CLIMATE SINCE THE LATE CRETACEOUS 65 



ning, and say that no tropical, warm-temperate, or even cool- 

 temperate, forest flora could possibly live and develop in high arctic 

 latitudes under present conditions. However, the conditions of the 

 early Tertiary at high latitudes were vastly difi^erent from those of 

 today. Durham (1950, pp. 1253-1254) concluded that during the 

 Eocene, the 18° C isotherm was at latitude 53° to 55° N., even farther 

 north than it was in the late Cretaceous. The fossil floras also indi- 

 cate that the temperatures of the Lower Tertiary may have been 

 higher than those in the late Cretaceous, since we have subtropical 

 floras, such as that at Raton, New Mexico (Knowlton, 1917, pp. 

 239-240), growing at moderate elevations, and this in spite of a 

 generally emergent continent and mountain building in the Cordilleran 

 region. The great inland seaway of the Upper Cretaceous was 

 drained at the close of that period. Intermittent mountain building 

 occurred throughout the area of the western Cordillera. The west- 

 ern mountains of the early Tertiary, with a few possible exceptions, 

 were of moderate elevation and did not seem to pose an insurmount- 

 able barrier to plant and animal dispersals, and the general elevation 

 of the continent was still comparatively low during the Lower 

 Tertiary. There seems to have been none of those great continental 

 upwarps that characterize the present Cordilleran region. 



By the beginning of the Middle Eocene there were at least three 

 botanical provinces in western North America (MacGinitie, 1941, 

 pp. 92-95). In the far- western states a subtropical forest extended 

 along the coast as far north as latitude 55° N., and, with some modi- 

 fications, inland at least as far as the present area of northwestern 

 Wyoming. The Eocene flora of Kupreanofi^ Island in southeastern 

 Alaska, at Chalk Bluffs on the west slope of the Sierra, and at 

 Yellowstone Park have many characteristic plants in common. 

 The low-lying shores of the Mississippi embayment were occupied 

 by a warmer, practically tropical flora of quite different composi- 

 tion. Far to the north were the summer-green Arcto-Tertiary 

 forests. 



Two Eocene floras are particularly significant with respect to 

 climatic trends: (1) the flora of the London Clay at latitude 52° N. 

 is of Lower Eocene age and is a tropical strand flora — not warm- 

 temperate, but tropical (Reid and Chandler, 1933, pp. 47-74); (2) 

 the Green River flora of Middle Eocene age, found at many localities 

 in northwestern Colorado and southern Wyoming, shows the 



