CLIMATE SINCE THE LATE CRETACEOUS 71 



The cooling continued until, in the late Miocene or Lower Pliocene, 

 winter snows may have occurred along the northern boundary of 

 the United States, but the outbreaks of polar air so characteristic 

 of the present winters appear to be a development of the late 

 Pliocene. The world climatic changes of the late Tertiary were 

 intensified and aggravated by the mountain building that reached 

 its peak at the close of the period. One of the effects of the climatic 

 changes that finally culminated in the Glacial period was an en- 

 largement of the area of polar climates and a compression or shrink- 

 age of the area occupied by temperate and tropical climates (Brooks, 

 1949, pp. 55-62). In the earlier Tertiary the area of warm-temperate 

 and tropical climates was enlarged at the expense of the area 

 occupied by polar climates, but this situation was gradually reversed 

 in the later Tertiary (Craig and Willett, 1951, pp. 381-382). The 

 zone of polar-front weather, marked by the succession of moving 

 high- and low-pressure areas, was greatly intensified, and moved 

 southward. In contrast, the polar front in pre-Miocene time must 

 have been nonexistent during the summer and of weak development, 

 far to the north, in the winter. During the Glacial stages the zone 

 of maximum cyclonic activity was even farther south than at present. 

 The southward movement of the polar front and the increased 

 temperature gradients caused the Glacial periods to be rainy or 

 pluvial periods in the zone just south of the glaciated area. The 

 large inland lakes of the Great Basin waxed and waned in response 

 to Glacial and Interglacial conditions. At the times of Glacial 

 advances the temperature gradient across the temperate zone, 

 between the Gulf and the northern states, must have reached a 

 maximum. Aianley (1955) has indicated that this temperature 

 gradient could have had at least twice its present value and has 

 estimated that at the time of maximum cooling the minimum annual 

 temperature, on the Fahrenheit scale, was lowered about 13° near 

 the Gulf, 16° at latitude 35° N., 20° in the Ohio Valley, and 27° at 

 New York. 



Glaciers form wherever snowfall exceeds summer melting, and 

 the maximum development of Pleistocene glaciers was in just 

 those regions where at present there is maximum winter snowfall 

 and cool, cloudy summers. A comparatively slight lowering of 

 summer temperatures from that of the present would suffice to 

 reinitiate glaciation in those areas. 



