Populations of Globigerina pachyderma (Ehrenberg) are consistently left-coiling, cool-water types in sections of the lower Pleistocene of the Los Angeles basin, California; right-coiling, warm-water populatons are predominant in the upper Pliocene. The base of the Pleistocene, defined in this way, indicates a major change in water masses of the southern California area at that time. Recent radiometric dates by Obradovich place this boundary about 3 m.y. ago.

Populations of Globigerina pachyderma reflect bathyal conditions generally; modern populations in

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typical form live suspended deeper than about 50-100 m. Basing the beginning of the Pleistocene on planktonic foraminiferal criteria, benthic Foraminifera indicate that outer-shelf conditions prevailed at the beginning of the Pleistocene in some areas, upper-bathyal depths existed in other places, middle-bathyal depths in some areas, and lower-bathyal depths at still other locations. Thus, a boundary based on upper limits of benthic species (Epistominella pacifica and Uvigerina peregrina), and defined by comparing sections from different parts of the Los Angeles basin, differs hundreds of meters in stratigraphic position.

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