PLEISTOCENE ECOLOGY AND BIOGEOGRAPHY 391 



Alaska (Hansen, 1947, 1953; Heusser, 1953, 1955). They have de- 

 veloped a sequence that parallels the Postglacial pollen zones of 

 eastern North America. Hansen (1947) found a rise in grass- 

 chenopod-composite pollen in south central Oregon in the Thermal 

 Maximum. Apparently, grassland habitats replaced forest along the 

 prairie-woodland border. Elsewhere in the west, archaeological cave 

 sites and pluvial lake beds of the Great Basin promise much to the 

 pollen analyst. Radiocarbon dating of lake sediments may be the 

 most profitable means of correlating pluvial and glacial events, and 

 radiocarbon dates of Searles Lake in southern California confirm a 

 direct relationship between Great Basin pluvial lakes and the classic 

 Wisconsin sequence (Flint, 1957, p. 232). To the ornithologist this 

 should signify abundant and highly productive aquatic environ- 

 ments at a time when glacial ice covered most of the present breed- 

 ing grounds of waterfowl and scolopacid shore birds. , 



An original analysis was made by Antevs (1954) of zonal dis- 

 placement in New Mexico during the Wisconsin. Snowline depres- 

 sion, the hydrography of pluvial Lake Estancia, and the former 

 distribution of Marmota indicate a lowering of life zones in New 

 Mexico on the order of 4,000 to 4,500 feet. The yellow-bellied 

 marmot is reported from Basket-maker burials at 7,000 feet in 

 northeastern Arizona (Lange, 1956). This, and its distribution in 

 grassland of Utah at 4,500 feet, make it a less reliable Pleistocene 

 thermometer than Antevs (1954), Stearns (1942), and Murray 

 (1957) have assumed. Nevertheless, the vertical displacement of 

 snowline and, apparently, of treeline (Martin, 1958b) seems to 

 require a major shift in montane vegetation gradients. 



Spruce (Picea) should be an ideal indicator of temperature change 

 in the Southwest and should afford some biological control on the 

 geological evidence of climatic change. Clisby and Sears' pollen 

 study of the San Augustin plains (1956) indicated Full-glacial spread 

 of spruce through central New Mexico at 7,000 feet, with a maxi- 

 mum frequency of 40%, sometime before 27,000 B.P. During the 

 Pleistocene, almost certainly in the Wisconsin, spruce reached the 

 Valley of Mexico (Sears et al., 1955). To enter the Mexican Plateau, 

 spruce, and any associated boreal animals and plants, had to descend 

 to lower elevation. The lowest point on the Continental Divide be- 

 tween the Rockies and the Sierra Madre lies at about 4,500 feet in 

 southern New Mexico. In the adjacent Chiricahua Mountains spruce 



