CAPE COD GEOLOGY 



77 



larly numerous on the south side of Cape Cod between Falmouth and the 

 east coast. 



There are marine marshes on the west side of the upper arm of Cape Cod 

 and elsewhere behind barrier beaches wherever shallow water gives opportunity 

 for the growth of vegetation. Marshes of this type occur between Plymouth 

 Heights and the Peaked Hill bar beach, as well as along the south shore of Cape 

 Cod Bay, near Barnstable and Sandwich. The largest tracts of marsh are those 

 of the Pamet River (partly converted into freshwater swamp by a dike at Truro 

 village) and the marshes behind the beach between Nauset Harbor and the 

 Nauset coast guard station in Eastham. 



Except where they are cut back by the sea or covered by beaches of shingle 

 the marshes are slowly increasing in area, and since 1620 have closed up several 

 small shallow embayments, particularly where the inwash of silt by the tide has 

 helped the growth of plants. The inner part of Wellfleet Harbor (see Plate 2) was 

 once a small port in which vessels rode at anchor. 



From point to point, along the coast, a fresh-water swamp deposit containing 

 logs may be seen at or just below sea level, where it has been exposed by the cut- 

 ting away of the drift around a kettle hole in which the swamp was formed, its 

 original position protecting it from the intrusion of salt water. Such a deposit 

 was exposed on the beach in front of the Nauset coast guard station in 1916. 

 Deposits formed long ago in salt water marshes are seen at many places along 

 the beach between high and low tide. Water-worn boulders and pebbles of such 

 deposits are thrown ashore along the outer side of Cape Cod and fishermen report 

 that they encounter marsh deposits on the outer bar at considerable depths below 

 tide when they are driving the poles for their fishing gear. Masses of peat cast 

 up by the waves are often seen on the south shore of Gay Head. 1 



WIND ACTION AND DUNES 



Cape Cod and the New England Islands are often exposed to violent storms. 

 The prevailing southwesterly wind is registered in the shape of the trees that 

 get a foothold on the coast. Wherever the scant sod on the sandy and gravelly 

 plains becomes broken, particularly near the edges of bluffs, where vortical 

 movements of the air are set up, the wind sweeps away the fine sand, leaving 

 saucer-shaped hollows strewn with lag gravel, the transported sand and dust ac- 

 cumulating now on one side, now on another of the hollow. In these hollows and 



1 On the nature of the New England marshes see Edson S. Bastin and Charles A. Davis, Peat deposits 

 of Maine, U. S. Geol. Survey Bull. 376. 



