forest was extensive in the western mountains and 

 on the Pacific slope, much as at present. 



Glaciation destroyed the coniferous forest over 

 vast areas in the north, but there is considerable con- 

 troversy as to the area south of the glacial boundary 

 in which the deciduous forest was thus affected. In- 

 terpretation of the probable climate, of past and pres- 

 ent distribution of plants and animals, and of the 

 pollen record in bogs indicates that, in North Amer- 

 ica, the deciduous forest was not extensively dis- 

 placed, but that it became mixed with coniferous 

 species to varying distances south of the margin of 

 the glacier (Epling 1944, Hansen 1947, Braun 1950, 

 Thomas 1951). 



Enormous amounts of cold water, melted from the 

 glacier during the summer months and, perhaps, 

 carrying chunks of ice, drained down the Delaware 

 and Susquehanna Rivers in the East, the Ohio, 

 Wabash, Illinois, Missouri, Platte, and Mississippi 

 Rivers in the central part of the continent (Hobbs 

 1950), and the Snake and Columbia Rivers in the 

 Northwest. Water filled these wide river valleys from 

 the present blufifs on one side to the blufifs on the 

 other, and doubtless extended the boreal microclimate 

 for many kilometers (Wolfe 1951), perhaps permit- 

 ting the establishment of coniferous trees and other 

 northern species on their banks. The Atlantic coastal 

 plain was exposed by the falling sea level. Cold gla- 

 cial waters draining southward between the coast and 

 the Gulf Stream probably created a microclimate of 

 a type permitting northern species, including conifers, 

 to invade the coastal plain as far south as Florida. 



A tree line existed above 1200-1500 meters 

 (4-5000 ft) on the higher Appalachian peaks. Conif- 

 erous forests that are now limited to the higher ele- 

 vations of the Appalachian Mountains descended the 

 mountain slopes perhaps as much as 600 meters 

 (2000 ft) and covered large areas. In the moun- 

 tains all over the world, the snow-line (Klute 1928) 

 and biotic zones (Murray 1957) were at least 500 

 meters (1600 ft) and in some humid localities possi- 

 bly as much as 1 500 meters (5000 ft) lower than they 

 presently are. The occurrence of pollen and the re- 

 mains of spruce and fir in bogs and glacial deposits 

 in Texas, Louisiana, Florida, North Carolina, and 

 elsewhere on the coastal plain can probably be ex- 

 plained as the result of southward boreal forest intru- 

 sions that did not completely displace the deciduous 

 forest. The ranges of many animal species also ex- 

 tended farther south during the height of glaciation 

 than they do at the present time. 



Loess was deposited extensively from Wisconsin 

 to southwestern Indiana and west into Nebraska and 

 Kansas (Flint 1957). This buff-colored, homogene- 

 ous, porous, calcareous, non-stratified deposit forms 

 only in arid or semi-arid regions, and is indicative of 

 grassland abutting directly on the glacial front. 



There is no evidence that extensive coniferous forest 

 existed in front of the glacier during any of its ad- 

 vances in this area. The occurrence of snails in 

 Pleistocene deposits in Kansas indicates that during 

 Wisconsin time, for instance, open prairie occurred 

 on the upland and deciduous woodland along streams 

 much as they do at the present time (Frye and 

 Leonard 1952). With the greater precipitation that 

 was generally prevalent, some of what is now desert 

 in the Great Basin and the Southwest was probably 

 grassland then. 



Evidence is scanty for the existence of tundra in 

 North America south along the ice front during its 

 advances, although tundra occurred in Alaska dur- 

 ing the last Wisconsin stage. Tundra mammals, 

 such as the muskox and woolly mammoth, are well 

 represented as fossils along the old glacial margins, 

 and a few fossils of these species have been found as 

 far south as Texas, Mississippi, and Florida (Potzger 

 1951). These species, however, are believed to be 

 derived from grassland forms, and it appears that the 

 distinction between grassland and tundra faunas did 

 not become sharply defined until late in the Pleisto- 

 cene (Hibbard 1949). 



In North America the coniferous forest survived 

 glaciation in four separated refugia (Adams 1905, 

 Halliday and Brown 1943). These refugia were in 

 the region of the northern Appalachians, the northern 

 Rockies, the Pacific slope of the Cascades, and 

 Alaska (Hulten 1937). The Appalachian refugium 

 during the Wisconsin epoch e.xtended westward 

 south of the Great Lakes, but was separated from 

 the Rocky Mountain refugium by grassland. Except 

 for the unglaciated pocket in southeastern Minnesota, 

 southwestern Wisconsin, and northwestern Illinois, it 

 appears that forests were absent along the ice front all 

 the way from Illinois to the Rocky Mountains. The 

 Rocky Mountain refugium was separated from the 

 Pacific refugium by the Great Basin, the high peaks 

 of the Cascades and Sierra, and by seasonal differ- 

 ences in precipitation. Probably this separation was 

 only partially effective as a narrow belt of coniferous 

 forest extended around the north border of the Great 

 Basin. Coniferous forest in the Pacific refugium 

 probably extended some hundreds of kilometers far- 

 ther south than it does at the present time. The 

 Alaskan refugium was probably connected by a land 

 bridge across the Bering Sea with unglaciated areas 

 in Asia. Fossil remains found in frozen muck and 

 silt indicate the probable occurrence in Alaska during 

 glacial periods of woolly mammoth, muskox, reindeer, 

 and many other forms (Flint 1952). 



Our description of how biotic communities were 

 affected by the Pleistocene glaciation is not univer- 

 sally accepted. The southward extrusions of conif- 

 erous forest along the Mississippi River and the At- 

 lantic coastal plains are conjectural. According to 



286 Geographic distribution of communities 



