890 EMILIANI AND FLINT [CHAP. 34 



do not permit the dating of earlier Pleistocene events by extrapolating rates 

 obtained from the short time span measurable with radiocarbon. Some attempts 

 have been made to date continental Pleistocene material by the potassium- 

 argon method, but important difficulties, both of sampling and of techniques, 

 are yet to be solved. Deep-sea sediments, which sometimes offer continuous 

 and undisturbed stratigraphic sequences and are more easily datable than 

 continental material by means of radioactive methods having a greater range 

 than radiocarbon, may make possible the estabhshment of a continuous, dated 

 thermal record covering the whole Pleistocene. Correlation of geophysical, 

 geochemical and biological events with this record should then afford insight 

 into the rates at which many primary processes operate. Although the Pleisto- 

 cene Epoch represents only a fraction of 1% of the time elapsed since the 

 beginning of the Cambrian Period, probably it is long enough to resolve the 

 activities of many of these processes. 



2. Continental Stratigraphy 



A. Character of Data 



Research in the stratigraphy of the Pleistocene is focused directly or in- 

 directly upon three lines of enquiry: sequence, climate and absolute dating. 

 The goal that can be visualized, but that is still far from being attained, is a 

 stratigraphic sequence correlated from one region and continent to another, 

 and time calibrated at significant points throughout its length. Attached to 

 the sequence would be absolute values of mean annual temperature for repre- 

 sentative stations at various latitudes. The maxima and minima for a station, 

 changing with time, would measure roughly the amplitude fluctuations, during 

 the Pleistocene, of temperature, a basic parameter of climate on which other 

 parameters appear to be dependent. 



This is a very long-term goal, but progress toward it is already substantial. 

 From evidence of many kinds there has been erected a series of regional strati- 

 graphic sequences to which new discoveries are fitted by correlation. The units 

 within the sequences carry labels that, through widening use, are approaching 

 standardization, although universal standards are still in the future. As much 

 of the early stratigraphic work was done in glaciated regions, the iniits recog- 

 nized first were glacial sediments consisting chiefly of drift, and interglacial 

 units consisting of soils, weathering zones and sediments with fossils of tem- 

 perate- rather than cold-climate aspect. These constitute the chief basis of 

 the sequences developed up to the present. 



The sequences follow reasonably well the principles on which the classification 

 of the earlier geologic strata has been built. These principles involve ideally the 

 recognition of time-stratigraphic units, both major and minor, with each unit 

 defined, at least in theory, in terms of the fossils it contains. In parts of 

 Africa, artifacts have been substituted for fossils, with results that, so far, 

 appear reasonable. 



