Jurassic Navajo sandstone forms the upper part of the bold cliff that defines the culmination of the structure and comprises much of the smooth-stripped structural surface around the northern, western, and southern flanks of the structure. The difference in dip of the rocks on east and west is shown by the wider bands of exposure on the more gently dipping west side and the less pronounced scarp face. The dark spots near the south end of the swell mark relatively young volcanic centers. These centers lie generally on trend with the narrow graben mentioned above and may mark the locus of a deeper seated fracture that controls both graben formation and the volcanic activity, each of which postdates the swell. Close examination of the topography in the core of the swell reveals linear and rectangular patterns, indicating sets of joints that erosion has emphasized by etching.
Joints trend northsouth, northwest, west-northwest, and northeast.
Several small faults parallel these joints. The joints appear to be related to the processes that produced the fold. In flowing across the axis of the swell, the San Rafael River disregards both structure and rock type. East of the swell in the vicinity of the La Sal Mountains lie the folded and faulted rocks of the Paradox basin. Ridges formed of more resistant strata and valleys carved in weaker rocks outline the structure around this central core.