THE GLACIAL OR PLEISTOCENE PERIOD 223 



Based upon the time which has elapsed since the cutting of the Niagara 

 gorge (that which is not preglacial) and the Falls of St. Anthony in the Missis- 

 sippi River, the time limit may be expressed in years as follows: 10 



Climax of Late Wisconsin 20,000 to 60,000 years ago" 



Climax of Early Wisconsin 40,000 to 150,000 



Climax of Iowan 60,000 to 300,000 



Climax of Ulinoian 140,000 to 540,000 



Climax of Kansan 300,000 to 1,020,000 



Climax of Xebraskan y to z 



These figures are only approximate at best and perhaps possess but little 

 scientific value, but they serve to indicate that a great period of time has 

 elapsed since the beginning of the last ice age, a period long enough for faunas 

 and floras to wax and wane and for many groups to become extinct. The ex- 

 tent of these changes in some groups of animals and the lack of such changes in 

 other groups appear in the pages which follow. 



4. SUMMARY 



The Pleistocene Period was followed by an interval of intense cold during 

 which British America and the northeastern portion of the United States was 

 successively covered by five or six huge ice sheets. The maximum area 

 covered was 4,000,000 square miles and the thickness is estimated at from 

 3 to 8 miles. The ice invasions completely changed the topography, replacing 

 the old rivers and streams by ponds, lakes, and swamps. All life was exter- 

 minated within the englaciated area, or driven south of it. At each successive 

 interglacial period (of which there were four and possibly five) the biota fol- 

 lowed the retreating ice and again occupied the devastated territory, only to 

 be again driven southward by a return of the ice. After the last ice invasion 

 (the Wisconsin) the aquatic life followed the waterways from the melting ice 

 and spread over the territory that we see today. The terrestrial life followed 

 closely the margin of the ice, Arctic, Subarctic, Cold Temperate, and Temperate 

 biota occupying the area in turn, as the climate became suited to each. 



The different ice sheets are imbricated at their edges and the drift, as well 

 as the interglacial deposits, maybe traced beneath the material of a later drift 

 sheet. The criteria upon which to build an interglacial biota is very fragmen- 

 tary, and must be selected with care to avoid serious errors, only material 

 from a known geologic horizon being available for use. For this reason 

 deposits outside of the area as well as much material lying upon the drift 

 sheets cannot be used. It is believed that upwards of a million years have 

 elapsed since the first field of ice covered the country, a period of sufficient 

 length to permit great changes in the species of plants and animals inhabiting 

 this region. 



" Chamberlin and Salisbury-, Geology, III, pp. 415-420. 



