Pinnacle Mountain is situated between the Big Maumelle River to the north and the Little Maumelle River to the south. Its proximity to the Arkansas River and steep south slopes make it a prominent landform. The U.S. Board on Geographic Names once held that any landform greater than 1,000 feet of local relief classified as a mountain, while any landform less than 1,000 feet of local relief classified as a hill. Though Pinnacle Mountain is 1,013 feet above mean sea level, it is only 763 feet of local relief, which would have made it a hill. Due to broad disagreement over this naming convention, the classification system was abandoned in the early 1970s. According to the Geographic Names Information System, Pinnacle Mountain is a summit, which is defined as "a prominent elevation rising above the surrounding level of the Earth's surface", but does not include mountains or hills, among other things.
Geology
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 PennsylvanianJackfork 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.
Hiking
Two routes ascend Pinnacle Mountain: the 1.5-mile West Summit Trail, which starts at the West Summit parking off of Highway 300, and the 1.5-mile East Summit Trail, which starts at the East Summit parking lot off of Pinnacle Valley Road. The West Summit Trail is more trafficked than the East Summit Trail, partly because the East Summit Trail involves scrambling up a steep slope. Both, however, can be quite overcrowded due to their proximity to Little Rock. The 2.5-mile Base Trail loops completely around the base of Pinnacle Mountain and connects the West Summit and East Summit trailheads. The 223-mile Ouachita Trail overlaps a 0.5-mile section of the Base Trail and continues another 1.5 miles to the visitor center after crossing Pinnacle Valley Road at the East Summit parking lot.
