Austin Chalk, Selma Chalk and numerous thin marl, chalk and sandstone layers. The Richton Salt Dome was briefly famous in the early 1980s as a proposed site for US nuclear waste disposal. A volcano built the Jackson Dome, uplifting the Eutaw Formation on a 184 square mile island. Magma and thermal gas created halos in the Smackover Limestone, Norphlet Sandstone and Cotton Valley Group rich in hydrogen sulfide and carbon dioxide and over-pressurized gas is a risk in the poor quality gas fields on the southeastern side of Jackson Dome. The Midnight Volcano in southern Humphreys County is another buried volcanic feature from 75 million years ago in the Cretaceous. Cenozoic (66 million years ago-present) During the Cenozoic, the Mississippi Embayment was filled by large river and delta systems, depositing sediments reaching up to 45,000 feet thick beneath the Gulf Coastal Plain and 35,000 feet in the Terrebonne Trench, west of the Mississippi River. In the Paleocene, the Midway Group limestone, marl and sand lenses formed, followed by the Wilcox Group into the Eocene. Shale, sandstone and limestone typified the Eocene Claiborne Group and Jackson Group, with a greater presence of limestone in the Oligocene Vicksburg Group. Thicker units such as the Catahoula and Hattiesburg Formations formed in the Miocene, followed by the Pascagoula Formation, Graham Ferry Formation and Citronelle Formation into the Pliocene. Alluvium, sand, gravel, barrier islands and loess define most of the Quaternary deposition from the past 2.5 million years. Soils Soils in Mississippi result from the weathering of bedrock, fine grained alluvial fill and loess (windblown glacial rock flour from the Mississippi River Alluvial Plain). The high fertility soils of the Loess Belt attracted many people to pursue plantation agriculture in the 1800s. Hardwood trees dominate in loess deposits north of Vicksburg, particularly sweet gum, basswood, water oak, cherrybark, poplar and bitternut. A few small prairies developed atop Cretaceous and Eocene chalk. Hydrogeology The state has up to 125,000 water wells, with the most significant pumping from the Mississippi River alluvial aquifer for agriculture. Analysis of 1369 water wells from 1989 to 2007 found only three wells with unsafe levels of pesticides and agricultural chemicals. The Miocene, Wilcox, Sparta, Lower Cretaceous and Tuscaloosa are the next most important aquifers after the Mississippi River aquifer. Historically, the thick Yazoo Clay made groundwater difficult to extract on the Jackson Prairie. Natural resource geology The discovery of the Amory