CAPE COD GEOLOGY 



137 



SECTIONS IN THE CLIFFS 

 The cliffs afford the best sections on the island. Many of them have been 

 almost continuously cut back by the waves and show the deposits well, except 

 where the nature of the material causes it to slide down. The structure of a cliff 

 as worked out one year may not be the same in later years. Many of the cliffs, 

 especially those in which the retreat is rapid and the dip is high, change so 

 greatly from one year to another, both as to contour and as to the beds ex- 

 posed, that none of the features first seen can be found again. 



SECTIONS IN THE INTERIOR 



The older deposits are exposed at but few places in the interior of the 

 island, and they show their character and structure only in fresh and fairly large 

 exposures. The exposures consist of sand and gravel pits, a few road cuts, and 

 pits from which clay was dug many years ago. As sand and gravel can be ob- 

 tained almost anywhere on the island and are not much used, the pits from 

 which they have been taken are small. A few cuts along the state road expose 

 the later deposits. The old clay pits in the western morainal area help the geo- 

 logist to trace the distribution of the Cretaceous beds, but they are usually filled 

 with water, so that only the material in the dump indicates what is under- 

 ground. 



LITHOLOGICAL SIMILARITY OF THE PLEISTOCENE DEPOSITS 

 The lithological similarity of the Pleistocene beds is particularly marked in 

 the waterlaid deposits of the Manhasset formation and in the Wisconsin de- 

 posits. These have about the same size of grain and are apparently composed 

 of much the same material. The upper and the lower members of the Man- 

 hasset formation could not be told apart were it not for the bed of till (Mon- 

 tauk till member) that separates them, nor could these two beds of Manhasset 

 gravel be distinguished from the outwash gravel of the Wisconsin stage except 

 by considering the difference in their distribution. The Wisconsin till, although 

 it is usually much sandier than the till member of the Manhasset, is at some 

 places so nearly like the Manhasset till that it can be distinguished only by 

 its position and by the fact that it is separated from the Montauk till by a 

 bed of gravel. Where this older till contains few pebbles and is made up chiefly 

 of reworked Gardiners clay it so closely resembles the Gardiners that it can 

 be identified only by its position. It is not possible to distinguish the various 

 Pleistocene deposits by differences in the amount of weathering. 



