280 



CAPE COD GEOLOGY 



an embankment as the mass of drift that skirts the west coast of Cape Cod 

 Bay and forms an irregular wall of drift that rises above the present sea level. 

 An estimate of the height and the extent of this outer bank may be made by a 

 theoretical reconstruction of the Wellfleet and Eastham plains on a hypothesis 

 supported by a comparison of such deposits — namely, that the Eastham plain 

 at its original summit along the ice contact rose as high as the Wellfleet plain. 

 Assuming that the higher plain stood at an altitude of 120 feet at its iceward 

 border, as it does just north of the Eastham plain, the eastward rise of the 

 Eastham plain would reach this altitude at a distance of about 3 miles off the 

 present coast. This estimate is of the same order of magnitude as that reached 

 by Davis. 1 If the assumed rate at which the coast was cut back approximately 

 during the present stand of the sea holds good, this work could hardly have 

 been begun more than about 3,150 years ago. As the minimum estimate of the 

 time since the ice sheet disappeared is more than twice this number of years 

 and as the maximum is many times longer, the Cape would now have been nearly 

 cut away by the sea if some cause had not prolonged its erosion. If the sea 

 began its work when the land stood about 30 fathoms higher than it now stands 

 in relation to sea level in the latitude of the middle of the arm of Cape Cod, 

 the discrepancy may be accounted for by assuming an attack of the sea, brought 

 about after a period of subsidence, on a prism of land that lay between the 

 initial plane of marine erosion at the end of the glacial epoch and the present 

 higher plane. 



Gulliver 2 describes Cape Cod as ' 'a winged beheadland which projects far 

 into the sea, so that the wings do not extend across the bays on either side." 

 Davis 3 has considered at length the change from the presumable original glacial 

 outline of the land to its present form under the action of the sea. Within 

 historical time the headland has receded more than a third of a mile. On the 

 south, in the zone of the winged bars of Chatham, small islands of glacial drift 

 have disappeared. Mitchell 4 gave an account of these changes in a report 

 concerning Nauset Beach and the peninsula of Monomoy, 5 and a brief summary 

 of them is given by W. C. Smith. 6 From these and other sources of information 



1 Davis, W. M., The outline of Cape Cod, Proc. Am. Acad. Arts and Sciences, 31, pp. 302-332. 



2 F. P. Gulliver, Shoreline topography, Proc. Am. Acad. Arts and Sciences, 24, 1899, p. 213. 



3 Davis, W. M., The outline of Cape Cod, Proc. Am. Acad. Arts and Sciences, 31, 1896, pp. 303-332. 



4 Mitchell, Henry, Nauset Beach and Monomoy peninsula, U. S. Coast Survey B,eport for 1871. Ap- 

 pendix 9, pp. 134-143, Sketch No. 3.5. 



6 For a list of the 39 different ways in which this aboriginal name has been spelled and pronounced, 

 see William C. Smith, A History of Chatham, Part I, p. 1, footnote, Hyannis, 1909. Monomoit and 

 Monomoy are variants of the same name. Monomoy is now restricted to the beach south of Chatham. 



6 Smith, W. C, A history of Chatham, Part I, pp. 6-11, 1909, with plate reproducing part of Cham- 

 plain's map of Fort Famine showing the beach at Chatham in 1606. 



