This feature is part of the Saddlehorn Creek Cluster of volcanoes, as defined by Hildreth and others (2004). From Hildreth and others (2004): "Folsoms Bluff (Knob 3800), an inhomogenously andesitic (55-60% SiO2) funnel-shaped vent complex, is a multi-lobate glassy lava mass 500 m wide and 200 m high that makes up part of the canyon wall just 2 km east of Fenners Saddlehorn. Marked by steep flow foliation and several sets of inclined, subhorizontal, or steeply curving glassy columns indicative of ice-contact emplacement, the lava has a brecciated base that overlies 8 to 15 m of stratified, poorly sorted proximal fallout, which includes scoria bombs to 75 cm and blocks of basement granitoid to 30 cm. This basal fallout drapes a steep paleoslope and extends uphill into a mass of agglutinated lithic-rich rubble more than 20 m thick, probably vent fill largely concealed by the overlying lava. All lithologies contain abundant small plagioclase phenocrysts as well as olivine, clinopyroxene, orthopyroxene, and magnetite. Its canyon-wall setting and eruptive facies relations suggest that, like Fenners Saddlehorn nearby, this undated glassy unit is younger than the three ridge-capping members of the cluster."
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