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Pollen analyses of siltstone beds in sedimentary rocks that overlie and closely resemble the basal sedimentary unit of the Eocene volcanic sequence in the Washburn Range, north-central Yellowstone National Park, have yielded pollen types and percentages indicative of an age no older than latest Pliocene. These sedimentary rocks are lithologically similar to others that lie conformably beneath the slightly more than 2.0-million-year-old Junction Butte Basalt and are therefore no younger than latest Pliocene in age. The geomorphic setting of the sediments in a north-trending paleovalley suggests that their source was an upland to the south in an area of central Yellowstone Park which has since foundered because of caldera collapse. Comparison of the pollen analyses of the sediments at two localities and the altitudes of the localities with the modern pollen rain and altitude limits of present day forest associations suggests that the vegetation zones and climate in latest Pliocene time were broadly similar to present conditions. The sedimentary rocks are correlated with others in and near Yellowstone Park on the basis of their overall relation to the Huckleberry Ridge Tuff, which overlies the Junction Butte Basalt and is also pottasium-argon dated as slightly more than 2 million years old.

Climatic implications of two pollen analyses from newly recognized rocks of latest Pliocene age in the Washburn Range, Yellowstone National Park, Wyoming