THE PLEISTOCENE 



Physical conditions 



The Pleistocene was marked by a series of 

 great climatic fluctuations throughout the world 

 (Flint 1957). Thirty-two per cent of the land area 

 of the world was buried under glacial ice ; 10 per cent 

 still remains ice-covered. In places, this ice reached 

 a thickness of at least 1500 m (5000 ft), roughly the 

 thickness of the ice sheets now on Greenland and 

 Antarctica. Outwash from the glaciers carried sedi- 

 mentation hundreds of kilometers beyond the farthest 

 ice boundaries. Sea-level fell to 138 m (450 ft) below 

 the present level as water became bound in glacial ice. 

 This resulted in exposed coastal plains all around the 

 world to an extent greater than at present (Russell 

 1957). There were widespread back and forth move- 

 ments of animal and plant species as glaciers alter- 

 nately advanced and retreated. 



There is good reason to believe that North Amer- 

 ica was connected by a land bridge to Siberia over the 

 Bering Sea and Strait at times of glacial advance, 

 but not during interglacial periods (Hopkins 1959). 

 The climate over this bridge was probably similar to 

 that associated with tundra vegetation and not warm 

 enough for forests to develop. According to one 

 theory (Ewing and Donn 1956), the Arctic Ocean 

 remained open water and largely free of ice during 

 the time of the glacial advance and its eventual 

 freezing over, thus cutting ofT the supply of moisture 

 as precipitation, brought the glacier to a halt and then 

 to retreat. A reconstruction of the appearance of 

 North America at the time of the last (Wisconsin) 

 glaciation is shown in Fig. 21-2. Possibly the Arctic 

 Archipelago was not covered with ice (Antevs 1929). 

 It is conjectured that the shores of the Artie Ocean 

 may have had a reasonably mild climate during the 

 advance of the glacier. Of the rest of the world, it is 

 interesting to note that the British Isles were con- 

 nected at this time to continental Europe (Antevs 

 1929) ; the Indo- Malayan Archipelago extended as 

 dry land to include Sumatra, Borneo, and Java : and 

 New Guinea was connected to Australia (Mayr 

 1944). 



There were a number of successive thrusts of 

 glacial ice, both in North America (Nebraskan, 

 Kansan, Illinoisan, Wisconsin) and in Europe (Giinz, 

 Mindel, Riss, and Wurm in the Alps; Pre-Elster, 

 Elster, Saale, and Weichsel in the northern coun- 

 tries), that came at different times and extended dif- 

 ferent distances southward. Glacial and interglacial 

 stages were probably synchronous in North America 

 and Europe. During the interglacial periods the biota 

 reoccupied the newly uncovered areas as the glacier 

 melted back, only to be driven out as the glacier again 



advanced. It is possible that at the present time we 

 are in an interglacial period, and in a few thousands 

 of years the northern parts of the continent will again 

 be covered with ice. 



We are chiefly concerned with the last glaciation, 

 which began perhaps 60,000 years ago and reached 

 its maximum extension 18,000 or more years ago. 

 In North America, the Wisconsin glaciation is di- 

 vided into the Iowa, Tazewell, Gary, Mankato, Vald- 

 ers, and Cochrane substages. Each substage repre- 

 sents a separate glacial advance, one separated from 

 its predecessor and successor by warm periods dur- 

 ing which the glacial front retreated various dis- 

 tances. The first two glacial stages were the most 

 extensive; the Mankato (at peak about 13,000 years 

 ago) and the Valders (about 10,700 years ago) 

 advances reached as far as the Great Lakes ; the 

 Cochrane glaciation (at peak about 7000 years ago) 

 reached only slightly south of James Bay. Glaciation 

 was extensive in northern Europe, but not in north- 

 ern Asia, and occurred southward in the higher moun- 

 tains. 



It is estimated that, in northern Ohio, the ice ad- 

 vanced during certain stages of the Wisconsin glacia- 

 tion at the rate of 100 m (350 ft) per year, and in 

 southern Ohio at from 12 to 33 m (38 to 108 ft) 

 per year (Goldthwait 1959). There is evidence that 

 the advancing ice lowered the temperature suffi- 

 ciently ahead of it — for a distance of 800 m (0.5 

 mile) — to decrease the annual growth of spruce and 

 other coniferous trees but not to kill them until the 

 glacier actually overrode and destroyed the forest 

 (Burns 1958). 



The drop in mean annual temperature over tem- 

 perate North America is estimated to have been 5° 

 to 10°C, but was probably greater at the edge of the 

 glacier (Dillon 1956). Superficial oceanic water lay- 

 ers in the tropics dropped approximately 6°C (Emil- 

 iani 1955). The gradient from low to higher tem- 

 peratures at the glacial front was probably steep. 

 Storm tracks in North America during maximum 

 glaciation extended from the west and southwest to 

 the east and northeast ; thus, warm winds were 

 brought against the front of the glacier. There is 

 controversy as to how far south of the glacier, the 

 high pressure anticyclonic conditions developed by 

 the great ice mass were felt (Hobbs 1926). 



Precipitation appears to have been comparatively 

 heavy during the glacial stages over much of the 

 world. Accumulation of unmelting snow in the 

 North, as the result of increased precipitation, built 

 up the great glacial masses. Even in areas where 

 continental glaciation did not occur (Africa and Aus- 

 tralia, for instance), variations between pluvial and 

 interpluvial periods probably coincided with the gla- 

 cial and interglacial periods and produced far-rang- 

 ing effects on the geographic dispersal of organisms 



284 Geographic distribution of communities 



