CAPE COD GEOLOGY 



109 



over Break will doubtless be again repaired by the sea that made them in its 

 transportation of the waste that it has wrested from the cliffs. 



Owing to the retreat of the south coast, the ponds there that lie in the beds 

 of glacial streams and that are held in by barrier beaches have been shortened 

 and must continue to decrease in length. The name Forked Pond, given to the 

 eastern member of this series of ponds, shows that within historic time it had a 

 branch, or branches, and resembled Hummock Pond, which lies farther west. 

 Of the Weeweeder ponds, now mere marshes, only the prongs remain. If we 

 compare the system of creases on the south shore of Nantucket with the system 

 on Marthas Vineyard we may infer that Nantucket has been cut back a mile or 

 more by the sea. The convergent group of creases in Nobadeer Pond and just 

 east of it would have had their confluence half a mile south of the present coast, 

 and if the Nobadeer crease and the Madequecham crease became confluent 

 they would have joined not less than a mile and a half south of the present coast. 

 Marindin, in the paper cited above, estimates the rate of recession of this coast 

 at 1.42 metres a year, which is equal to 1.42 kilometers (0.88 miles) in a thousand 

 years. The southward inclination of the surface of the outwash plain would 

 carry it below the level of the sea in the region west of Tom Nevers Head about 

 a mile south of its present position. If the land had been stable while this cutting 

 has been in progress this part of the island would have been cut back in 1,200 

 years, but the plain evidently turned southeastward at the place where Tom 

 Nevers Head now stands, and this part of it protected the western coast from 

 the strong southeast storms. Erosion was then probably slower, and the time 

 consumed by it was probably more than 1,200 years. 



The ponds in the channels of glacial streams indicate submergence since 

 the streams ran across the plain, at the end of the first "Wisconsin ice advance. 

 Peat and swamp material are found submerged at several places along the coast, 

 and are mentioned by Shaler. 1 Submerged stumps led him to believe that the 

 island had undergone a sinking, a positive deleveling, of 2 feet since the trees 

 grew, but this subsidence may have occurred many hundred years ago. The 

 old beach on the west side of Coskata, as Dr. J. Howard Wilson points out, 

 appears to have been but little depressed within the time it has taken Coatue 

 Beach to be formed in front of it, though the beach may have sunk two feet with- 

 out affording proof of a change of level. Aside from the evidence presented and 

 considered by Shaler I am not aware of any fact that would warrant the state- 

 ment that subsidence is now going on along the coast of the island. 

 1 Shaler, N. S., Geology of Nantucket, pp. 28-30. 



