CAPE COD GEOLOGY 



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FRESH WATER SWAMPS AND PEAT BOGS 



Most of the swamps and peat bogs lie on the south side of the island, along 

 the west coast, and along the east coast between Harbor Pond and Sands Pond. 

 They occupy depressions that are presumably of less depth than those which 

 form the site of the numerous so-called ponds or glacial lakelets of the upland 

 parts of the island. All the depressions now occupied by swamps were originally 

 lakelets in which aquatic plants took root. By the addition of other plants, in- 

 cluding some trees as soon as the vegetable mat permitted their growth, the 

 depressions were filled to the level at which water will stand in them. 



Nearly all the swamps yield peat. No exact estimate of the quantity of this 

 peat has been made, but peat was used as fuel from about 1750 until after 1875, 

 in which year 544 cords were dug on the island. According to Livermore l peat 

 has been found on the west coast from highwater mark to a line a quarter of 

 a mile out in the sea. 



MARINE MARSHES 



The vegetal deposits in tidal water are limited to small patches, most of 

 them along the shore of Great Salt Pond. This body of water appears to have 

 had no direct connection with the sea when the island was settled in 1662. 

 About 1686 an opening was made in the beach on the west side in an attempt 

 to form a harbor, which was made but not long maintained. In 1873 another 

 opening was made, which has been enlarged, and the water of the so-called pond 

 has fallen to sea level, with consequent effects on the growth and the area of the 

 marsh bordering it. 



WAVE AND CURRENT ACTION 



The striking effect of the action of the sea on the coast of the island is seen 

 in the steady erosion of Mohegan Bluffs, on the south coast, and of the cliffs at 

 Clay Head. Wave cutting has been everywhere somewhat abated by the bould- 

 ers and smaller erratics that accumulate on the beach. Some of these were 

 derived from the Wisconsin drift that lies on the surface of the island, but many 

 more were derived from the wasting of the bed of Montauk till. At certain 

 points where this bed stands at a high angle in the bluffs the boulders that have 

 accumulated on the beach reveal at once their source. 



The large quantity of clay in the Montauk till and in the underlying Gar- 

 diners clay causes numerous landslides. Large slices of the bluff slip down. 

 Certain gullies show that rainwater helps to carry the bluffs back above the 



1 Livermore, S. T., A history of Block Island. Hartford, Conn., 1877, p. 29. 



