Contrary to rumors, Pinnacle Mountain is not a volcano. Despite its resemblance to a cinder cone, Pinnacle Mountain is composed of deep-water sedimentary rock, the Pennsylvanian Jackfork Sandstone. Named for Jackfork Mountain in Pittsburg and Pushmataha counties, Oklahoma, the Jackfork Sandstone at Pinnacle Mountain is massive, fine- to coarse-grained, mostly tan quartzitic sandstone of great hardness. Cinder cones, on the other hand, form from the eruption of mafic lavas and are composed of extrusive igneous rocks such as basalt. Research proposes that the Jackfork Sandstone was deposited in a developing foreland basin during the early stages of the Ouachita Orogeny. It is conventionally interpreted as a sequence dominated by turbidites in a submarine fan setting, however, diagnostic internal features, such as inverse size grading, indicate sand emplacement by debris flows and slumps in a slope setting. The terrane has been deeply eroded since its deformation during the late Paleozoic.