542 r. G. CLAPP GLACIAL PERIOD IN NEW ENGLAND 



contains more abundant boulders than the underlying clay, and, so far as 

 known, is destitute of fossils. The presence of boulders in this part of 

 the clay more abundantly than in the lower part indicates that the upper 

 part of the clay has been reworked l)y the ice; in other words, it is in 

 reality till. 



A good example of this superficial stratum is found in Edmester's 

 brick-yard in Eevere, Massachusetts, where the section is as follows : 



Section in Clay Pit at Revere, Massachusetts 



Material Feet 



3. Peat 4 



2. Butt", very stouy till, with botalders up to one foot in diameter 2 



1. Buff, blocky clay, with a few pebbles G 



Total 12 



Similar exposures were seen in other pits near Eevere and in Lynn, 

 Saugus, and Danvers, Massachusetts. 



An important question which must be answered in every observation of 

 till over clay is whether the upper material is positively "till." We know 

 tliat deposits of gravel, both angular and rounded, occur in places as beach 

 deposits overlying cla}^, and in such cases it is frequently difficult to dis- 

 tinguish their true mode of origin. Certain till-like deposits seen on 

 Mount Desert island and in Washington county, Maine, are of this tyjie. 

 The exposures described at Newburyport, Exeter, and many other places, 

 however, are in situations where the deposits could not have been formed 

 through wave action. Both of these deposits are too extensive to be ice- 

 berg-dropped. It is true, as has been said, that the latest till (Wisconsin) 

 contains rounded pebbles much more abundantlj^ than striated ones, but 

 such an occurrence is to be expected on account of the peculiar nature 

 and relations of the Wisconsin ice-sheet (page 545). Moreover, Avhile 

 striated boulders are not abundant in Wisconsin till, they do ocmr. and 

 for this reason Wisconsin till on the lee side of rock hills can frequently 

 not be distinguished in lithological characteristics from, Montauk till. 

 This is illustrated in numerous exposures in the vicinity of Boston, where 

 the only method of recognizing the till as Wisconsin is by its slighter 

 amount of oxidation than the Montauk. In this region the exceptional 

 clayey nature of the Wisconsin till is due to the movement of the ice over 

 broad clay plains and Montauk till deposits farther north. 



6. Older topography and greater elevation of some clays than of certain 

 clays believed to be of Wisconsin age. — Some of the best evidence of the 

 pre-Wisconsin age of the high-level marine olay is derived from its dif- 



