CHAP, 34] THE PLEISTOCENE RECORD 899 



North America both from core borings. One is a pair of cores from lake sedi- 

 ments beneath Mexico City (Sears et al., 1955). The pollen stratigraphy shows 

 fluctuation of moisture-indicating trees through a range of about 40% of total 

 plants represented, and imj)lies significant climatic variation. However, 

 although it seems likely that the sequence might extend as far back as the last 

 major interglacial age, the fluctuations have not yet been correlated with those 

 derived from other localities. Another sequence is derived from a core in western 

 New Mexico (Chsby et al., 1956, 1957). It shows fluctuation of the proportion 

 among subalpine trees, other woodland and semi-desert plants, and is calibrated 

 by i^C dates in its upper part. This sequence, too, remains uncorrelated with 

 sequences elsewhere. There is a distinct possibility that climate at the locality 

 may have been altered by tectonic movement. 

 . Fluctuation of the evaporation-precipitation ratio is implied also by the 

 results of study of a long core taken from beneath Great Salt Lake in Utah 

 (Eardley and Gvosdetsky, 1960). No pollen is involved, and correlation is only 

 tentative, but it is thought possible that much of the Pleistocene is represented. 



In South America pollen stratigraphy is being elaborated by Thomas van 

 der Hammen from cores taken from basin fill near Bogota, Colombia. Results 

 to date include curves showing fluctuation of temperature and precipitation 

 continuously back into the glacial age preceding the last major interglacial 

 (van der Hammen and Gonzalez, 1960, p. 305). The curve, located close to the 

 equator, resembles curves for localities in Europe, North America and the deep- 

 sea floor. 



As to both range of temperature and departures from today's values, then, 

 the record is meager and spotty. However, two generalizations seem justified. 

 First, the ranges derived from physical data are of the same order as those de- 

 rived from organic data — several degrees centigrade. Secondly, today's values 

 lie within the Pleistocene ranges, usually near their upper limits. But the rela- 

 tion of the temperature fluctuations to the units of the stratigraphic column 

 remains incompletely defined. The view is commonly held that there are major 

 units (the stages of North American usage) and within them secondary units 

 (some of them called substages in North America), but a clear distinction be- 

 tween them has never been agreed on. In European practice glacials and inter- 

 glacials (essentially the stages of North American usage) are distinguished from 

 stadials and interstadials (essentially substages); here, at least by implication, 

 the diff"erence lies mainly in span of time and amplitude of temperature change 

 represented, rather than in differences among the related fossils, if fossils are 

 present at aU. As knowledge grows it may appear that this classification is 

 too rigid; that fluctuations of various amplitudes and durations have occurred; 

 and that a more flexible framework of terminology may be needed. 



Whatever the ami^litude of fluctuations derived from continental data, the 

 Pleistocene changes of temperature appear to have been rather distinctly 

 localized in time, following a long antecedent period of generally falling tem- 

 perature. Table IV illustrates the falling trend through a period of some 50 

 miUion years. Within the record of that period no evidence of conspicuous 



30— s. m 



