36 CAPE COD GEOLOGY. 



osseous conglomerate on Gay Head; but this deposit contains no feldspar-bearing 

 pebbles, and it carries remains of fossil animals that range in age from Miocene 

 to early Pleistocene, which have not been found on Long Island. The glacial 

 deposits on Block Island and Marthas Vineyard below the unconformity men- 

 tioned may represent the whole of the Mannetto, but they seem to begin with 

 signs of glaciation in ice-transported boulders, which are followed by gravel and 

 sand deposited in water long before the epoch of close folding of the preexisting 

 beds in those islands at the time of the Mannetto ice advance. 



A striking feature of the glacial deposits on these islands is their content of 

 undecomposed feldspathic minerals, the essential constituents of such rocks as 

 granite, diorite, and other igneous and metamorphic rocks of the adjacent main- 

 land on the north, from which they were derived by glaciation. The underlying 

 deposits, including the Aquinnah conglomerate (here assigned to the oldest 

 Pleistocene because it contains traces of a horse of that age) and the still older 

 Miocene and Cretaceous deposits, contain no quartzite pebbles, though quartz 

 pebbles are abundant, and none of the kinds of rock that make up the bulk of the 

 glacial series. This lack of feldspathic deposits appears to be due to the weather- 

 ing, in preglacial time, of certain beds, such as those of kaolinite, which is made 

 up of grains of quartz and clay, the clay representing decomposed particles of 

 feldspar. 



Excluding the superficial glacial material of the Wisconsin stage, whose 

 thickness reaches 50 feet in the more massive morainal knobs and in the gravel of 

 the outwash plains fronting the terminal moraine, the older Pleistocene deposits 

 have a maximum thickness in the islands of about 250 feet, as follows : 



Feet 



Manhasset formation 50 



Jacob sand 10 to 50 



Gardiners clay 50 



Jameco and Mannetto formations 50 



Weyquosque formation and Dukes boulder bed ...... 50 



AQUINNAH CONGLOMERATE (OSSEOUS CONGLOMERATE) 



The Aquinnah conglomerate (the osseous conglomerate of Hitchcock) is a 

 local deposit of preglacial age. It is known only on Marthas Vineyard, where it 

 forms a bed in Gay Head cliffs. It has already been mentioned here because it 

 contains vertebrate remains ranging in age from Miocene through Pliocene to 

 early Pleistocene. The bed exposed to view in Gay Head cliffs has been greatly 

 eroded by the ice in its earlier advances. Scattered boulders of the conglomerate 



