was developed along a north-south alignment or perpendicular to the compressional forces. As the stress level became too great, the rocks were first folded and then faulted. Initial faulting of the rocks relieved some of the stress; however, with continued application of compressive forces many stages of folding and faulting were generated. Because the alignment of the compressive forces remained about the same throughout deformation (i.e., east-west) and because the strongest relative compressive forces were from the west, successively younger folds and faults were generated in an eastward direction.

The structural evolution of the Fossil Butte area may be interpreted as follows from the schematic diagrams:

1. The Rock Creek-Needles Anticline is developed at the western edge of the Fossil Basin. The present topographic highs developed on this structure are the Crawford Mountains and Tunp Range (Fig. 9-B).

2. With continued application of compressional forces, Paleozoic and Mesozoic rocks to the east of the Rock Creek-Needles Anticline are folded (Fig. 9-C).

3. The fold becomes sharply asymmetrical and when the rocks can no longer accommodate the compressional forces by further folding, a low-angle fault is developed (Absaroka thrust fault). This fault probably began as a fracture parallel to bedding which rose to topographic surface at a low angle at the point of greatest strain in the fold. Most likely, this fracture was an eastward extension of the fault underlying the Rock Creek-Needles Anticline, as depicted in Fig. 9-D.

4. The rocks to the west of the newly developed fault overrode those to the east. This was accompanied by further downwarping in the syncline to the east of the fault and the erosion of uplifted sedimentary rocks on the west side of the fault (Fig. 9-E).

5. Another cycle of folding and faulting was initiated to the east of the Absaroka fault. This post-Absaroka deformation instigated further downwarping of the Fossil Basin area between the Tunp-Crawford Mountains and Oyster Ridge. Tertiary sediments began to accumulate in the Fossil Basin (Fig. 9-F).