and Deposits of Flooded Rivers. 49 



area, most abundantly during stages of glacial recession, has 

 been observed and often well described and mapped in all 

 glaciated countries which have received study, but perhaps 

 nowhere else so fully as in the valley of the Connecticut river.* 

 This beautifully terraced valley, the terraces and plains of the 

 Merrimack river, and the osars and associated plains of Maine 

 which Stone has so well described, illustrate the formation of 

 the modified drift deposits where free drainage could take 

 place from the border of the ice-sheet. Beds of gravel, sand 

 and clay filled the Connecticut and Merrimack valleys to the 

 level of their highest terraces, which are remnants of the glacial 

 flood-plain, and the lower terraces record stages of the subse- 

 quent erosion. Every large river valley running southward 

 within our glaciated area has had an interesting history, which 

 may be discovered by study of its stratified drift. Owing to 

 the northern depression of the land under the ice-weight, the 

 descent of these valleys when first uncovered from the ice was 

 generally less than now. In the Hudson valley the changed 

 levels appear to have included not only subsidence at the north, 

 which admitted the sea to the basin of lake Champlain, but also 

 elevation of the present coast above the sea on the latitude of 

 New York city and southward, so that while the ice-front was 

 receding along this valley from south to north it held a lake 

 from Manhattan island to Albany and Lake Champlain, until 

 the farther glacial retreat allowed this long narrow lake to be 

 mainly drained northward into the Champlain arm of the sea. 

 In the Mississippi basin the northern subsidence probably pro- 

 duced broad shallow lakes along the border of the earlier ice- 

 sheet, preventing the accumulation of terminal moraines and 

 extending a mantle of loess over the till as the ice melted. 

 Basins now draining northward, also, as of the Contoocook 

 river in New Hampshire, the great lakes tributary to the Saint 

 Lawrence, and the Red river of the North, held glacial 

 lakes due to obstruction on the north by the ice-barrier. The 

 old shore-lines of these glacial lakes are found to have now an 

 ascent from south to north, caused by differential rise of the 

 land after it was relieved from its load ; and the successive 

 beaches of Lake Agassiz, the largest of these lakes, which oc- 

 cupied the basin of the Red river and Lake Winnipeg, show 

 that this upward movement was in progress simultaneously 

 with the departure of the ice. 



*C. H. Hitchcock, Geol. of Vt., vol. i, 1861, pp. 93-118. J. D. Dana, Trans. 

 of the Conn. Acad, of Arts and Sciences, vol. ii, 1870, pp. 45-112 : and many 

 papers in this Journal, III, vols, i, ii, v, x-xiii. xv. xxii-xxvii, 1871-1884, etc. 

 Warren Upham, Geol. of N. H., vol. iii, 1878, pp. 3-61 ; and this Journal. Ill, 

 vol. xiv, pp. 459-470, Dec, 1877. 



Am. Jour. Scr. — Third Series, Vol. XLI. No. 241. — January, 1891. 

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