QUATERNARY AGE. — CHAMPLAIN PERIOD. 557 



3. Tlie height of the upper terrace of river valleys and lakes was 

 largely an effect of the height of the flood, and not necessarily of a sub- 

 sequent change of level in the continent. — So enormous was the supply 

 of water that where now the river floods rise to 25 feet, they rose then, 

 in many cases, to 100, and sometimes to 250 feet or more. At these 

 heights the upper terraces were made, and heuce they mark approxi- 

 mately the upper flood-level. The flood caused an increase in the 

 pitch of the stream on an average probably of a foot a mile, and not 

 uncommonly of five feet for small obstructed streams. In Southern 

 New England the change of level — 10 to 25 feet — which has since 

 taken place, may have affected this much the height as far up the riv- 

 ers as tide-level extended, but only little, when any, beyond this point. 

 The St. Lawrence was one of the great rivers, whose terraces are for 

 a long distance due almost solely to the elevation of the land. But 

 this was true only so far up the river as the open ocean and oceanic 

 life entered. Above this point the level of the waters was raised by 

 the floods from the back country, and the height of the terraces was 

 consequently due in part to the height of the flood. About the upper 

 of the Great Lakes, the flood was almost, if not quite, the sole cause 

 of the height, as has been urged by N. H. Winchell. 



The waters of the Connecticut at Hartford, 45 miles (in an air-line) from its mouth 

 are, when low, at mean tide-level. The river rises 30 feet at its highest modern floods; 

 but it rose 180 feet during the Glacial flood, all but 10 to 15 feet due to the greatness of 

 flood, and the damming by ice at the Narrows below Middletown. The obstructions at 

 the Narrows have the same effect now, as then, but vastly less. In the case of one of 

 the rivers emptying at New Haven, Conn. (Mill River), the terraces 11J miles from the 

 Sound have a height of 115 feet above the sea-level, indicating a pitch in the flood- 

 waters of 10 feet a mile; but here the valley was shallow and rather broad and the 

 waters were greatly obstructed by the sand and gravel given them to transport. 



The Mississippi waters, from the mouth of the Ohio to the Gulf (550 miles), have at 

 high water a pitch of about six inches to the mile; the level at high water adds, at the 

 Ohio, fifty feet to the height. If the supply of waters were sufficient to increase the 

 slope to eleven inches per mile, the height of water would be great enough to deposit all 

 the loess at its present level. But the land was probably depressed, in the latitude of the 

 Ohio, at least fifty feet below the present level ; and, in that case, with less than nine 

 inches to the mile, the existing Champlain depositions could have been made. Much 

 greater changes of level actually took place, in the vicinity of the Gulf, according to 

 Hilgard (Am. Jour. Sci., II. xlviii. 331, and III. ii. 398). 



The subsiding of the flood and the drying up of the waters were 

 attended with great changes in the surface of the continent. The im- 

 mense interior lake became the five Great Lakes, and hundreds of 

 smaller lakes along the rivers and elsewhere disappeared. The Kan- 

 kakee Swamp country, 25 miles wide and 50 long, in Western Indiana, 

 is described by F. H. Bradley as one of these obliterated lakes. The 

 rivers dwindled to one tenth their former magnitude and became nar- 

 row threads of water, with contracted flood-grounds between the wide 

 terraced alluvial plains which had been their flood-limits. 



