222 LIFE OF THE PLEISTOCENE 



was forced into courses which were partly new. In other cases, the ice, by 

 encroaching on the middle course of the valley, as in the case of the Ohio, forced 

 drainage around its front, and the drainage lines thus established by force, 

 were often held after the ice melted. " 



These changes in topographic relief produced profound changes in the habi- 

 tats of the fauna and flora. Where previously were rivers and their tributaries, 

 now were ponds, lakes, and swamps (Plate LVII, figures 1, 2). Many fresh 

 water species, upon re-entering the glaciated territory, were compelled to change 

 their normal habitats from river to pond, lake, or swamp. It is probably this 

 change in ecological relationship that has so greatly increased (apparently) 

 the number of species of certain fluviatile animals within the englaciated terri- 

 tory (the fresh water mollusks for example). 



It is believed that the old drainage basins of the Laurentian River have 

 been occupied by lakes since an early invasion, perhaps since the Nebraskan 

 and Jerseyan, as old shore lines and biota are known from several interglacial 

 periods. 



f . Length of Time since the Earliest Ice Invasion 



The length of time which has elapsed since the beginning of the Glacial 

 Period and especially the time involved in each interglacial stage is of consider- 

 able value in estimating the changes which have taken place in the biota during 

 this long period. McGee 8 thus graphically portrays this time element: "Let 

 the period of written history be represented by a day; then a month or a year 

 of such days will measure the period that has elapsed since the first Pleistocene 

 ice sheet invaded northwestern Iowa. The uncertainty as to the date of the 

 invasion is great; but it was so long ago that the date would be but vaguely 

 conceived if it were possible to write it. " 



Chamberlin and Salisbury, 9 collating the judgment of five of the glacial 

 geologists who have most studied the available data, give the following table: 

 "The time datum for each sheet of till is the stage at which it began to suffer 

 erosion, which, of course, would be slightly after the beginning of the ice 

 retreat. The time-unit is the period which has elapsed since the Late Wiscon- 

 sin began to be exposed for erosion: 



From the Late Wisconsin to the present 1 time unit 



From the Early Wisconsin to the present 2 to 2Y 2 time units 



From the Iowan to the present 3 to 5 



From the Ulinoian to the present _ 7 to 9 



From the Kansan to the present 15 to 17 " 



From the Nebraskan to the present x 



' 11th An. Rep., U. S. Geol. Sun-., p. 567. 

 » Geology, III, p. 414. 



