168 



CAPE COD GEOLOGY 



Character 



On Marthas Vineyard the Gardiners clay consists entirely of fine, grayish- 

 black silt or clay, which in places has a bluish tint, giving rise to the local name 

 of ' 'blue clay." (Plate 23, fig. 2). On this island it lacks inter-bedded sand except 

 near its top, where it grades into the Jacob sand. Its color is probably due in 

 part to organic matter, for fragments of lignite have been found in it at several 

 places. These fragments appear to be considerably waterworn. When wet the 

 clay is very plastic; when dry it may be reduced to a light-colored powder by 

 slight rubbing or pressure. 



Thickness 



The thickness of the Gardiners clay on Marthas Vineyard cannot be deter- 

 mined accurately because it has been greatly disturbed, but it is probably not- 

 more than 35 feet thick, much thinner than it is near Brooklyn, on Long Island, 

 where, according to Fuller, 1 its aggregate thickness (including layers of sand 

 and gravel) is 150 feet. Wherever its thickness on Martha's Vineyard is greater 

 than about 30 feet, it has been highly contorted. 



Source of Material 



Fuller holds that the Gardiners clay is of interglacial origin because the 

 marine fossils it contains indicate a moderate climate and its thickness indi- 

 cates a long period of deposition. On Marthas Vineyard only one fossil has been 

 found in it and its thickness is much less than that on Long Island. On Cape 

 Cod Woodworth has found large lenses of clay of possible Gardiners age, a 

 fact that does not indicate extensive interglacial deposition. 



The evidence obtainable on Marthas Vineyard and elsewhere therefore 

 shows that the Gardiners clay along the south shore is of marine origin. Whether 

 the clay is of interglacial or glacial origin cannot be determined, but it is not 

 necessarily interglacial, for all the conditions requisite to its deposition would 

 have existed when the ice front stood some distance back of this area. The 

 distribution of the clay and its marine origin indicate that the Gardiners stage 

 was a time during which the main shore line lay somewhat back of the area 

 in which clay was deposited. On this shore line the waves and currents were 

 working on deposits left by the retreat of the ice and were carrying away the 

 finer part of these deposits from the shore and depositing it as clay. Thus the 

 Gardiners clay is considered a glacial or an interglacial marine deposit, which 



1 Fuller, M. L., op. tit., p. 93. 



