| Summary: | Organ Pipe Cactus National Monument preserves an area of 1300 km² typical of the Lower Sonoran Desert ecosystem of southern Arizona, encompasses part of the ancestral lands of the Tohono O’odham Nation, and adjoins the international border between United States and Mexico. In 1976, the Monument was declared a UNESCO Biosphere Reserve; in 1977, 95 percent of the Monument area was designated Wilderness. The hallmark species of OPCNM is the organpipe cactus (Stenocereus thurberi). This columnar cactus, though common in northwest Mexico and widely scattered through parts of southwest Arizona, is most abundant in the United States in OPCNM. Another arboreal Mexican cactus, the senita (Lophocereus schottii), is found in the U.S. only in one small area within OPCNM. OPCNM geology is characterized by northwest-southeast trending mountain ranges separated by broad alluvial plains, typical of the southern Basin and Range tectonic province. Diverse volcanic, intrusive, metamorphic, and sedimentary rocks exposed within and near the Monument record nearly two billion years of Earth history, weather to picturesque geomorphic features, and support several distinctive plant communities. Rocks near the town of Ajo (18 km north of the Monument) host ores of copper, molybdenum, and gold. This report is a much-revised version of data originally compiled for the National Park Service Geologic Resource Inventory. The report and accompanying map collect decades-long work in geologic mapping of OPCNM and analytical studies of its rocks; document the geologic history of the Monument; and provide a complete, current geologic map for scientific, educational, and public use. Documents in the AZGS Documents Repository collection are made available by the Arizona Geological Survey (AZGS) and the University Libraries at the University of Arizona. For more information about items in this collection, please contact azgs-info@email.arizona.edu.
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