104 



CAPE COD GEOLOGY 



HYPOTHETICAL CONSTRUCTION OF THE CORRUGATED NANTUCKET 

 MORAINE BACK OF THE FOSSE 



The evidence now available of the crumpling of the beds in the so-called 

 submarginal moraine makes it possible to offer a better explanation than has 

 yet been given of the mode of action of the ice sheet back of the outwash plain. 

 If the marginal ice rested on the relatively even older Pleistocene surface, as soon 

 as the sand and gravel washed out from the ice front had been banked up to a 

 height that would obstruct the forward motion of the bottom part of the ice, 

 the ice above a certain prism lying back of the head of the plain would begin 

 to shear up. The shear plane would dip inward and downward toward the ice, 

 cutting off a prism of stagnant ice from the live ice back of it and above it. Ac- 

 cording to this conception the topography of the head of the outwash plain 

 and the breadth of the fosse occupied by the prism of dead ice indicate that this 

 shear or thrust plane had an inclination from the horizontal of about 50 feet to 

 the mile from the present head of the plain, which stands at an elevation of 60 

 feet. The pliable material behind this thrust plane, below the bottom of the 

 ice, would be dragged and kneaded by the motion of the glacier. It is conceiv- 

 able that successively higher slices of the ice at its front, such tongues of ice 

 as the Mer de Glace, may have overridden the lower parts of its mass, leaving 

 relatively inert, detached sections beneath it. This theory would explain the 

 succession of ridges and hollows of the Nantucket moraine, especially as the 

 alignment of the under part of the ice front could hardly have been more orderly 

 than the mounds and hollows seen in the Shawkemo Hills. 



Some evidence that the ice sheet overrode the outwash plain is afforded by 

 beds of sandy-gravelly till, containing small striated pebbles, seen in railroad 

 cuts southeast of the town of Nantucket. It is evident that the ice advanced 

 over many deltas, although the ice contact slope at the head of the plain is sharp 

 and distinctly preserved, showing the mean position of the ice front during most 

 of the time it stood along this general line. 



The gravelly or rubbly layer at the top of the cliff near Sankaty Light con- 

 tains fragments of the following kinds of rocks : 



Igneous rocks: A diabase with pronounced ophitic structure; hornblendic granitite; 

 diorite. Rocks of these varieties crop out along the east coast of Massachusetts north and 

 south of Boston. 



Sedimentary rocks: Pebbles of Upper Cambrian rocks containing casts of Obolus sp. 

 [Westonia rogersi Walcott] identical with pebbles found in the Carboniferous conglomerates 

 of Massachusetts and Rhode Island. Fine red sandstone resembling the Carboniferous red 

 rocks about North Attleboro, Mass. Gray compound conglomerates identical with the con- 



