CAPE COD GEOLOGY 



173 



tortion, and therefore the irregularity of the surface, varies, for at some places 

 the clay was exposed to the full force of the ice action and at others it was 

 protected by remnants of older and more resistant beds. The weight of the 

 overriding ice also modified the surface, partly by abrasion, but even more be- 

 cause the plasticity of the clay allowed it to be flattened out where the pressure 

 of the overlying ice was greatest. In places a whole mass of the clay was evi- 

 dently pushed or dragged from its original position over itself or over other 

 beds. Thus at some places the Gardiners clay presents a level, undisturbed sur- 

 face and is overlain by the later Jacob sand, and at others its surface is a 

 well-defined ridge, which may or may not include the Jacob sand. At still other 

 places the surface was kneaded into undulations by the overriding ice and the 

 Gardiners clay is usually accompanied by the conformably overlying Jacob sand. 

 At still other places the surface was originally broken, but its irregularities 

 were smoothed off by the action of the ice. 



* Fossils and Age 



The Gardiners clay is of Pleistocene age, a fact shown by its relation to 

 earlier deposits of unmistakable glacial origin at the north end of Gay Head 

 cliffs, at Nashaquitsa cliffs, and at Norton Point. This clay also overlies earlier 

 Pleistocene deposits on Long Island. As it was laid down later than two earlier 

 beds, one of gravel and the other of till, and probably later even than a second 

 bed of gravel, it evidently does not belong in the earliest Pleistocene. The 

 Gardiners clay overlies these beds unconformably, so that it was probably laid 

 down after a period of erosion in addition to periods of deposition. 



Since the Gardiners clay was deposited, the Jacob sand, the Manhasset 

 formation, and the Wisconsin moraine and outwash have all been formed, 

 and there has been also the Vineyard period of erosion. Thus the Gardiners 

 clay, according to the stratigraphic evidence in this region, appears to be of 

 about mid-Pleistocene age. 



No fossils have been found by the present writer in the Gardiners clay 

 in this area. One is mentioned by Dall, 1 — Modiolaria nigra (now known as Mus- 

 culus nigra), which he stated had been found at "Chilmark, in gray clay." 

 Professor Woodworth has told the writer that this fossil came from the Gardiners 

 clay. Musculus (Modiolaria) nigra, a marine form, is clearly Pleistocene and 

 Recent. Fossils consisting chiefly of marine shells have been found in the 

 Gardiners clay on Long Island. 



1 Dall, W. H., Notes on the Miocene and Pliocene of Gay Head, Marthas Vineyard, Mass., etc., 

 Am. Jour. ScL, 3d set., 48, p. 297, 1894. 



