The Oligocene to early Miocene transition in southern Alaska perhaps represents one of the most dramatic floristic changes in the entire Tertiary of Alaska. The basic modification is from a dominant deciduous, broad-leaved forest biome in the late Eocene-early Oligocene (early Zemorrian) to a dominant moist, temperate, coniferous forest biome in the early Miocene (Saucesian). A similar change can be seen between the deciduous broad-leaved forests and the montane boreal coniferous forest of China today.

This change in flora--and from a palynological perspective this change in microflora--reflects the onset of global cooling in the Neogene and the concurrent change from a dominant marine transgressive to a dominant nonmarine regressive mode of sedimentation.

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