The Gardiner thrust fault has been mapped in detail from the west side of Cinnabar Mountain, Montana, where it appears from under a cover of Tertiary volcanics, southeastward to a point 3 miles east of Mount Everts, Yellowstone National Park, where it passes under the volcanics. The fault plane dips to the northeast at approximately 350, and is characterized by numerous subsidiary "slicings," also dipping to the northeast, beneath the major thrust plane. This high-angle, northeastward-dipping thrust forms the southwestern margin of the Beartooth Mountain uplift of which the low-angle, southwestward-dipping Beartooth thrust forms the northeastern margin. A genetic relationship is thought to exist between the thrusting and the intrusion of the dacite and andesite porphyries found within a close proximity of the fault plane. At some time following the development of the present topography basaltic extrusions came up along the fault plane. At a still later date hot springs issued from the fault plane, resulti...