Pinnacle Mountain (Arkansas)

Pinnacle Mountain is located between the Maumelle River to the north and the Little Maumelle River to the south. The Maumelle River empties into the Arkansas River at Maumelle Park approximately 2.8 miles (4.5 km) east of Pinnacle Mountain. The Little Maumelle River empties into the Arkansas River at Two Rivers Park approximately 6.6 miles (10.6 km) east of Pinnacle Mountain.

The United States Board on Geographic Names once defined a mountain as any landform greater than 1,000 feet (305 m) of local relief and a hill as any landform less than 1,000 feet (305 m) of local relief. Though Pinnacle Mountain is 1,011 feet (308 m) above mean sea level, it is only 761 feet (232 m) of local relief, i.e., 761 feet (232 m) higher than the nearby Arkansas River, which is 250 feet (76 m) above mean sea level. Due to broad disagreement over this naming convention, the classification system was abandoned in the early 1970s. According to the Geographic Names Information System (GNIS), 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.^[3][4] Its proximity to the Arkansas River and steep south slopes make Pinnacle Mountain a prominent landform observable from many areas in Pulaski County.

Pinnacle Mountain is not a volcano. Despite resembling a cinder cone (a small conical volcano), Pinnacle Mountain is composed of sedimentary rock, the Jackfork Sandstone.^[5] Named after Jackfork Mountain in Oklahoma, the Jackfork Sandstone is an exceptionally hard quartzitic sandstone; weathered surfaces are usually beige in color, while fresh surfaces tend to be much lighter.^[6] The Jackfork Sandstone can exhibit conchoidal fracturing, a defining characteristic of quartz, which, along with its great hardness, suggests the possibility of a metasandstone.

The sediment that eventually formed the Jackfork Sandstone was deposited in the ocean prior to the Ouachita Orogeny, the event that formed the Ouachita Mountains.^[7] During the early Pennsylvanian, slope collapse on the edge of the continental shelf emptied vast quantities of sand into a deep trough, which was slowly buried and transformed into sandstone. During the middle Pennsylvanian, the collision of two continents, Laurasia and Llanoria, resulted in uplift of the Ouachita Mountains, completing the assemblage of the supercontinent Pangea. The Ouachita Mountains have been above sea level undergoing erosion since the late Pennsylvanian, approximately 300 million years ago. Pinnacle Mountain is a remnant of the rock deep within this ancient mountain range.^[8]