The Gas Hills uranium district has produced about 12 percent of the United States total. Surface drilling was the most effective tool for finding and developing the below-water-level ores. Sedimentary rocks exposed range from Cambrian to Miocene and are over 14,000 feet thick. Source beds for uranium are arkosic sandstones interstratified with lensing mudstones and shales. Two types of sandstone are present in the Wind River Formation, a yellowish-orange to yellowish-gray arkose, and a pale yellowish-gray to pale olive; both derived from Precambrian rocks. The most important type of uranium deposit in the district is solution-front deposits, formed along margins of highly altered, tabular sand beds. Ore minerals are coffinite and uraninite, and occur as a coating on, or interstitial filling between, quartz sand grains. Several quite large transitional bedded deposits have also been mined.