180 THE GLACIAL LAKE AGASSIZ. 



where series extending- 100 to 150 miles have been described by Prof. 

 George H. Stone.' They are well exhibited also in the valleys of the Mer- 

 rimac and Connecticut rivers and elsewhere in New England, but are less 

 frequent on the nearly flat expanses whicli are drained to the Laurentian 

 lakes, to the Upper Mississippi, and to the Red River of the North. Occa- 

 sional plains or plateaus of gravel and sand, associated with the eskers, ' 

 and, like them, terminating in steep escarpments which descend to adjacent 

 lower land, were deposited in broad embaymeuts of the waning ice border. 



In the valleys of hilly and mountaint)us districts, and on certain belts 

 and tracts of nearly flat areas, the departure of the ice-sheet supplied Inroad 

 flood-plains of gravel, sand, and clay, brought by the waters of the glacial 

 melting and of the accompanying abundant rains. These deposits, when 

 inclosed in valleys, are named valley drift, and are seen to slope with the 

 present streams, but often somewhat more rapidly ; and they continue along 

 the coiu'se of the larger rivers to the sea or to the areas of lakes that were 

 pent up against the receding ice-sheet, and there form deltas and farther off"- 

 shore sediments. Since the departure of the ice, river erosion has carved 

 the valley drift into terraces, and the streams now flow far below their 

 levels of the closing- portion of the Ice ag-e. 



A very fine variety of the valley di-ift, especially where it contains some 

 glacially comminuted rock flour from calcareous formations, is called loess. 

 In the Mississippi and Missouri valleys and on the Rhine tliis deposit is 

 clearly in large part of glacial origin, being directly supplied from the ice 

 melting; but very similar fluvial beds are now being formed by the Nile 

 and were formerly spread in great thickness by the rivers of China, where 

 the origin of the silt is refei-able wholly or chiefly to subaerial denudation. 

 The allu^num derived from erosion of the drift sheet and deposited by the 

 Red River along its valley after Lake Agassiz was di-ained away resembles 

 the loess in fineness, in its occasionally inclosing fresh-water and land shells, 

 and in containing- here and there small calcareous concretions. 



1 Proceedings, Boston Society of Natural History, Vol. XX, 1880, pp. 430-469, with map. Proc, 

 Am. Assoc, for Adv. of Science, Vol. XXIX, for 1880, pp. 510-519, with map. Am. Jour. Sci. (3), Vol. 

 XL, pp. 122-144, Aug., 1890. 



