86 BULLETIN OF THE ESSEX INSTITUTE. 



that it was built against the edge of melting ice ; the ab- 

 sence of boulders fioni the plain shows that the boulders 

 on the surrounding mounds did not come to their positions 

 from floating ice, else some erratics must have dropped on 

 the plain. While boulders are rarely found on the actual 

 surface of sand-plains, they are frequently found at the 

 same level on the surface of till continuous with the sand- 

 plain topography, and boulders have been seen sparingly 

 in the sand-plain itself, particularly near the head, as at 

 Woodland, Mass., where a boulder probably floated out 

 on ice in the early stages of deposition. One of the plains 

 in the Narragansett Bay region is coated with angular 

 blocks and some till indicating clearly an advance of the 

 ice-sheet over the field. Even on the hypothesis that 

 plain level was marginally at water level, it is rather sur- 

 prising to note the absence of boulders from characteristic 

 wash-plains. 



The iceward mai'gin of wash-plains. — The head or 

 highest part of wash-plains is towards the ice or the source 

 of the detritus. There are two classes of plains as re- 

 gards the topographic features of their iceward margin, 

 viz. : (a) plains with a terrace confronting low interglacial 

 ground north of them; {h) plains, without terraces, con- 

 fronting till-covered areas usually rising above plain level. 

 These types are illustrated by the Nantucket plain on the 

 one hand, and that of Martha's Vineyard on the other. 



We sometimes find kames and eskers associated with 

 plains having an iceward terrace ; but kames and eskers are 

 quite as frequently absent as present. We must, therefore, 

 conclude that there is no necessary relation between the 

 formation of kames and eskers and the pouring out of 

 gravels and sands from the ice to make plains. It is im- 

 portant to perceive this want of dependence l)etween intra- 

 glacial and extraglacial deposits in formulating an hypoth- 

 esis for the stream action which produces the wash-plain. 



