The Ambrosia Lake district in northwestern New Mexico is the most important U mining and milling district in the United States. Together with the nearby Laguna district it contains more than 50% of the nation's reserves. Most of the ore occurs in the Morrison Formation of Late Jurassic age as elongate, tabular, mantolike bodies principally in the upper half of the Westwater Canyon Sandstone Member and near the base of the Poison Canyon sandstone tongue. Individual deposits are distributed along 2 easterly trending belts 2 to 3 mi. apart. The ore bodies are as much as 3000 ft. long, several hundred feet wide, and 100 ft. thick. Depths to the ore range from 0 to 2200 ft. Some ore is also mined from the Todilto Limestone [Member] of Late Jurassic age and from the Dakota Sandstone of Early(?) and Late Cretaceous age. Two types of unoxidized ore are recognized: pre-fault ore, which is considered to be primary, and post-fault ore, which may be redistributed. The pre-fault ore shows no obvious relationship to tectonic structures but appears to be controlled by a variety of sedimentary structures. Post-fault ore is controlled by a combination of sedimentary and tectonic structures. Disseminated carbonaceous matter, believed to be plant derived, appears to be the dominant control in the localization of the U. The ore mineralogy is comparatively simple, and coffinite is by far the most abundant ore mineral. Mo, Se, V, and Fe occur in anomalous quantities in the deposits in both oxidized and unoxidized minerals. U/eU ratios and radioisotope distribution indicate almost universal disequilibrium and fairly recent migration of radioisotopes in all deposits that have been sampled. Further studies on the organic carbonaceous matter, sandstone alteration, age determinations, and S isotope composition are required to obtain a better understanding of the source, transportation, and deposition of U and other elements in the deposits.