reportedly minor. Groundwater The Denver Basin aquifer system consists of a layered sequence of four aquifers in beds of permeable conglomerate, sandstone, and siltstone. Layers of relatively impermeable shale separate the aquifers and impede the vertical movement of ground water between the aquifers. The northern part of this aquifer system underlies the surficial aquifer of the South Platte River. Although the Denver Basin aquifer system and the surficial aquifer are hydraulically connected in part of this area, they primarily function as separate aquifer systems which serve as important sources of water supply in the region. Denver and its suburbs such as the communities in Douglas County draw ground water from the aquifers in the Denver Basin. There is recharge of about 40,000 acre-feet per year from the broken land to the west of the basin and withdrawals of about 10 times as much. Ground water levels have dropped in the aquifers especially near population centers which draw on the aquifers for water. Cement, construction aggregate and dimension stone Raw materials are mined from the Niobrara Formation and Pierre Shale and made into cement at the Cemex plant near Lyons, Colorado. Sand and gravel for construction are a major mineral resource in the Denver Basin. The Lyons Formation provides flagstone from quarries in Boulder and Larimer counties, along the western edge of the basin. Gold Small amounts of gold have been mined from sands and gravels in the Denver area since the Pikes Peak Gold Rush of 1858. Some sand and gravel pits still recover gold in their washing operations. Uranium A small amount of uranium ore has been mined from the Dakota Sandstone at Morrison, Colorado, where the sandstone is impregnated with petroleum. Uranium is known to exist in roll-front type deposits in the Denver Basin, but the basin has never been a major source of uranium. References Roadside Geology of Colorado, Halka Chronic, Mountain Press Publishing Company, Missoula, Montana (1980). External links and further reading Emmons, Samuel Franklin, 1841-1911; Cross, Whitman, 1854-1949; Eldridge, George Homans, 1854-1905, Geology of the Denver basin in Colorado (1896) Washington D.C, Government Printing Office Kansas Geological Survey: Geological Overview of the Niobrara Chalk Natural Gas Play 2002 USGS Assessment of Oil and Gas Resource Potential of the Denver Basin Petroleum Province of Colorado, Kansas, Nebraska, South Dakota, and Wyoming Colorado Geological Survey: Sedimentary Rock Aquifers: Denver Basin Structural basins of the United States Sedimentary