CAPE COD GEOLOGY 



111 



may not be more than a very few centuries old, but even two or three hundred 

 years ago the head of the harbor and the beach barring out the sea between 

 Coskata and the main island must have been several hundred feet east of their 

 present position, so that the shell heaps were laid down on the side of the present 

 harbor rather than at its extreme eastern corner, where Haulover Break now 

 joins Coskata islet. The erosion of the clayey till on the south side of Coskata 

 would have made the bottom of the harbor muddy and, therefore, favorable to 

 the growth of the clam (Venus mercenaria) , and pebbles here and there would 

 have afforded seats for oysters. The addition of sand from the coast on the 

 ocean side as the head of the harbor moved westward would have in time changed 

 the character of the bottom off the point where the shell heaps are now found, 

 and the change from clam shells to oyster shells may mark this incident in the 

 history of the island. 



In a small heap of shells at the top of the bluff at Squam Head, near Wau- 

 winet, I collected shells of Helix alternate;,, a species said to be extinct on the 

 island. The snail now living on the island (Helix hortensis) has been brought in 

 by Europeans. If Binney is right in saying that Helix alternata is not found near 

 the seacoast, the sea must have been much farther out when the shell heaps 

 were made and this species was living on Marthas Vineyard and Nantucket. 

 The land may have since been depressed in relation to the sea or erosion may 

 have brought the seashore to its present place. Both depression and erosion 

 may have occurred. 



RELATION OF SHELL HEAPS OR KITCHEN MIDDENS TO DUNES 

 AND BLOWN-SAND TRACTS 



All the small patches of Indian shell heaps or kitchen middens found in the 

 bluff along the coast crop out beneath layers of fine wind-blown sand. Thus it 

 appears that the coastal strip of wind-blown sand that surmounts the bluffs 

 on the eastern and northern sides of the island is more recent than the shell 

 heaps. This natural relation is a result of the retreat of the bluff and the blowing 

 up of fine sand to its top and a few yards inland. The occurrence of kitchen 

 middens beneath a layer of sand at the present coast line probably means that 

 the site of the shell heaps was not far from salt water and that the retreat of the 

 bluffs has not exceeded a few hundred feet. At some places, however, it may 

 have amounted to as much as half a mile, if the position of certain shell heaps on 

 Marthas Vineyard and Chappaquiddick Island are significant. 



