CLIMATE SINCE THE LATE CRETACEOUS 63 



(1942, pp. 100-103) has shown a definite change from subtropical 

 to warm-temperate floras between southern Colorado and Montana. 

 It appears clearly established that the area between the temperate 

 northern forests and the tropical forests at the southern border of 

 the United States was occupied by an ecotone characterized by a 

 gradual transition from the one type to the other. In considering 

 the "problem" of the northern floras (which is really no problem at 

 all) we have to remind ourselves again of conditions in the past. 

 All the evidence points to the fact that the seas were much warmer 

 than at present. As an example of this kind of evidence the attention 

 of the reader is called to a paper on Cenozoic marine climates of the 

 Pacific Coast by Durham (1950), who based his conclusions on an 

 intensive study of fossil reef-building corals and the associated 

 invertebrate faunas. His fundamental postulate was that living reef 

 corals cannot endure a minimum temperature much below 18° C. for 

 any length of time. Thus, he took the figure of 18° C for the coldest 

 month (February) as the limiting isotherm beyond which reef- 

 building corals cannot exist. According to the fossil corals of the 

 West Coast, the February isotherm of 18° C must have been located 

 at about 53° N. Lat. in the late Cretaceous. That is approximately 

 1,500 miles north of its present location on the coast of Lower 

 California. It is difficult to appreciate the full significance of such 

 a difference in ocean temperatures from that of the present. 



Today a large proportion of the ocean is close to the freezing 

 temperature. Polar waters are denser than equatorial owing to the 

 low temperatures ; cold water settles in the polar regions and slowly 

 creeps along the sea floor to rise to tropical and equatorial regions 

 where it is warmed. Warm water from within the tropics moves 

 north at the surface to complete the thermal circulation. The layer 

 of warm water is comparatively shallow on account of the slow up- 

 welling of cold water from below. Thus, "the temperature of 

 abyssal waters in the open ocean basins is conditioned by the tem- 

 perature of surface waters in the polar regions" (Emiliani, 1954, p. 

 854). Even in equatorial regions the abyssal waters are close to the 

 polar waters in temperature. All the evidence that we now possess 

 indicates that this type of circulation existed throughout the 

 Tertiary. Emiliani (1958, pp. 57-58), from his tudy of a deep-sea 

 core, has indicated that the abyssal temperatures in the equatorial 

 Pacific in the Middle Oligocene were about 10° C, which implies a 



