CAPE COD GEOLOGY 



183 



Fresh exposures of the till are bluish-gray, but this color is seen only in the 

 lower part of a bed. The upper part is usually brownish, or even chocolate- 

 colored, a shade due to the oxidation of some of the iron in the till. Where 

 the till is made of largely reworked Gardiners clay it has the dark gray or nearly 

 black color of that bed. Most of the boulders in it are granite and gneiss, but 

 it contains many other kinds of rock. Some of the boulders and pebbles are 

 angular and show strong glacial striation. The largest boulders are 6 feet in 

 diameter, but many exposures show scarcely a boulder over a foot long. 



Thickness. — The maximum thickness of the Mont auk till on Marthas 

 Vineyard is 40 feet. 



Source of material. — The material of the Montauk till is obviously glacial. 

 The heterogeneous mixture of boulders, gravel, and clay and the shape of the 

 larger constituents and their characteristic markings show conclusively a glacial 

 origin. The coarser constituents of this till must have come from the mainland 

 on the north, for such rocks are not found in place nearer than the north side 

 of Buzzards Bay. The finer constituents were probably derived from the older 

 Pleistocene beds, all of which show considerable erosion by the Montauk ice. 

 The gravel must have been in large part derived from the Herod gravel, and 

 perhaps in less amount from earlier gravel. The clayey elements and the rock 

 flour produced by the Montauk ice were probably derived from the Gardiners 

 clay and the Jacob sand; in fact, parts of the Montauk till closely resemble 

 the Gardiners clay and are undoubtedly made up of reworked clay of that 

 member. It is impossible, however, to determine the sources of all the material. 

 Some of it may have been brought from the area now occupied by Vineyard 

 Sound and Nantucket Sound. 



Relation to other deposits. — At some places the Montauk till rests con- 

 formably upon the Herod gravel; at others it rests unconformably upon the Jacob 

 sand or the Gardiners clay. The contact is usually marked by disturbances 

 such as would be made by an ice sheet overriding unconsolidated beds. These 

 disturbances consist of the removal of some part of the lower beds and the 

 warping, folding, and even faulting of the part of the bed that lies directly in 

 contact with the till. 



The Hempstead gravel rests conformably on the Montauk till. This gravel, 

 however, is not nearly so extensive as the Herod, and at some places the Mon- 

 tauk is overlain directly by the Wisconsin. At many places where the contact 

 between the two cannot be seen it is impossible to distinguish the Hempstead 

 from the Wisconsin. If the contact is conformable the gravel must be the 



