176 



CAPE COD GEOLOGY 



clay, namely, in the Squibnocket and Nashaquitsa cliffs, along the shore of 

 Menemsha Pond, and along the shore of Vineyard Sound. In the central part 

 of the Nashaquitsa cliffs the Gardiners clay for some distance is not overlain 

 by the Jacob sand. In places on the northwest coast, particularly at the mouth 

 of Roaring Brook, the Jacob sand is absent and the Gardiners clay is present, 

 but in practically all places where the Gardiners clay is exposed the Jacob sand 

 overlies the greater part of the clay. The best exposures of the Jacob sand, 

 both in development and in variation of structure, are seen in the Nashaquitsa 

 cliffs. In places here it has been very much disturbed and compressed and it is 

 locally overturned and removed from the Gardiners clay. Where the lateral 

 compression has been great it has been thrown into small plications, and where 

 it has been exposed to the action of the wind the thin beds, which contain more 

 clay, stand out in relief, so that the deposit has a fluted appearance. 



The general distribution of the Jacob sand on Marthas Vineyard may like- 

 wise be considered the same as that of the Gardiners clay. The Jacob sand 

 flanks the northwest and southeast sides of the western area, surrounding a core 

 of the older, more highly folded beds that form the backbone of this area. It 

 is not seen in the town of Gay Head, and it probably lies below sea level in the 

 northeastern part of the island. It has not been found below the outwash gravel 

 of the great plain area, where there are no exposures deep enough to display 

 it, but it may exist in this area. 



Form of Surface 

 The form of the surface of the Jacob sand is due largely to the action of 

 the Montauk ice. In places where the sand has been but slightly disturbed it 

 passes upward gradually or fairly abruptly into the Herod gravel member of 

 the Manhasset. In such places the surface of the Jacob has been undulated by 

 the weight of the overlying ice. Where the Herod gravel was removed by the 

 Montauk ice, or where it may never have been deposited, the upper surface 

 of the Jacob sand is usually more irregular. In such places the action of the 

 Montauk ice has been stronger and the Jacob sand has been pushed and drag- 

 ged as well as eroded. At a few places till overlies the sand conformably, but 

 generally the Jacob beds are greatly disturbed, as in the Nashaquitsa cliffs. 

 As the Jacob sand overlies the Gardiners clay it was more exposed to the action 

 of the ice and so has been more disturbed than the clay, but, owing to its con- 

 tent of silt, it acted in much the same way as the clay, being bent and folded 

 rather than crumbled, as a coarser sand would have been. 



