THEORETIC DISCUSSION 601 



and gravel above is seen in the Hudson Valley, but such structure might 

 be more readily produced in a valley with lateral inflow than in the open 

 sea. It is possible that imconformities of similar origin might be inter- 

 preted as evidence of multitple giaciation. 



Within the range of vigorous rivers the earlier marine deposits would 

 be more or less buried under the shallow water and stream detritus, the 

 latter grading upward from fine to coarse, in this respect being the 

 reverse of the deep-water deposits. Over wide areas unaffected by rivers 

 or by tidal scour the deep-water silts and clays would be lifted into the 

 air as the surface material, as in the lowlands of the Boston district. 



The complete ideal succession of the deposits initiated in deep water 

 may rarely be exposed, but two elements are common, namely, the pell- 

 mell gravels and the superficial clays, especially in eastern and south- 

 eastern Massachusetts, where the oceanic waters were deepest. 



When the ice-front had receded to higher ground, so as to be faced 

 by shallow water, the glacial outwash would build sandplains, the deltas 

 of the glacial streams ; and Avhen the ice-front had backed away entirely 

 from the sea and lay on the exposed land, all the streams, of glacial or 

 of land drainage, would build deltas in the standing water. Such deltas 

 were inevitable, and they mark the highest stand of the sea or lowest 

 of the land at the time of the ice removal in any district. These sum- 

 mit-level deltas are the heaviest deltas of the streams, for the reason that 

 each stream then had its greatest burden -of detritus, the glacial outwash 

 being added to the land drainage, and the latter then having its heavies i. 

 load, derived from the newly exposed, drift-covered land surface. 



Subsequently, during the slow rise of the land, the reduced load of the 

 streams, and this of finer material, was distributed along their course- 

 in the sealevel waters, being commonly spread about as filling of low 

 areas instead of forming distinct deltas. The summit-level delta, the 

 record of the deepest submergence, is practically the only heavy, well 

 marked gravel plain in each river valley. These early gravel plains are 

 the most inevitable and the most reliable features for determinino- thc^ 

 approximate amount of land uplift since the ice-sheet deserted the 

 district. 



With the rise of the land and the shallowing of the sea the zone of 

 effective wave action was slowly lowered over all the inferior land surface. 

 The localized glacial deposits of gravel (kames), consisting of coarse 

 and well rounded stuff, and therefore sensitive to wave impulse, w^ero 

 more or less leveled, truncated, or planed. The Newtonville sandplain 

 is an example of a partially leveled kame area. In eastern Massachu- 



