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CAPE COD GEOLOGY 



head the beds are underlain by a glacial gravel. Finally, if the rock at the 

 bottom of the Provincetown well is really a boulder it indicates that glacial 

 erratics lie beneath the Sankaty sand. If this view should be supported by 

 further evidence, it would place this marine invasion in an interglacial stage 

 following what was probably the first ice invasion of southern New England. 

 The shell beds carrying Macoma incongrua appear to lie below the blue clay 

 at Highland Light, which is of supposed Gardiners age, for a pebble containing 

 that species was found in the underlying gravel, of supposed Jameco age, in the 

 bluff south of the light-house. 



The Sankaty sand may extend northward into Massachusetts Bay and 

 Boston Harbor, and may have furnished the fossil shells and pebbles of agglu- 

 tinated sand carrying shells such as are found in the drumlins in that region. 

 The deposits supplying these shells there apparently lie beneath the glacial 

 blue clay found in the valleys of the Mystic and Charles rivers. The Dana 

 brothers 1 noted, as early as 1818, fragments of Mya arenaria found 40 feet 

 below the surface at Jamaica Plain, and the fragment of a clam shell found 

 at the depth of 107 feet in digging the well at Fort Strong, an earthwork built 

 at the end of August, 1814, in East Boston, at the south end of Noddle's Island. 2 



JAMECO (?) GRAVEL 



The end of Cape Cod peninsula from near Wellfleet to Pilgrim Heights, 

 which overlooks the expanded sand hook on which Provincetown stands, includes 

 the two plains described by Grabau as the Truro plain and the Wellfleet plain. 

 For several miles along the ocean side of this part of the Cape there is a bluff 

 whose structure is well exposed at Highland Light and at other points where 

 the sea is undermining it and removing the talus of gravel and sand. Distinct 

 stratigraphic members in this section can be recognized only at Highland Light, 

 where, beneath the surface rubble and wind-blown sand, about 40 feet of fine 

 yellowish sand resembling the unfossiliferous Jacob sand overlies about 40 feet 

 of dark clay containing no stones. This clay has the appearance and stratigraphic 

 associations of the Gardiners clay. It rests on a coarse glacial gravel, which 

 Fuller regarded as the Jameco gravel, and it is overlain by fine yellowish sand 

 that resembles the Jacob sand. Under the lighthouse at the "clay pounds" 



1 J. Freeman Dana and Samuel L. Dana, Outline of the mineralogy and geology of Boston and its 

 vicinity, with a geological map, Boston, 1818, p. 96. The drifted shells are described by W. O. Crosby 

 and H. O. Bullard: Distribution and probable age of the fossil shells in the drumlins of the Boston 

 Basin. Am. Jour. Sci., 48, 1894, pp. 486-496. Includes a list of 55 species of marine animals collected 

 by Messrs. Stimpson, R. E. Dodge, W. W. Dodge, Warren TJpham, W. W. Herman, and others. 



2 Justin Winsor, Memorial history of Boston, 3, pp. 309-310, Boston, 1881. 



