15 Million Years Ago, the rocks of the Mascall were laid down in a series of wide, level basins following a ferocious volcanic period. The Mascall landscape consisted of several broad basins with lakes and meandering streams that formed atop the last of the Picture Gorge Basalt flows.
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.”
If you would like to view Mascall’s geological timeline, check out the generalized stratigraphic column!
Today the buff to tan colored ashes of the Mascall formation show up in outcrops across central to eastern Oregon (shown here) 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.