Possible origin of the block stream in Tom Mays Canyon, Franklin Mountains, TX
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Sometime during the Pleistocene, a relatively small mass movement event occurred in which material detached from the canyon wall just north of Franklin Mountain, traveled down the canyon, and was super-elevated along the opposite side of the canyon. Determining the actual mode of transportation and the proper classification of this landform has been challenging. Near the end of the 20th century, two researchers debated, in print, the mode of emplacement for the deposit. One researcher suggested transport similar to a flash flood. The second researcher suggested several alternative possibilities: landslip accompanied by washing out of fines, frost-induced deposition or rubble flow and sieve deposition. We reexamined the deposit and based on the fabric of clasts, topographic profiles, cross-sections, estimated volume, mean gradient (H/L), and local geomorphology and geologic setting, suggest that the Tom Mays block stream was most likely formed as a rock avalanche. Freeze-thaw weathering is suggested as the trigger mechanism for the avalanche of material, which originated from preferentially weakened source rock within a large fault line that cuts the rhyolite cliffs of the west canyon wall just north of Franklin Mountain. Blocky debris was transported by means of grainflow or fragmental flow along a diverted runout within the confines of the canyon. Gebrder Borntraeger,.
