3 16 MYRON L. FULLER 



(p. 991). According to the evidence presented, the clays are 

 contemporaneous with an earlier ice advance, and are clearly 

 older than the last, but nothing definite as to the length of time 

 intervening is known. 



In 1898, Shaler in his paper on "The Geology of the Cape 

 Cod District," 1 again recognized the existence of two tills, 

 between the deposition of which a period of great length inter- 

 vened. In this interval he recognized the deposition of three 

 sedimentary formations: 2 the Nashaquitsa, the Barnstable, and 

 the Truro, each of which was followed by prolonged periods 

 of aqueous erosion. This interglacial time was regarded as 

 vastly longer than that which has elapsed since the disappear- 

 ance of the ice of the last invasion. 



DESCRIPTION OF TILL EXPOSURES 



The ordinary till exposures in southeastern Massachusetts 

 present the following characteristics. At the top lies a light buff 

 till consisting of the usual heterogeneous mass of clay, sand, and 

 bowlders. The percentage composition of this till varies within 

 wide- limits, especially in regard to the quartz-flour and clay con- 

 stituents which range from a combined amount of perhaps 10 

 per cent, or less in some of the tills in the southern portion of 

 the state to an average total of some 55 per cent, in the drumlins 

 about Boston. 3 In most sections the till is moderately oxidized 

 from top to bottom, as indicated by its buff color, but where 

 natural or artificial cuts have exposed it to any considerable 

 depth it is found to pass downward into an unoxidized portion 

 of a gray or bluish-gray color, usually designated as blue till. 

 The depth to which the oxidation extends presumably depends 

 somewhat on the percentage of the clay constituent of the till. 

 Though the oxidation is very much less conspicuous in tills high 

 in sand, the depth to which the oxidation extends is probably 



'U. S. Geol. Surv., Eighteenth Ann. Rept., Pt. II, pp. 497-593. 



2 Loc. cit., pp. 535-538. 



3 W. O. Crosby : Composition of the Till or Bowlder Clay. Boston Soc. Nat. 

 Hist. Proc., Vol. XXV, p. 25. 



