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in Cohasset and their almost complete absence in the Nantasket 

 peninsula is due to the general absence of barriers capable, 

 especially at the higher levels, of retaining the water from the 

 melting ice-sheet. The temporary lakes indicated by the sand- 

 plains of the South Shore must, however, have owed their exist- 

 ence in every instance to a wall of ice on the north — the front of 

 the I'ctreating ice-sheet. Durino; the recession of the ice the 

 imprisoned waters north of the water-parting found outlets at 

 successively lower levels, and it is undoubtedly to the 

 somewhat abrupt changes of level in the lakes thus determined 

 that we owe the broadly terrace or step-like arrangement of the 

 sandplains which is observed on passing up from the Coastal 

 Plain across the Lower and Glad Tidings Plains to Liberty 

 Plain. It follows from this that, as in the case of ordinary 

 terraces, the upper plains are the oldest and the lowest were 

 formed last. At each level the glacial lakes received the 

 natural drainage of the low water-shed on the south and of 

 the ice-sheet itself, with its superficial rivers, on the north. 

 The detritus deposited in the lakes, like the water, must have 

 had a double origin, coming pai'tly from the washing of ordi- 

 nary till from which the ice had retreated and partly from the 

 englacial till of the ice-sheet. The glacial streams had 

 undoubtedly, in many cases, deeply grooved and divided the 

 marginal portion of the ice, and in these grooves or channels 

 was accumulated, after the manner of rivers, the narrow 

 deposit of sand and gravel, which, when the ice melted away, 

 became the steep, winding ridges which we call kames and 

 eskers. At the mouths of these glacial rivers, delta-plains 

 were formed in the lakes, adjacent deltas often becoming con- 

 fiuent. The plains are thus naturally coincident in height with 

 the eskers, and often appear as expansions of the latter. The 

 remarkably steep marginal slopes presented by the plains at 

 some points are believed not to be the free, growing front — 

 the foresets — of the deltas, but to indicate supporting walls of 

 ice, the melting of which, as with tlu; kaincs, allowed the 



