REPRESENTATIVES OF PRE-WISCONSIN TILL 3 I 7 



greater than in the more clayey tills. In the Boston drumlins, 

 which are high in clay, the oxidation has usually reached a depth 

 of some twenty feet. Where the bedrock upon which the tills 

 rest is exposed to view it is ordinarily found to present well gla- 

 ciated surfaces, showing no traces of decomposition beyond a 

 thin superficial zone seldom more than half an inch in thickness, 

 and often represented only by a slight surface discoloration. 



CENTER STREET EXPOSURE, BROCKTON 



In marked contrast to the section just described are the sec- 

 tions exhibited by the older tills observed by the writer. The 

 first of these old tills to be observed was exposed several years 

 ago during excavations for the foundations of one of the heavy 

 stone arch bridges which were necessitated by the abolishment of 

 the grade crossings of the New York, New Haven and Hartford 

 Railroad. (Fig. 2, Exposure I.) The first six feet or so from 

 the surface was of the ordinary slightly oxidized buff till of the 

 last, or Wisconsin invasion, but on going deeper, instead of 

 becoming lighter and gradually merging into theunoxidized blue 

 till as in the ordinary typical section, the buff till gave place 

 abruptly to a mass of distinctly red and yellow till. This till at 

 the time it was seen by the writer was exposed to a depth of 

 about four feet, but was later excavated to a depth of from two 

 to four feet more, at which point it was found to rest on a deeply 

 decomposed and highly oxidized conglomerate of Carboniferous 

 age. Besides its high colors, due to the advanced state of oxi- 

 dation, the lower till was found to differ in a marked degree in 

 composition from the ordinary buff till. Clay, including quartz- 

 flour, which in the overlying till forms less than one half of its 

 bulk, constitutes nearly the whole of the lower till. The pebbles 

 of the upper till comprise some 25 per cent, or more of its 

 mass and are varied in type, complex in composition, fresh 

 in appearance, and have often been transported considerable dis- 

 tances. In the lower till, on the other hand, the pebble compo- 

 nent probably never exceeds 10 per cent, of the mass, and only 

 the resistant quartz and quartzite pebbles from the underlying 



