he Reef Creek fault is in northwestern Wyoming, a few miles east of the northeast border of Yellowstone National Park. It lies within the area covered by the more extensive Heart Mountain fault. Like that fault, it is a décollement or detachment fault in which strata became detached along a basal shearing plane and moved laterally on a slightly inclined fault surface. At the most northwesterly exposures the Reef Creek fault parallels the bedding; southeastward it cuts upward across the bedding as a transgressive fault and becomes a fault in which the allochthonous blocks moved on the surface of the ground. The fault blocks consist chiefly of the Madison Limestone of Mississippian age and volcanic tuffs and breccias of the Cathedral Cliffs Formation of early or middle Eocene age. The Reef Creek fault blocks are scattered over a 7- by 14-mile area, but a considerable part of the scattering is due to “piggy-back” transportation on the Heart Mountain fault blocks.

The Reef Creek fault is younger than the Cathedral Cliffs Formation and older than the middle Eocene early basic breccia. Consequently, the rocks transported by it were emplaced either in late early Eocene or in early middle Eocene time. Both the Reef Creek fault and the South Fork fault are older than the principal movement along the Heart Mountain fault. The movement of all three fault masses was southeastward, and the mechanics of their emplacement is believed to have been similar. Movement was due in part to gravity, but considering the low slope involved, gravity alone seems to be inadequate. The shaking motion of many earthquakes is suggested as a contributing force which acted in conjunction with the constant force of gravity.