Cape Cod, beds of clay that are regarded as 

 probably of the same age rise about 120 feet above the present sea level and are 

 only slightly deformed. The submergence at that place may have exceeded 120 

 feet. The beds of glacial brick clay near Belmont, northwest of Boston, which 

 r are probably older Pleistocene deposits, show an eroded surface at a height of 

 about 120 feet above sea level. If the Gardiners clay and the correlative deposits 

 were laid down in water that was not more than 100 fathoms deep, a reasonable 

 assumption, there should have been formed at the same time, between them and 



i Dall, W. H., Am. Jour. Sci., 48, p. 297, 1894. 



