CAPE COD GEOLOGY 



53 



The typical localities of these three beds are on Long Island. The beds are 

 well exposed in the contorted sections on the south shores of Block Island, No 

 Mans Land, and Marthas Vineyard, but they have not been identified with 

 certainty north and east of Marthas Vineyard. Where a series of beds of like 

 character lies in more or less rhythmic succession, it is uncertain which member 

 is present, unless a fairly complete section is exposed. From bottom to top the 

 series consists of deposits made by torrential streams that poured sand and gravel 

 (the Herod gravel) out from the front of an advancing ice sheet. Upon these beds 

 were deposited the till laid down by the ice sheet, and finally the beds of out- 

 washed gravel laid down over the same region during the retreat of the ice front. 

 Viewed in this light the deposits are the product of a single stage of glaciation 

 corresponding to the Manhasset of Long Island. 



The Manhasset formation has suffered less deformation from ice thrust than ■ 

 any of the older divisions of the Pleistocene series in the islands, but in western 

 Marthas Vineyard and on No Mans Land its beds are strongly folded. 



Herod gravel member.— Much of the gravel of the Herod member is coarse, 

 the largest cobbles measuring 6 inches or more in diameter, but at some places 

 the deposit is prevailingly sandy. The beds are 5 to 10 feet thick and have not 

 been disturbed by the deposition of the overlying Montauk till, though that till 

 and all the underlying beds have been in places much folded by the action of ice 

 subsequent to the deposition of the Manhasset beds. From analogy with other 

 beds of glacial gravel, it is presumed that the Herod gravel was laid down in the 

 open air on plains of the Jacob sand just at or above sea level. On the southwest 

 coast of Marthas Vineyard and thence westward to places on Long Island the 

 beds of gravel at this horizon are readily recognized in the sections, where they 

 lie between the Jacob fine sand and the boulder-bearing beds of the Montauk till. 

 The beds of sandy gravel seen near the tops of bluffs on Nantucket, overlying a 

 fine sand taken to be the Jacob sand, may be eastward extensions of the Herod 

 gravel, but the sections are not well exposed. The bed has not been certainly 

 identified on Cape Cod, nor has it been recognized about Boston, though certain 

 thin gravels in Third Cliff, near Scituate, are thought to belong at this horizon. 



Montauk till member.— On Long Island a well-defined bed of boulder clay 

 lies near the middle of the Manhasset gravel and here and there shows a lateral 

 passage into layers of assorted gravel and small boulders. At Montauk Point, 

 on Long Island, the till is thicker and was traced to Block Island and Marthas 

 Vineyard. On western Long Island there is a boulder bed that is at few places 

 more than 5 feet thick, but farther east, in Mohegan bluffs, on the south coast 



