CAPE COD GEOLOGY 



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(1) The line of disturbance is coincident with the line of the terminal moraine 

 from Nantucket to northern New Jersey; (2) the beds are folded and crumpled 

 only where the moraine has advanced over some part of the coastal plain; 

 (3) folding and crumpling cease abruptly where the moraine bends away from 

 or leaves the plain. To these reasons may now be added another: deep borings 

 on Long Island show that the beds below those that are folded are much less 

 disturbed. The chief obstacle seen by Shaler to the theory of ice folding was 

 the fact that the pre-Wisconsin surface, formed during the Vineyard interval, 

 was very little disturbed by the Wisconsin ice. Shaler thought that if the 

 ice could override the region without altering the surface, it was hardly possible 

 that it would compress the underlying beds into close folds. The fact that 

 there had been earlier and more vigorous ice advances before the Wisconsin 

 stage was then not clearly recognized. 



CONDITIONS DURING DEPOSITION 



The conditions that prevailed on Marthas Vineyard while the Cretaceous 

 deposits were being laid down, as shown by the character of the successive 

 beds, indicate progressive subsidence and the formation of land wash. Early 

 in Upper Cretaceous time the land that stood on the site of the island was low 

 and poorly drained and contained fresh-water swamps. Plants grew in and 

 around these swamps and their remains formed most of the material deposited. 

 As time went on, more clay was brought in, probably because of differential 

 tilting of the land, which increased the gradient of the streams that ran into 

 the areas of deposition. The land was doubtless sinking, for in the later de- 

 posits signs of vegetation gradually became scarcer, and then only horizontal 

 layers of clay containing a few leaves were laid down, the deposits indicating 

 an increasing distance from the area of erosion. At this time the streams must 

 have been made very turbid by the material they brought in, evidently from 

 a region that had long lain at a low level and that had been subject to atmos- 

 pheric weathering. Gradually the land must have stopped sinking and the areas 

 of deposition must have become increasingly shallower, for the succeeding 

 deposits show cross-bedding. The erosion of the land was accelerated, for the 

 deposits became coarser, and grains of quartz and flakes of mica were mixed 

 with the clay. Finally parts of the region must have subsided and have been 

 invaded by the sea, for the deposits contain marine fossils. The record of Cre- 

 taceous time on the island is incomplete; it includes, perhaps, not much more 

 than the first half of Upper Cretaceous time. 



