CAPE COD GEOLOGY 



75 



glacier, or where material was carried out on the plain from the ice front by 

 glacial streams. 



Boulders are particularly abundant in the morainal hills on Cape Cod from 

 the vicinity of Orleans westward to Buzzard's Bay and thence southwestward to 

 Woods Hole, on the iceward border of the mass of sand that forms the highest 

 part of the land. This belt of country lies along the ice contact, where boulders 

 dumped from the melting ice front would naturally be found. A similar line of 

 bouldery moraine is traceable over the Elizabeth Islands. 



On Nantucket and Marthas Vineyard the surface mapped as till or frontal 

 moraine was originally much more bouldery than it is now, for many boulders 

 have been broken up to make foundations for houses. As a result of this use of 

 the only available supply of large blocks of stone on Nantucket, very few boulders 

 remain there. In the settled parts of Cape Cod and of the islands the smaller 

 boulders and blocks of rock have been gathered into stone fences; and in some 

 areas, particularly on Block Island, the smaller stones that were not suitable for 

 building fences have been thrown into the numerous pits that indent the surface. 

 Notwithstanding these uses of the blocks, boulders still encumber the surface 

 of large tracts in a way to impede or prevent the use of modern methods of culti- 

 vating the soil and reaping grain. 



None of those high heaps of boulders which many existing glaciers accumu- 

 late in their frontal moraines are found in the district, or indeed anywhere along 

 the Wisconsin frontal moraines in southern New England. The nearest approach 

 to such a pile of boulders along the ice front is seen near the middle of Marthas 

 Vineyard, in North Tisbury, on the south side of the northern ridge of the island, 

 on land once owned by Professor Shaler. Certain tracts in the ice contact at the 

 head of the Falmouth outwash plain in and about Bournedale, on the south side 

 of the Cape Cod Canal, display a marked huddling of blocks, but no distinct 

 pile of boulders along the ice front is seen in the Cape Cod morainal tract. 



On Marthas Vineyard, No Mans Land, and Block Island, where boulder 

 beds of older Pleistocene age crop out in the cliffs and appear to be truncated at 

 the surface of the ground by erosion, erratics brought to the islands at different 

 times may be commingled on the present surface. On the beaches at the bases 

 of cliffs in which older beds of boulders lie beneath a top sheet of Wisconsin 

 drift, it is impossible to distinguish boulders from till. It is, therefore, not possible 

 to determine the direction of the motion of the ice by studying the boulders in 

 connection with their parent ledges on the mainland, for these erratics may have 

 been carried some distance forward during one advance of the ice and finally laid 

 down after a short journey by ice moving in a different direction. 



