The early stages of the Cenozoic Cool Mode started with the cooling in the early Eocene (55 Ma). From that time onwards the climate of the Earth gradually cooled from the Warm Mode of the late Cretaceous to early Tertiary to the cool glacial climates of today. Important changes which occurred during this phase include the enhancement of climatic zonation and the development of a thermally stratified ocean. Unlike the Palaeozoic record, the early part of the Cenozoic cooling is not recorded simply by the presence of ancient glacial deposits. In fact, any direct evidence of extensive glacial ice at the poles during the Tertiary is scarce, principally because the rocks in these regions are now covered with ice. However, the presence of ice-rafted debris in deep ocean cores provides positive evidence for the presence of at least seasonal ice.

The principal evidence for Tertiary cooling is documented in the oxygen isotope record of calcareous foraminifera from the oceans. This illustrates the decline in ocean temperatures and the build-up of ice at the poles (mainly the South Pole) from about 55 Ma onwards (Fig. 9.1). Climate evidence from fossil plant assemblages reflects the same cooling trend. Documentation of the Tertiary cooling is important because it illustrates the crucial transformation phase from a non-glacial to glacial state, as recorded by several geological parameters. This trend or transformation can be used as a model to determine the history of glacial build-up during former Cool Modes, such as those in the Palaeozoic, for which data are less reliable.