326 MYRON L. FULLER 



character, and the same distinct or even sharp division from the 

 overlying tills, all seem to point to an origin similar, and 

 probably contemporaneous with that of the Brockton tills. 

 The deposition of the tills is believed to date from the time of 

 earliest Pleistocene ice advance. 



POSSIBLE INTERGLACIAL ROCK DISINTEGRATION 



A further reason for considering the tills composed of 

 highly oxidized or disintegrated material as representing the first 

 ice invasion lies in the fact that the weathering is distinctly 

 more advanced than in the exposures of what, seems likely to 

 prove to be examples of interglacial weathering. It has been 

 seen that at the advent of the first ice sheet the rocks of the 

 region were deeply decomposed as, for example, the conglomer- 

 ate at Brockton. It is also known that where the conglomerate 

 was so situated as to receive full benefit of the erosive action of 

 the ice of the last advance the ledges are perfectly fresh. 

 Between these two extremes there are numerous examples of a 

 partial breaking up of the ledges by atmospheric agencies, and a 

 partial disintegration. Such a case is illustrated in Fig. 6. 



The moderate amount of decay exhibited by ledges of this 

 class, as compared with ledges known to be pre-glacially decom- 

 posed, or with tills formed from such decomposed material, and 

 the considerable amount which they show as compared with the 

 freshly glaciated ledges of the last ice advance, seem to make a 

 plausible case in favor of the view of interglacial weathering. In 

 this case we have a rough measure of the time from the earliest 

 of the Pleistocene ice advances to the present time, for both the 

 field relations and weathered character show that the conditions 

 mainly antedate the last of the ice invasions. The evidence of 

 this weathering, if it be accepted as interglacial, is indicative of 

 the great length of such time as compared with that which 

 has elapsed since the final disappearance of the ice. 



The preservation of these ledges evidently depended in many 

 cases upon the character of the topography, but this is not 

 always the case. The general explanation probably lies in the 



