that are mostly covered by much younger gravel and sand layers deposited in the last several million years. The region is incised by canyons that drain south and west to McElmo Creek; the creek falls from the east at about west at the Utah border. The oldest rocks are exposed in McElmo Canyon and are sedimentary layers of Jurassic age. In and near the Monument, these sedimentary rocks have been bent upwards to form a dome, the McElmo Dome: most of the Jurassic rocks are below the surface in the regions immediately bordering the area. The southern boundary of much of the Monument lies just north of McElmo Creek. The Ute Mountains to the south are cored by igneous intrusions and rise to . Formations from the Cretaceous and Jurassic periods The following description of the geology of the Monument is derived mostly from contributions to Anderson et al. (1997), unless otherwise referenced. The nomenclature of the rock units here may not represent the present consensus. The sedimentary sequence is over thick and is represented by the stratigraphic units listed below. Carboniferous Period Carbon dioxide is produced from rocks of Mississippian age (Carboniferous Period 359.2–299 million years ago) in the McElmo Dome, from wells drilled to a depth of about . The dome contains one of the largest deposits of carbon dioxide in the United States, and the extracted gas is used for enhanced oil recovery. This carbon dioxide reservoir has been studied to learn about possible underground storage (carbon sequestration) of carbon dioxide. Oil and gas exploration In 1986, the area was designated as the Anasazi Area of Critical Environmental Concern by the Bureau of Land Management to ensure there are no new leases for oil and gas industry; Leases will only be given to promote the conservation of oil and gas resources. 85% of the monument was under lease by ranchers and oil and gas exploration entities in 2000. With only 2,000 acres of monument land that is not leased, and that land home to ancient archaeological sites and wildlife habitat, four environmental groups have been active to ensure there is no further oil and gas exploration and extraction. However, off-road vehicles and an increase in road construction for oil and gas exploration, allow greater access to archeological areas. Large trucks known as "thumpers" were found searching for oil and gas pockets by pounding the earth and recording