CLIMATE SINCE THE LATE CRETACEOUS 67 



types and climates from Oligocene to Pleistocene as we can trace 

 in the western states. This slow but inexorable climatic change 

 (MacDonald, 1953) profoundly affected the evolution of vegetation 

 types and their associated faunas. The Middle and Upper Oligocene 

 mark the influx from the north of Arcto-Tertiary species over the 

 western states. The trend toward cooler climates brought about the 

 southward dispersal of a host of temperate-forest trees and the 

 beginnings of the modern vertebrate faunas. By the Upper Oligocene 

 we find much of the earlier tropical or subtropical elements of the 

 floras displaced by northern forms whose ancestors were members 

 of the Eocene Arcto-Tertiary forests. A striking aspect of the Upper 

 Oligocene floras of the western states is the large number of species 

 related to those now living in the forests of eastern Asia. This 

 Asiatic aspect is one of the characteristic features of the western 

 Oligocene floras and is due, of course, to the fact that the Arcto- 

 Tertiary complex, as it dispersed southward, came down into both 

 eastern Asia and the western part of North America. Dozens of 

 common species in the western fossil floras have their living counter- 

 parts in the forests of eastern China (Chaney, 1947, p. 145). Among 

 the common genera are Acer, Ailanthus, Castanopsis, Celastrus, 

 Dipteronia, Exbiicklandia , Glyptostrohus, Holmskioldia, Afetaseqiwia, 

 Koelreuteria, Paliiirus, Pterocarya, Quercus (Asiatic types), and 

 Zelkova. The widespread Bridge Creek flora of late Oligocene age 

 occupied a large area from northern California into British Columbia 

 and eastward to Montana and Colorado. It is characterized by an 

 abundance of Metasequoia and Asiatic oaks. The Florissant flora of 

 Lower Oligocene age, in central Colorado, is extraordinarily rich in 

 warm-temperate species. It comprises many forms derived from the 

 earlier Green River flora, together with representatives of the flora 

 moving down from the north. 



It is a curious fact that practically none of the western Oligocene 

 species having living relatives in eastern Asia was able to get into 

 Mexico or into the Appalachian region. A climatic barrier that 

 prevented migration either southward or eastward arose in the 

 Eocene and was well established in the Oligocene. The evidence 

 seems clear that this barrier was one of reduced rainfall. There is 

 good evidence, obtained from a study of the Green River and 

 Florissant floras (MacGinitie, 1953, pp. 52, 58), that an area of 

 subhumid scrub extended across northern Mexico and northward 



