The 16-13 million year old Mascall assemblage caps the Picture Gorge Basalts with a sequence of ashy layers and paleosols (fossilized soils). It records a gradual yet influential climate change event that took place over millions of years, the mid-Miocene climatic optimum. Paleosol evidence from the Mascall assemblage indicates a period where forests returned, outcompeting the sagebrush steppe of the mid-Miocene.

These wooded environments were similar to modern temperate forests found in the Eastern United States, filled with swamp cypress along bodies of water, deciduous forests in the lowlands, and coniferous forests in upland habitats. Though forests dominated the area, the decline in shrub land allowed for the growth of short sod grasslands. Grazing animals lived alongside the more common, larger mammals that dominate the Mascall fauna.

Dominant Fossils Found in this Assemblage: Mylagaulus (rodent) Tephrocyon (bear-dog) Dromomeryx (giraffe-deer, hooved animal) Merychippus (three-toed horse) Gomphotherium (trunked, four tusked elephant relative) Parahippus (three toed horse) Archaeohippus (three toed horse) Taxodium (Swamp cypress) Aphelops (rhino) Miolabis (camel) Pseudaelurus (panther relative, tree climbing cat) Amphicyon (bear-dog) Mixed forest (Celtis, Liquidambar, Quercus, Ulmus, Acer, Fagus, etc.) Leptarctus (weasel) Clemmys (turtle) Falcon, unidentified