Mascall Formation

The rocks of the Mascall Formation were laid down in a series of wide, level basins following a ferocious volcanic period 15 million years ago. The Mascall landscape consisted of several broad basins with lakes and meandering streams that formed atop the last of the Picture Gorge Basalt flows.

masc-ratt wide
Buff to tan layered ashes of the Mascall Formation

NPS Photo

These deposits were subsequently covered by successive falls of ash from volcanoes to the west and from the much closer Strawberry volcanics to the east. Alternating between the tuffs (consolidated volcanic ash) are layers of ancient soils and stream deposits that provide evidence of a dynamic floodplain. Many of the vertebrate fossils from the Mascall formation are found in close association with a prominent layer, the 15 million-year-old “Mascall Tuff.”

spring at the Mascall formation
Spring at the Mascall overlook

NPS Photo

Changing Climates

Although dramatic fluctuations in the global climate and regional volcanic activity continued, there were enough phases of moderate climate with ample rainfall and fertile soil to allow the growth of lush grasses and mixed hardwood forests. This savanna-like landscape was characterized by broad, level floodplains with scattered lakes.

Fossils from the Mascall Formation
Today the buff to tan colored ashes of the Mascall Formation show up in outcrops across central to eastern Oregon and preserve ancient rodents, elephants, and horses. Evidence from the Mascall Formation indicates a period where forests returned, outcompeting the sagebrush steppe of the mid-Miocene.

Last updated: December 14, 2017

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