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The Henry Mountains is a mountain range located in the southeastern portion of the U.S. state of Utah that runs in a generally north-south direction, extending over a distance of about . They were named by Almon Thompson in honor of Joseph Henry, the first secretary of the Smithsonian Institution. The nearest towns are Bicknell, Torrey, and Hanksville, which is north of the mountains. The Henry Mountains were the last mountain range to be added to the map of the 48 contiguous U.S. states (1872), and before their official naming by Thompson were sometimes referred to as the "Unknown Mountains." In Navajo language, the range is still referred to as Dził Bizhiʼ Ádiní ("mountain whose name is missing").Linford, L. Navajo Places. History, Legend, Landscape. University of Utah Press. 2000.
Most lands within the Henry Mountains are federally owned, public lands administered by the U.S. Bureau of Land Management. A herd of 350 American Bison roams freely in the Henrys.
The Henry Mountains are drained by several canyon systems which radiate away from the isolated range, flowing north into the Fremont River, east into the Dirty Devil River, or south into Lake Powell. Surface water is scarce in the Henrys. Precipitation is about annually at an elevation of at the foot of the mountains, increasing to about at elevation and then diminishing to about at the highest elevations.
The geology of these mountains was first studied in 1875-1876 by Grove Karl Gilbert. He coined the term "laccolite" (now laccolith) to describe the characteristic shapes of some of the igneous intrusions that core the mountains. The main type of igneous rock is porphyritic diorite. This continues to be a unique area of study for geologists who focus on intrusion emplacement processes, since the near-horizontal sedimentary strata are clearly folded to accommodate the magma.
The ages of igneous rocks are important for understanding the evolution of the Colorado Plateau. The ages of these rocks measured by potassium–argon dating between 1969 and 1990 were discordant, ranging from 25 to 45 million years. However, uranium–lead dating of zircon, argon–argon dating of hornblende, and fission track dating have yielded consistent ages of about 23 to 31 million years. The intrusions are hosted by Permian to Cretaceous sedimentary rocks. The geology of these mountains is similar to the geology of the La Sal Range and of the Abajo Mountains, both also on the Colorado Plateau in southeastern Utah.
"The conditions of aridity, isolation, and rugged surroundings . . . have always kept settlement and development in the Henry Mountains to a minimum . . . . The landscape is just too harsh, the distances too vast."
The Henry Mountains Bison Herd was created in 1941 when 18 bison, including three bulls, were moved from Yellowstone National Park and released near the Dirty Devil River, south and east of Hanksville, Utah. An additional five bulls were added to the population in 1942. The herd has gradually moved toward the Henry Mountains, frequenting elevations up to . The Henry Mountain herd has been brucellosis-free since 1963.
Utah wildlife biologists set a population objective of 325 bison by 2012 for the Henry Mountain herd. As bison reproduce rapidly and the herds have been larger than this in the past, a decision was made to reduce the size of the herd. To achieve this objective and increase overall genetic diversity, breeding animals are transplanted from the herd to other locations. In January 2008, Utah Division of Wildlife Resources officials transplanted 31 animals to the Book Cliffs in eastern Utah. The new group joined 14 animals previously released in August, 2008 from a private herd on the nearby Uintah and Ouray Indian Reservation. In addition, special licenses are issued annually to hunt the animals and help reduce the excess population. In 2009, 146 public once-in-a-lifetime Henry Mountain bison hunting permits were issued.
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