268 



CAPE COD GEOLOGY 



on both sides of the drainage crease at Craigsville show that till lies either at or 

 very near the surface. Great Island, whose southern part forms Point Gammon, 

 is also a till-covered tract. Another small tract makes up "Fish Hills," called 

 Harbor Bluff on the map, at the entrance to Hyannis harbor. All these till- 

 covered tracts stand out because the sand of the plains on the north does not 

 cover them. 



Large boulders lie at some places along the shore. Those south of Harwich 

 have been noted by Julien. 1 There are small patches of till on the beach at the 

 east entrance of Herring River ; at Dennis Port ; on the shore of Lewis Bay, east 

 of Hyannis; at three points on the beach at Succonnesset and farther north, 

 toward Poponesett Beach; and on Great Neck, in the southern part of Mashpee. 

 These are tentatively referred to the Nantucket substage, but some or all of 

 these deposits may be older. 



It is not possible to distinguish the series of plains on the south side of the 

 Cape that were formed or finished by the wash of sand from the remnant of the 

 Nantucket ice from the series of higher plains farther north, which owe their 

 origin to outwash from the ice sheet at the Falmouth substage ; for the two series 

 are so intricately interlocked that the attempt to make distinctions based not 

 on difference in material but on slope, position, or slight differences of level 

 would be long and laborious. A study either of the topographic map or of the 

 ground shows that the large tracts in front of the high plains that form the 

 southern border of the Falmouth moraine in Chatham, Yarmouth, and Barn- 

 stable were held open by masses of ice outside of the steep-sided ponds. Hun- 

 dreds of small pits and saucer-shaped depressions too shallow to be expressed 

 by the 20-foot contour interval of the topographic map dimple the surface, yet 

 on the map these areas appear to be smooth seaward-dipping plains. 



Amid all the variety of relief due to the deposition of gravel and sand 

 between and around melting masses of the ice sheet in this region of broken 

 plains there are no traces above the present level of the sea of its former action 

 on the surface. It may therefore be inferred that neither at the time of the 

 Nantucket substage of the Wisconsin ice nor since that time has the sea level 

 here been higher than it now is. The ice blocks would have prevented tides and 

 currents from sweeping sand into the kettles and holes in the plains, but after 

 these ice blocks disappeared, tidal action upon these surfaces would have 

 built deltas backward into ponds having excurrent channels, and waves would 



1 Julien, A. A., On the pebbles at Harwich (Cape Cod), Mass., and on rude arrowheads found among 

 them, Science, new ser., 26, pp. 831-832, 1907. 



