CAPE COD GEOLOGY 



255 



For a time it seemed that this Devonian erratic must have been brought 

 by an ice sheet from Maine or from an area north of Boston to Marthas Vineyard ; 

 but the pebble lay in the field of the lobate moraine west of the Plymouth inter- 

 lobate axis and was associated with striae of a glacier that swept down from 

 central New Hampshire across the interior of the Boston and the Narragansett 

 areas and lay quite out of the field of action of the glaciers that ran southeastward 

 across Maine. Shortly before his death Prof. A. C. Packard showed me a quartz- 

 itic pebble from the Carboniferous conglomerate east of Providence, in the 

 Taunton syncline, containing a species of Spirifera closely resembling the form 

 described by Williams. The Devonian pebbles of southeastern Massachusetts 

 therefore evidently also came into the Pleistocene drift from Carboniferous con- 

 glomerate. The Devonian and Cambrian pebbles in the supposed Jameco 

 gravel under Highland Light on Cape Cod and any that may be found in the 

 higher drift beds of the region can therefore hardly be used as a basis of definite 

 conclusions as to the direction of motion of the Pleistocene ice sheets. These 

 pebbles appear to have been shifted northward in Pennsylvanian or Permian 

 time into the gravel beds at a time of local mountain glaciation, 1 and to have 

 been carried southward during the Pleistocene epoch. Upper Cambrian pebbles 

 could not have been brought to southeastern Newfoundland during Pleistocene 

 time because an ice sheet coming from that small gathering ground to Nantucket 

 and Cape Cod would have to maintain its position and course directly along the 

 front of a great regional glacier that was pushing down over New Brunswick, 

 Maine, and eastern New Hampshire, on the edge of the continental platform, 

 in a position that seems physically impossible. The fact that no ice mass radiated 

 out far to the southwest and west is shown by the absence of glacial deposits 

 from the Magdalen islands, in the St. Lawrence Gulf, except some deeply weath- 

 ered early Pleistocene till discovered by Goldthwaite 2 in 1913. The position of 

 Upper Cambrian fossiliferous sandstone containing Obolus on the Avalon penin- 

 sula, in southeastern Newfoundland, is quite consonant with the probable 

 extension of a band of these rocks southwestward off Nova Scotia and the 

 present southern coast of New England in late Paleozoic time. 3 



GARDINERS (?) CLAY 

 Beds of dark clay may be seen along the shore of Cape Cod above sea level 

 at several places, as at Sandwich and thence eastward to West Barnstable; 



1 Robert W. Sayles, The Squantum tillite, Bull. Mus. Comp. Zool. Harvard College, 66, 1914, pp. 

 141-175; 12 pis., including map of country south of Boston showing localities. 



2 James Walter Goldthwaite, The occurrence of glacial drift on the Magdalen Islands, Museum Bull. 

 Canada Geol. Survey, No. 14, geol. ser. No. 25, May 12, 1915, p. 11, with maps and illustrations. 



3 See C. D. Walcott's chart showing the distribution of Cambrian formations in the twelfth Annual 

 Report of the Director, U. S. Geol. Survey, for 1890-91, pt. I, pi. XLII, p. 532. 



