its half-life is 5760 years. The age of any sample can 

 be determined on the basis of the extent to which it 

 has degenerated (Libby 1960). 



The clisi-n- 



As the glacier retreated, vast areas were freed 

 for reinvasion by plants and animals (Adams 1905, 

 Gleason 1922). The land must have been a barren, 

 sterile expanse of raw parent soil material, deficient 

 in nitrogen. The first plants to invade were jirobably 

 species the root nodules of which l)ore bacteria, fungi, 

 or actinomycetes capable of fixing nitrogen from the 

 air, thereby enriching the soil (Lawrence 1958). The 

 recession rate of the glacier was probably faster than 

 the advance of vegetation and animal life. A belt of 

 tundra developed ; coniferous forest broadened to a 

 much greater width than existed at the peak of glacia- 

 tion. Deciduous forest, requiring a better soil, ame- 

 lioration of climate, and competitive displacement of 

 the already established coniferous forests, moved 

 northward rather slowly. There is evidence that this 

 northward movement of the biota is still in progress, 

 and that the great belts of vegetation are not yet 

 stabilized in respect to each other and to the climate. 



Fossil or pollen evidence for the existence of 

 tundra along the retreating glacial front is scant in 

 North America, except for certain areas in Maine ; 

 existence of tundra is better established in Europe. 

 Special difficulties are involved in the identification 

 of tundra pollen in core samples from bogs. Further- 

 more, deep kettles in which bogs later formed sus- 

 tained large blocks of ice, well insulated by being 

 nearly buried in glacial till, for a long time after the 

 main mass of ice had withdrawn northward. De- 

 posits of pollen could not settle in the kettles until 

 the ice blocks had melted, which was usually not until 

 spruce-fir coniferous forests had become the pre- 

 vailing vegetation of the region. This is doubtless the 

 reason for the almost universal occurrence in North 

 America of spruce-fir pollen in the deepest layer of 

 peat cores. The tundra belt may have been 160 km 

 (100 mi) wide as the ice retreated through New 

 England, but was probably much narrower West of 

 the Appalachians. Presumably, however, forest vege- 

 tation continued to advance onto the tundra along its 

 soutiiern margin, but at a rate slower than the tundra 

 expanded northward as the glacier retreated. Tundra 

 gradually, therefore, became more extensive in the 

 North, and permitted its fauna to expand to its jjres- 

 ent-day form. 



The occurrence of pine pollen in peat cores above 

 the spruce-fir indicates the emergence of a warmer 

 climate, drier as well. Beginning with the first ap- 

 pearance of deciduous tree pollen, there is differen- 

 tiation of the pollen spectrum in different parts of the 



FIG. 215 The Great Lakes 

 (Hough 1958). 



time of the Valders glaciation 



country, the nature of which apparently reflects a 

 drier climate in the Mid-western states and a more 

 humid one in the East. 



During the warm moist climatic optimum, when 

 conditions for forest growth vi^ere most favorable, 

 eastern hemlock spread from the northern Appala- 

 chians and became firmly established in New Jersey, 

 New York, New England, and, to a lesser extent, 

 in Ohio. Beech appeared early in New Jersey and 

 spread through New York and Ohio. Animal species 

 extended their ranges northward and withdrew from 



FIG. 21-6 Glacial Lakes Agassii (A) and Ojibway-Barlow (O-B). 

 and the outlets of each (after Flint 1947). 



Paleo-ecology 289 



