CAPE COD GEOLOGY 99 



Kimball farms, southwest of the town of Nantucket, shells were encountered at 

 a depth of 50 feet. This well affords some evidence as to the thickness of the 

 last glacial outwash gravel deposits on the outwash plain. 



The reported depths at which shells are found, presumably in the Sankaty 

 sand, confirms the indications seen in the cliffs along the shore, that the last 

 glacial drift of Wisconsin time, in the morainal northern half of the island and 

 in the creased plain forming its southern half, is relatively thin. 



Edward Hitchcock 1 collected a shell of Pyrula carica (Busycon carica) from 

 a depth of 47 feet below the surface of Nantucket. The Sankaty sand and its 

 fossil shells probably have a rather broad extent beneath the outwash plain at 

 about the present sea level. 



COARSE GRAVEL 



Between the lighthouse at Sankaty Head and Siasconset there is a coarse 

 rubbly gravel bed, from 10 to 12 feet thick at the upper part of the section, 

 containing pebbles of Upper Cambrian quartzife, casts of Obolella sp., a diabase 

 with pronounced ophitic structure, a fine-grained red sandstone resembling the 

 red Carboniferous rock about North Attleboro, a gray conglomerate that is 

 common in the Carboniferous coal measures of southern Massachusetts, 'a red- 

 banded felsite, a dark schist resembling varieties of the metamorphosed Car- 

 boniferous rocks, hornblendic granitite of eastern Massachusetts type, diorite, 

 fragments of reddish Cretaceous nodules and sandstone and lignitic sandstone, 

 and some gneissic and amphibolitic rocks. Most of these rocks are of types com- 

 monly found in the drift as far west as Block Island and in southeastern Massa- 

 chusetts. The felsite is the same that is found in the Pleistocene gravel from 

 Nantucket northward, on Cape Cod, and presumably came from the Lynn- 

 Saugus area or an eastward extension of it now beneath the sea. The bed lies 

 unconformably on the fossiliferous sands of the Sankaty section. 



SECTIONS ALONG THE NORTH SHORE 

 Sections along the north shore from Sherburn bluffs westward are discon- 

 tinuous partly because of folds that carry the beds below sea level and partly 

 because of accumulations of talus. An instructive section is seen at Sachem 



Spring, in Nantucket cliffs. 



Section at Sachem Spring 



Thickness 



Rubbly drift and sandy soil at top in feet 



Brown pebbly clay or till 10 



Fine yellowish sand (Jacob sand) 6 



Blue clay (Gardiners clay) 15 



1 Catalogue of the collection of rocks, minerals, and fossils in the State Cabinet, Sixth Ann. Rept. 

 State Board of Agriculture for 1858, Boston, 1859; Appendix, Agricultural Museum, p. xi, specimen No. 

 106. 



