page_title: Hot Springs National Park
section_title: Rock types
hierarchical_section_title: Hot Springs National Park / Geology / Rock types
caption_attribution_description: English: Hot Springs National Park
context_page_description: Hot Springs National Park is an American national park in central Garland County, Arkansas, adjacent to the city of Hot Springs, the county seat. Hot Springs Reservation was initially created by an act of the United States Congress on April 20, 1832 to be preserved for future recreation. Established before the concept of a national park existed, it was the first time that land had been set aside by the federal government to preserve its use as an area for recreation. The hot spring water has been popularly believed for centuries to possess medicinal properties, and was a subject of legend among several Native American tribes. Following federal protection in 1832, the city developed into a successful spa town. Incorporated January 10, 1851, the city has been home to Major League Baseball spring training, illegal gambling, speakeasies and gangsters such as Al Capone, horse racing at Oaklawn Park, the Army and Navy Hospital, and 42nd President Bill Clinton. The area was made a national park on March 4, 1921. Until the redesignation of Jefferson National Expansion Memorial as Gateway Arch National Park in 2018, Hot Springs was the smallest national park by area in the United States.
context_section_description: The exposed rock types in the vicinity of the thermal springs are sedimentary rocks of Mississippian to Ordovician age, with the exception of younger igneous rocks (Cretaceous age) exposed in two small areas about  6 and 11 miles (9.7 and 17.7 km) southeast of the thermal springs (Potash Sulphur Spring and Magnet Cove, respectively), and in many small dikes and sills. Most dikes are less than 5 ft (1.5 m) wide. There have been 80 dikes noted about 4 miles (6.4 km) southeast of Hot Springs, on and near the Ouachita River.  There is no indication that igneous rock occurs where the thermal springs discharge.
  The sedimentary rocks in the vicinity of the thermal springs consist of shale, chert, novaculite, sandstone, and conglomerate.
  The Stanley Shale is predominantly a clayey, thinly fissile, black to green shale, with large amounts of sandstone interbedded throughout the formation. The sandstone, when freshly exposed, is a hard, fine-grained, feldspathic, silty sandstone, but weathers easily to a soft, clayey porous material ranging from green to brown in color. Almost all of the low-lying areas in the city of Hot Springs are composed of the Stanley Shale, and it surrounds Hot Springs Mountain on the south, east, and west sides.
  The Hot Springs Sandstone Member of the Stanley Shale, hereafter referred to as the Hot Springs Sandstone, consists of fine- to medium-grained sandstone with some shale and conglomerate. The sandstone is gray, hard and quartzitic, reaching thicknesses up to 6 ft (1.8 m). The shale predominantly occurs at the top of the unit, and the principal bed of the conglomerate occurs at the bottom.
  The Arkansas Novaculite consists of lower, middle, and upper members. The lower member is a massive fractured novaculite, and is the dominant member on Hot Springs Mountain, with a thickness of about 275 ft (84 m) The middle member is a black clay shale interbedded with novaculite, about 10 ft (3.0 m) thick on Hot Springs Mountain. The upper member is chiefly a massive, highly calcareous light gray to black novaculite. It reaches a maximum thickness of 180 ft (55 m) in the area.
  The Polk Creek Shale and Missouri Mountain Shale overlie the Bigfork Chert and generally consist of shale with minor thin layers of quartzitic sandstone. The Polk Creek Shale is a black, fissile, graphitic shale. The Missouri Mountain Shale varies in color, and is soft and argillaceous.
  The Bigfork Chert overlies the Womble Shale and consists almost entirely of chert and silty chert in layers  2 to 12 inches (51 to 305 mm) thick, separated by minor thin beds of black shale. The chert is very brittle and intensely fractured from folding.
  The Womble Shale is the oldest geologic unit that underlies all other exposed units. It is black, hard, and argillaceous shale.
  During most of the Paleozoic Era, what became the Ouachita Mountains was the bottom of a shallow sea, where several sedimentary layers were created. About 500 million years ago a collision of the South American Plate with the North American Plate caused the shale and sandstone layers to fracture and fissure, creating mountains of the folded rocks. The thermal springs emerge from the plunging crest line of a large overturned anticline in the Zigzag Mountains of the Ouachita anticlinorium. The overturned anticline plunges toward the southwest into the Mazarn Basin. There are two recognized major thrust faults trending nearly parallel to fold axes that define the northern and southern limits of the thermal springs discharge area. The northern fault extends nearly parallel to Fountain Street northeastward about 9,200 ft (2,800 m) onto the southeast flank of North Mountain, and dips about 26 degrees north. At the northern extent of the thermal springs, this fault is suggested to form along the bedding contact of the Hot Springs Sandstone and Stanley Shale, with the Stanley Shale forming the hanging wall of the fault. The southern fault extends northeastward about 9,000 ft (2,700 m) roughly along the axis of the Hot Springs anticline, and dips about 44 degrees north. It has been proposed 
caption: Remaining natural hot springs
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