>m' 



FIG. 

 the 1 



-2 Conjectural map of vegetation in North An 

 e of the maximum Wisconsin glaciation (from 



(Moreau 1933). Lakes Bonneville and Lahontan, as 

 well as many smaller ones, were formed in the Great 

 Basin of North America during these pluvial periods 

 (Meinzer 1922, Hubbsand Miller 1948). 



There was considerable alteration of drainage pat- 

 terns over the northern part of the continent. Old 

 river valleys were filled or dammed by ice or mo- 

 raines. New outlets were formed. Rivers previously 

 separated became connected. The retreat of the gla- 

 cier left vast level areas without drainage so that 

 many lakes, swamps, and bogs remain in northern 

 glaciated areas. In other places the large quantities 

 of melt water cut new channels or widened old val- 

 leys, through which the surplus water was trans- 

 ported to the sea. Silt, sand, and gravel were spread 

 out in outwash plains, from which winds picked up 

 the finer material and deposited it elsewhere as loess 

 in layers up to 2.5 or 3 meters thick (100 in.) over 

 hundreds of square kilometers. The treatise of 

 Thienemann (1950) is an extensive account of what 

 happened to the fresh-water fauna. 



Deciduous forest 

 lllllll Coniferous forest 

 •'.•/.■ Desert 



Glacier 

 IV; Grassland 



Bd forest 

 Woodlond.ctioporral 

 ■V. Tundra 



tlon given by Flint 1952, 1957, Meinier 1922, Hobbs 1950. Braun 

 1950, Hansen 1947). 



Terrestrial biota and communities 



Virtually all the fossil plants and mollusks dur- 

 ing the Pleistocene are represented by living relatives 

 (Baker 1920). Changes of ecological significance 

 are, for the most part, in point of geographic dis- 

 tribution rather than organic evolution. But insects, 

 especially beetles, which are well represented in the 

 fossil record evolved rapidly into new forms. Many 

 mammals became extinct. Large mammals present 

 during early stages of the Pleistocene, but no longer 

 occurring in North America, include camels, horses 

 (one species later re-introduced), ground sloths, two 

 genera of muskoxen, peccaries, a giant bison, a giant 

 beaver-like animal, a stag-moose, several kinds of 

 cats, mammoths, and the mastodon. 



By the Pleistocene, there was doubtless a broad 

 zone of coniferous forest across the northern part of 

 the continent, and perhaps some tundra. Deciduous 

 forest covered the eastern states ; grassland occurred 

 in the central part of the country ; and coniferous 



Paleo-ecology 285 



