More than 7,000 feet of uppermost Cretaceous and Tertiary continental strata in the northern Fossil basin, along the southern part of the western Wyoming thrust belt, is here subdivided into newly defined formal and informal rock-stratigraphic units. Recognition of these units makes possible the dating of some thrust-fault movement of the development of the Fossil structural basin, and of some later block faulting, as well as reinterpretations of the origins of the strata. The Evanston Formation, which is more than 2,000 feet tick and more extensive in the Fossil basin than formerly recognized, consists of an unnamed lower member, the Hams Fork Conglomerate Member, and the main body. The lower member consists of as much as 500 feet of gray to dark-gray mudstone, siltstone, claystone, and carbonaceous fine-grained, but partially gritty and conglomerate, sandstone and is of latest Cretaceous, probably Lance, age. The Hams Fork Conglomerate Member, previously unmapped erroneously as the Atly Formation along the east side of the Fossil basin, consists mainly of 450-1,000 feet of Intest Cretaceous (Lance) boulder conglomerate and interstratified partly conglomeratic brown and gray sandstone and gray partly carbonaceous mudstone; sources of the well-rounded boulders and pebbles were the nearby upper Paleozoic and Mesozoic formation and lower Paleozoic units of the Paris thrust sheet in Idaho. The main body of the Evanston, 400 to more than 1,400 feet thick, consists mainly of Paleocene but also latest Cretaceous light to dark-gray carbonaceous sandy to clayey partly quartzitic siltstone, gray, tan, yellow, and brown sandstone and conglomerate, carbonaceous to lignitic claystone, ironstone, lignite, and, locally, thin beds of coal.