TABLE 21-1 Late glacial and post-glacial chronology (modified from Deevey 1949, Deevey and Flint 1957, Flint 1957). 



the South. When later forced to withdraw from 

 over-extended ranges to the North, some species left 

 relict populations, which persist to the present time. 

 Following the climatic optimum came a warm, 

 dry climate, called the xerothermic period. The for- 

 est vegetation prevailing from the Mid-west into 

 New England consisted of aridity-tolerant oaks and 

 hickories. The most part of beech withdrew from 

 Ohio, but became well established in the East, where 



hemlock suffered for want of moisture. A prairie 

 peninsula penetrated at least as far as Ohio, and 

 probably scattered patches of prairie occurred be- 

 yond. Grassland animals penetrated far to the East 

 (Schmidt 1938, Smith 1957). Boreal forest retreated 

 northward ; sugar maple-basswood forests extended 

 far into Manitoba (Jenkins 1950). In Saskatchewan 

 and Alberta, the northward withdrawal of coniferous 

 forest left groves of aspen trees in the moister and 



290 Geographic distribution ot communities 



