The tectonic and sedimentary history of this structurally complex area in eastern Idaho and western Wyoming is summarized. Tectonic development is divided into three stages: The first, during Paleozoic and early Mesozoic, was a time of shifting patterns of tectonic elements; sediments, mostly marine, came from both east and west. The second started in the west in latest Jurassic, in part overlapped the first, ended in the east perhaps as late as early Eocene, and produced folds overturned toward the east and thrust faults dipping gently west. The third, started in Eocene time, has continued to the Recent, and is characterized by block faulting. Movement along many faults has been recurrent with possible reversed vertical movement along some north-trending faults. Present topographic relief is tectonic.