High level, early to mid-Pleistocene piedmont and pediment deposits and surfaces along the southern Franklin Mountains extend out toward bolson fill deposits and surfaces. On the east side of the range, 400 ft (121 m) of post-bolson fill displacement has been reported by earlier workers. There has been no concomitant uplift along the west side of the range. Thus, the range has been tilted during Pleistocene deformation, and uplift has been mainly along the eastern fault. The block rotation of about 2.3 degrees from 0.25 to 2.0 m.y. represents about 6.6 percent of the total block rotation since inception of range tectonics. A constant rate of block rotation indicates a date of inception between 4 and 30 m.y. ago, which agrees reasonably well with the accepted Miocene date of inception of the Rio Grande tectonic trench. Crustal shortening due to tilting indicates tectonic compression associated with orogenic uplift.

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