The Henry Mountains are located in the southeastern portion of the U.S. state of Utah and run in a generally north-south direction, extending over a distance of about 30 miles (50km). They were named by John Wesley Powell in honour of Joseph Henry, the first secretary of the Smithsonian Institution. The nearest town of any size is Hanksville, Utah, 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, and before their official naming by Powell, were sometimes referred to as the "Unknown Mountains."
The range is clustered into two main groups, with Highway 276 dividing the two portions. The northern group is by far the taller of the two with Mount Ellen: 11,506 feet above sea level; Mount Pennell: 11,371 feet; and Mount Hillers: 10,723 feet. The southern group is much lower in elevation. The southern group has two peaks: Mount Ellsworth: 8,235 feet and Mount Holmes: 8,000 feet. The southern group is also known as the "Little Rockies".
The Henry Mountains are drained by a number of 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.
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.
Ages of the igneous rocks are important for understanding the evolution of the Colorado Plateau. Ages of these rocks were reported to be about 45 to 50 million years in older geologic literature. However, it has been established that these intrusions formed in the period from about 23 to 31 million years ago, using uranium-lead dating of zircon and argon-argon dating of hornblende; fission track dating also has yielded consistent results (the older, erroneous ages were by potassium-argon dating). 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: locations are shown on a satellite image presented with information about the La Sal Range.
The Henry Mountain herd was created in 1941 when 18 bison, including three bulls, were moved from Yellowstone 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 above 10,000 feet. The Henry Mountain herd has been brucellosis-free since 1963.
A population objective of 325 bison by 2012 has been set by biologists for the Henry Mountain herd. To achieve this objective, and increase overall genetic diversity, breeding animals will be transplanted to other locations. In 2008-2009, Division of Wildlife Resources officials hope to transplant 25 animals to the Tavaputs Plateau in eastern Utah. The new group will be supplemented by up to 20 animals from a herd held privately 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 2007, eighty-one public once-in-a-lifetime bison hunting permits were available in Utah. (SL Tribune, January 10, 2008, pp. E1 and E2)