CLIMATIC CHANGES 87 



A similar succession of floras seems to have occurred in North 

 America, but the evidence has not been fully gathered. Over the 

 great plains of Canada, between the international boundary and 

 the forest region which stretches northwestward through Manitoba 

 and Saskatchewan and westward across Alberta, the climate on the 

 melting of the glaciers was probably much like that of the barren 

 lands farther north at the present time, where the mean summer 

 temperature is below 10° C, with permanently frozen subsoil and 

 consequently a complete absence of trees. As the climate became 

 warmer on the disappearance of the ice, it also became drier, so that 

 forests were unable to grow and Sphagnum swamps unable to form. 



Evidence of a warmer climate preceding the present has been 

 obtained from the Atlantic coast, where the Talbot formation of 

 Maryland and Virginia, believed to be post-glacial in age, holds a 

 flora which is more characteristic of southern portions of the same 

 region. Thus the bald cypress {Tax odium distich mn), found fossil 

 as far north as Long Branch, New Jersey, has its present northern 

 limit in southern Delaware and on the eastern shore of Maryland; 

 the loblolly pine (Pinus tccda), also found at Long Branch, does 

 not extend north of southern New Jersey at the present time, with 

 its maximum development west of the Mississippi ; the tupelo 

 {Nyssa biflora), found fossil in New Jersey, ranges to-day from 

 North Carolina to Louisiana. 



2. Aniuials. Marine and fresh-water mollusca are among the best 

 available indicators of climatic changes, so far as species are con- 

 cerned, which are still existing, and the geographic range of which is 

 known. In the marine species it must, however, always be borne in 

 mind that bathymetric distribution may counteract the influences of 

 climate and that hence the evidence must be carefully scrutinized. 

 Even better indications of change of climate are furnished by the 

 distribution of land animals, especially insects and mammals, though 

 this evidence is generally less readily available. Examples showing 

 changes of fauna, most probably due to change of climate, have 

 been obtained in numerous late glacial and post-glacial deposits. 

 A highly significant section of these deposits has been studied by 

 Jensen and Harder on the west coast of Greenland in the Orpiksuit 

 fjord, Disko Island (about lat. 70° N.). In the lowest clays occurs 

 a fossil fauna with Balanus Jiamcri. indicating a period during 

 which the climate was not high arctic, but rather resembled that 

 of the present time. This is followed by a series of clay beds 

 averaging 10 meters in thickness, with a rich fauna, among which 

 Mya truncata cf. ovata and i^o/rfio arctica must be noted, indicating 

 that the climate gradually refrigerated until high arctic conditions 



