English: Volcanic tuff (bentonite) (ash-fall tuff) (Split Fish Layer, Fossil Butte Member, Green River Formation, Lower Eocene; Promised Lands Quarry, Hams Fork Plateau, southwestern Wyoming, USA. 67 mm across along the base. Volcanic tuff interbed in succession of fossiliferous lacustrine marlstones, Split Fish Layer, Fossil Butte Member, Green River Formation, Lower Eocene, ~49 to 51 Ma.

Locality: wall of Promised Lands Quarry (SE1/4 sec. 22, T22N, R17W), Hams Fork Plateau, ~13.5 to 14 km northwest of the town of Kemmerer, southwestern Wyoming, USA.

This is a volcanic tuff, also known as a bentonite. It formed during an ash fall event following an explosive volcanic eruption. The tuff is probably a magnesian rhyolite or dacite. Tuff beds such as this occur as interbeds in the famous fossil fish-bearing beds of the Green River Formation in southwestern Wyoming. The source volcano for this particular volcanic tuff is undetermined, but published research has shown that it likely derived from the Absaroka Volcanic Field (= modern-day Yellowstone National Park area and adjacent Absaroka Mountains of northwestern Wyoming) or the Challis Volcanic Field of central Idaho.