266 



CAPE COD GEOLOGY 



that stand nearly perpendicular to the front of the lobe, but in none of the 

 stagnant glaciers in Alaska do such crevasses widen to form broad channels in 

 which deposits of drift have accumulated. In the channels on the south side of 

 Cape Cod such deposits were formed 25 to 30 miles back of the ice front while 

 the ice was at its maximum extent, when it covered the northern half of the site 

 of Nantucket. Certain ponds, such as those at the west base of Great Hill, in 

 Chatham, occupy ice-block holes whose sides are in part formed of much older 

 drift, showing that remnants of the ice there held open ancient channels in 

 what is here called the Chatham plain. Chains of ponds and waterways on the 

 south side of the Cape within these plains appear also to be old channels that 

 were in part filled with more recent drift that gathered around masses of ice, 

 which eventually melted and left depressions. The alignment of the lakelets 

 and ponds may therefore be a reflection of the topography of the Vineyard 

 interglacial stage just before the Wisconsin stage of glaciation. Such channels 

 extend north of the Falmouth moraine in Orleans and are well shown by the 

 trend of Town Cove. Lakelets did not lie in these old channels, because the ice 

 was there deepest and thickest and therefore remained longest. 



Estimates as to the thickness of the ice which stood in these holes depend 

 on evidence as to whether the water supplied by the melting ice built up gravel 

 fans or formed outflowing streams. Many existing depressions are shallow saucer- 

 shaped pits, the ice in which may have been covered by gravel and sand. Such 

 pits are found in Alaska. 1 Great Pond is flanked by fans of drift, which was 

 probably derived from ice that rose above the level of the plain. The small, 

 high tables of drift about Long Pond, in Brewster, appear to have had the 

 same origin. From several ponds water now flows out through channels that are 

 much too large to have been cut by the streams that flow in them. Mashpee 

 and Cotuit rivers are examples of such drainageways. These shallow valleys 

 appear to have been cut by streams that flowed through them when the melting 

 ice in the ponds added water to the run-off arising from rainfall. Many lakelets 

 have such excurrent channels, which start out much above the level of ground 

 water. Ashumet Pond, on the line between Mashpee and Falmouth, is a typical 

 example of this type of glacial lakelet. Whether the water of a lakelet formed an 

 excurrent stream depended upon the height of the water table in the gravel and 

 sand, for the lakelets and ponds may be regarded as great open wells. A depres- 

 sion that does not descend to the depth at which water stands in the sand remains 

 essentially dry. Such dry kettles are found in the northern part of the outwash 

 plain about Falmouth. 



1 Tarr, R. S., and Martin, Lawrence, Alaskan glacier studies, Washington, 1914, PL 76. 



