174 



CAPE COD GEOLOGY 



Stratigraphic evidence shows that the Gardiners clay is of the same age 

 as the similar clay on Long Island, Gardiners Island, Block Island, and Nan- 

 tucket. It cannot be correlated certainly with the Pleistocene deposits of the 

 Mississippi Valley, although Fuller, 1 who considered it an interglacial formation, 

 has suggested with much caution that it may be assigned to the Yarmouth 

 interglacial stage. 



JACOB SAND 

 Name 



The name Jacob sand, according to Fuller, 2 its author, is derived ' ' from 

 Jacob Hill, a high point on the north shore of Long Island, 8 miles northeast of 

 Riverhead, near which the formation is well exposed." On Long Island the 

 Gardiners clay grades upward through interlaminated clay and sand into the 

 fine silty sand of the Jacob formation, whose deposition followed that of the 

 Gardiners clay without a break. Nevertheless, this sand and clay show suf- 

 ficient lithologic difference from the Jacob formation to permit their recogni- 

 tion throughout Long Island and the islands farther east. The position and 

 character of this sand on Marthas Vineyard establish its identity with that 

 on Long Island. 



Character 



The Jacob formation is composed of extremely fine sand, made up of grains 

 of quartz and of some flakes of muscovite and grains of dark minerals. This 

 silty sand is somewhat plastic when wet and thus resembles the Gardiners 

 clay. In color the Jacob sand is usually a light gray, but at some places it 

 shows yellowish shades and grades into a buff sand. It is generally well strati- 

 fied. At some places the sand contains interbedded layers of clay; at other 

 places it contains beds of coarser sand. The Jacob sand is rarely over 12 or 15 

 feet thick, but as it is transitional from the Gardiners clay its limits are not 

 everywhere easily established. Wherever it is seen it rests on the Gardiners 

 clay, whose irregularities and distortions it follows closely. 



Source of Material 



A slight change of conditions from those that prevailed when the Gardi- 

 ners clay was formed would cause a gradual change from the deposition of clay 

 to the deposition of the fine sand of the Jacob formation. Fuller 3 suggests that 

 the change might be due to any one of three causes : a change in level, a change 



1 Fuller, M. L., The geology of Long Island, N. Y.', U. S. Geol. Survey Prof. Paper 82, p. 106, 1914. 



2 Op. tit., p. 107. 



3 Op. tit, p. 107. 



