86 PRINCIPLES OF STRATIGRAPHY 



eludes that "already in a period immediately after the melting of the 

 ice . . . the vegetation period was four months [in length] and 

 the July temperature about + 6° C. This increased, during the 

 period of the tundras, to a season of five months' vegetation and 

 a July temperature of -|- 9° C." (Andersson-i :xxv.) The arctic 

 flora of these countries was followed by forests in which three trees, 

 the pine (Piniis sihcstris), the birch (Betula odorata), and the 

 asp or poplar (Fopiilns tremnla) became the dominant types. The 

 July temperature at this time averaged probably 10° to 12° C. 

 These first forest trees were, however, not uniformly distributed. 

 Thus in the western regions the pines were absent, the birch and 

 poplar alone predominating. This corresponds to the distribution 

 of the trees in the present arctic region where Betula odorata forms 

 the dominant forest tree around the fjords of South Greenland, in* 

 Iceland, the whole of Scandinavia and the peninsula of Kola to the 

 White Sea. This may be explained by the greater humidity of this 

 region. In Finland and North Germany the birch and pine oc- 

 curred together in the first post-glacial forests, which represented 

 the drier, more continental type of arctic forest, such as is found 

 to-day in the remainder of the arctic regions, where forests, consist- 

 ing primarily of pines (Pinus), spruces (Picea) and larches 

 (Larix), abut against the treeless arctic plains. 



With the increase in temperature, the coniferous woods were re- 

 placed by those requiring a higher summer temperature, such as 

 small leaf lindens {Telia europcca), hazel {Corylus avcllana), 

 maple {Acer platanoides), elm (Ulmus montana), etc. Finally, 

 the oak (Quercus pcdnncnlata) made its appearance, and displaced 

 the pines almost entirely. Gunnar Andersson and others have fur- 

 nished evidence from the former distribution of the hazel, oak, 

 linden and several aquatic plants, to the effect that the increase in 

 warmth culminated in a period of higher temperature than the pres- 

 ent over the whole of western Scandinavia, and less markedly over 

 North Germany. In northern Sweden the temperature during the 

 warmer period averaged 2.5° C. higher than now, though the win- 

 ter temperature could not have been higher than the present, for 

 plants requiring a warmer winter apparently did not extend fur- 

 ther north than they do now. In the central parts of southern 

 Norway. Holmboe finds that the border line of the fir was once 

 about 300 meters higher than at present. In general, the appear- 

 ance of the spruce {Picea excelsa) in the northern and the beech 

 {Fagus silvatica) in the southern part of the glaciated region seems 

 to indicate a gradual deterioration of the climate of Europe since 

 the maximum of post-glacial temperature. 



