Petroglyph National Monument stretches along Albuquerque, New Mexico's West Mesa, a volcanic basalt escarpment that dominates the city's western horizon. Authorized June 27, 1990, the 7,236 acre (29.28 km2) monument is cooperatively managed by the National Park Service and the City of Albuquerque. The western boundary of the monument features a chain of dormant fissure volcanoes. Beginning in the northwest corner, Butte volcano is followed to its south by Bond, Vulcan, Black and JA volcanoes. Petroglyph National Monument protects a variety of cultural and natural resources including five volcanic cones, hundreds of archeological sites and an estimated 24,000 petroglyph images carved by Ancestral Pueblo peoples and early Spanish settlers. Many of the images are recognizable as animals, people, brands and crosses; others are more complex. Their meaning was, possibly, understood only by the carver. These images are the cultural heritage of a people who have long since moved into other areas and moved on through history for many reasons. The monument is intended as a protection for these lands and sites from and for visitors to see and appreciate for generations to come. The national monument is managed in a manner that allows recreational use. The monument has four major sites that visitors can access, Boca Negra Canyon, Rinconada Canyon, Piedras Marcadas Canyon, and the Volcano Day Use trails. Geologic history Approximately 200,000 years ago, six volcanic eruptions created a cliff containing thick basalt layers of rock and cooled lava. When the volcanoes erupted, molten lava ranging in depth from 5 to 50 feet flowed downhill using old water ways, called arroyos, which eventually formed triangular, peninsula shaped channels that flowed around hills. The hills have long since eroded away over time, while the stronger basalt rocks remained, which eventually cracked and formed canyons and escarpments. As time progressed, more eruptions occurred and thicker lava cooled to form the now-extinct volcanic cones to the west of the monument; these cones can be seen from the top of the mesa. This unique formation of the landscape is called reverse topography. The basalt rocks' geologic nature allows for the creation of the petroglyphs, or rock carvings, on their surface. The rocks contain high concentrations of iron, manganese and calcium; this combination creates rocks of a gray-like color. However, over thousands of years of exposure to the desert's rough environment, a "desert varnish" forms on the surface. The varnish is formed from the