The rocks on either side of Firehole Falls are two separate, Pleistocene-aged, rhyolite lava flows.

The cliff on the left side of the photo consists of rhyolites of the Nez Perce Creek Flow (Central Plateau Member, Plateau Rhyolite, upper Middle Pleistocene, ~152 ka (148-160 ka)).

The rocks on the right side of the photo are rhyolites of the West Yellowstone Flow (Central Plateau Member, Plateau Rhyolite, lower Upper Pleistocene ~111-114 ka).

The course of the Firehole River through Firehole Canyon represents the original contact between these two thick rhyolite lava flows.

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