CAPE COD GEOLOGY 



11 



inland, above the mean position of the outermost points on the land having an 

 elevation of 500 feet, the slope varies widely from the angles indicated by the 

 measurements given, being steeper along certain lines and less steep along others. 

 Thus from central-western Rhode Island the restored contours reveal a steep 

 slope between the 500 and 700 foot contours which extends northward through 

 the Kent, Burrillville, Blackstone, Marlboro, and Groton quadrangles in Massa- 

 chusetts into New Hampshire. The southeastern versant of the upland west of 

 this slope, which declines southward from the high ground about Worcester, 

 has an altitude of 800 feet in the southwest corner of the Burrillville quadrangle, 

 in Rhode Island. The slope from this point southward to the position of the 

 100-foot contour is about 22 feet to the mile ; thence northward over the upland 

 the land rises at about half that rate to the base of the monadnocks on the Wor- 

 cester plateau. 



From the eastern halves of the Groton, Marlboro, Blackstone, Burrillville, 

 and Kent quadrangles or, in general terms, east of meridian 71° 35 W., no rocky 

 summits rise above 500 feet, except Nobska Hill in the Franklin quadrangle, 

 and the Blue Hills of Milton, south of Boston, in the Dedham quadrangle. Rem- 

 nants of an old eroded surface that stood between 400 and 500 feet above sea 

 level extend eastward across the Franklin quadrangle and include small areas 

 in the southwestern part of the Framingham quadrangle, in the northwestern 

 part of the Providence quadrangle, and in the Dedham quadrangle. The gabbro 

 hills of Sharon and the Blue Hills of Milton are residual masses that mark the 

 outer limits of this old surface. One small point of rock in the northeastern part 

 of the Franklin quadrangle, near Concord, is also a remnant of this surface. 

 Northeast and southeast of the 400-foot generalized contour there is a broad 

 extension of the coastal lowland of Massachusetts in which the summits rise to 

 300 feet in the Salem quadrangle and in the Fall River quadrangle. Between the 

 traces of this 300-foot line and the present shore line the descent is much steeper 

 than it is between the 400-foot line and the 300-foot line. 



The topographic features of the mainland west of Cape Cod have been 

 described by Davis 1 and later by Keith, 2 who states that it falls into four plateau- 

 like surfaces which become successively lower towards the east, and each group 

 forms bays projecting westward into the higher ones. As a whole they are im- 

 mense steps or platforms ascending to the central upland. The highest group 



1 Davis, W. M., Nat. Geog. Mon. No. 9, pp. 269-304, 1895. See p. 273, where the lowland about 

 Boston appears to be regarded as the Cretaceous peneplain sloping eastward toward the sea rather than 

 as a surface produced by erosion below the Cretaceous level. 



2 Keith, Arthur, U. S. Geol. Survey Water Supply Paper 415, pp. 8-23, 1916. 



