Interaction of the Rocky Mountain foreland and the Cordilleran thrust belt has taken place in the Snider Basin area, west of Big Piney, Wyoming. Here, the central segment (La Barge Platform) of the Moxa Arch, a Late Cretaceous, basement-cored uplift, plunges northwestward beneath, and disrupts the continuity of, the Darby, Hogs-back, and Prospect thrusts. Although to the north and south of the La Barge Platform, basement shortening appears to have been along low-angle thrust faults, this was probably not the case for the platform. The 2,600 m of west-verging structural relief along the southwest limb of the La Barge Platform occurs over about 5 km of horizontal distance and might be the result of shortening of the basement across a zone of discrete, closely spaced reverse faults, generally restricted to basement. This shear zone (here termed the La Barge shear zone) dips about 60°NE. Shortening in the basement appears to have been matched by shortening of the cover rocks along a pair of west-dipping reverse faults, the upper of which is the Tip-Top thrust. According to our interpretation, Tip-Top thrusting was synchronous with the emplacement of the La Barge Platform and thus predated the Hogsback thrust. Our work confirms earlier interpretations that the Darby and Prospect thrusts link in the subsurface below Snider Basin and continue to the south as the Hogsback thrust. According to our interpretation, Hogsback thrusting is a composite of Paleocene Prospect thrust movement and later Darby thrusting. In the area of the La Barge Platform, the Hogsback thrust was localized by following west-dipping Cretaceous beds on the monocline above the La Barge shear zone.

Paleomagnetic investigations throughout the overthrust belt in the Wyoming and Helena salients show that buttressing effects of the Rocky Mountain foreland have caused local rotations of thrust sheets along the margins of the salients. Two previous studies of Triassic, Upper Jurassic, and Lower Cretaceous rocks in the Wyoming-Idaho overthrust belt involved rocks from the Prospect, Bear, Darby, and Absaroka thrust sheets. These studies documented as much as 60° counterclockwise rotation in the northern edge, and as much as 30° clockwise rotation in the southeastern edge of the Wyoming salient. It has been suggested that these rotations were buttressing edge effects, since the rotations do not appear to have been transferred back into the interior of the Wyoming salient. New work in the southern portion of the Helena salient in southwestern Montana has documented a similar situation. Here, samples from the Lower Cretaceous Kootenai Formation show a maximum of 54° clockwise rotation in the region of the McCarthy Mountain salient, 23° clockwise rotation within the southwest Montana transverse zone, and 35° clockwise and 30° counterclockwise rotations in the nose of the salient. These rotations vary greatly from site to site, suggesting that the thrust sheets were not deformed in a coherent fashion, but rather, they broke and rotated as individual pieces where the effects of buttressing against the Rocky Mountain foreland were the greatest.

The Gros Ventre, southern Teton, northern Hoback, and Snake River ranges and Grayback Ridge of the Wyoming Range, and the buttes of southern Jackson Hole display a remarkably long and nearly complete stratigraphic column. Archean granites and gneisses are strikingly displayed in the eastern fault scarp and high peaks of the Teton Range and the southeast front of the Gros Ventre Range. Paleozoic rocks, representing all periods with the possible exception of the Silurian, aggregate about 6000 feet, and the Triassic, Jurassic, and Cretaceous are represented by more than 12,000 feet of strata. The Paleocene and Eocene are represented by a maximum of about 20,000 feet in the Hoback basin between the Hoback and Gros Ventre ranges, and in the Mt. Leidy highlands north of the Gros Ventre ranges and east of Jackson Hole. The Oligocene is apparently unrepresented except farther north in Jackson Hole, but the Miocene and early Pliocene are represented by the Camp Davis formation several thousand feet thick in Bryan Flats between Hoback Range and Grayback Ridge, in Jackson Hole, and in Star, Grand, and Swan valleys west of the Snake River Range. Volcanic rocks of Tertiary age are intercalated with the sediments in Jackson Hole and Grand and Star valleys, and there are late Tertiary intrusives in the Snake River Range. The Pleistocene is recorded by the Buffalo, Bull Lake, and Pinedale drifts of Kansan, Iowan and Mankato ages. The Gros Ventre and Teton ranges are within the stable platform or foreland area with Paleozoic and early Mesozoic sections of moderate thickness and comparatively simple structures. The faults are largely high-angle reverse faults with pressures from the north and east. The Hoback and Snake River ranges and Grayback Ridge belong to the borders of the southern Idaho geosyncline with a Cambrian (Gros Ventre) to Cretaceous (Frontier) section nearly 90 per cent thicker than the foreland section. Structurally this area is marked by a series of subparallel low-angle thrusts from the south and southwest including the Jackson, Little Granite, Game, Bear, Darby, Absaroka, Ferry Peak, and St. John thrusts. Although the amount of crustal shortening involved in the individual thrusts of this belt is unknown, the effects of this shortening, combined with marked southward and westward thickening of the sedimentary columns, bring about significant differences in succession and thickness in several formations even between points now only a few miles apart. During several field studies at the University of Michigan Rocky Mountain field station, at Camp Davis, Wyoming, detailed stratigraphic sections were measured in the exposed formations in each of the ranges. These sections show numerous persistent thin diagnostic strata not previously noted, and give a clearer picture of the facies changes between the foreland and geosyncline. These variations are shown by graphic sections of the strata of each system. Single measured sections of the Devonian, Mississippian, Pennsylvanian, and Permian successions and of the Bear River and Aspen formations of the Upper Cretaceous are included in the text, and other measured sections have been placed on open file at the office of The Geological Society of America, where they may be consulted.