SOME GLACIAL WASH-PLAINS. 109 



low grounds alon*^ essentiall}'^ contemporaneous ice-fronts 

 shows that sea-level could not have afforded the control 

 which has limited the upward growth of wash-plains. 

 This view of course docs not exclude the possibility' of 

 certain low-lying plains near the coast being deposited 

 under the marine limit ; but the wash-plains themselves 

 have not as yet, it seems to the writer, been made to fur- 

 nish the criteria of marine deposition. Beaches, fossils, 

 and wave modified glacial deposits are much better indi- 

 cations of submergence than deltas which are in this 

 region identical in form and surroundings with similar 

 glacial accumulations found under circumstances where 

 no submergence is supposed to have taken place. 



STAGNATION OF ICE-SHEET. 



The mode of deposition of the wnsh-plains and accom- 

 panying morainal deposits above outlined in this paper 

 affords a clew to the relative areas of stagnant and live 

 ice during the retreat of the glacier across this field. 

 The facts demanding stagnation are found in the numer- 

 ous ice-block depressions and in wash-plains with heads 

 which show no forward movement of the ice-sheet, either 

 by the failure of shoving in the gravels or by the lack of 

 morainal deposits in the terrace at the wash-plain head. 

 The facts demanding live ice at intervals during the retreat 

 are the lines of boulder-belts, positions marking halts of 

 the ice-front during which backward melting equalled for- 

 ward movement. A similar demand is made to explain 

 displaced and overridden glacial deposits, as in the case 

 of the Fresh Pond area in Cambridge. It will also be 

 shown that the distribution of prominent belts of wash- 

 plains can only be explained on the supposition of a forward 

 movement of the ice. 



