Rockfall-Avalanche and Rockslide-Avalanche Deposits at Sawtooth Ridge, Montana

Abstract

Rockfall avalanches and rockslide avalanches are here defined as exceptionally large rockfalls and rockslides composed of rock debris that moved rapidly as dry flows, forming tonguelike or lobate masses. A review of distinctive features of some of the modern rockfall- and rockslide-avalanche deposits was necessary to identify the character of the movement of the prehistoric deposits. Sawtooth Ridge in northwestern Montana was the source of three rockfall avalanches on its east side and two rockslide avalanches on its west side. The three rockfall-avalanche deposits cover about 2870 acres and had an original volume of 850 million cubic yards. These deposits, although differing in some ways, consist of a heterogeneous mixture of relatively coarse angular rock fragments and some exceptionally large blocks. They are characteristically very porous and hummocky. They resulted from structural conditions in carbonate rocks that permitted extremely large cliff segments to fall simultaneously from Sawtooth Ridge, possibly because of the following factors that occurred singly or in combination: earthquakes, tilting related to regional uplift, and loss of stratigraphic support. Repeated falls caused the eastern edge of Sawtooth Ridge to recede almost 1200 feet. First, broad open folds with widely spaced, steeply dipping strike joints probably permitted very large segments of the cliff to drop simultaneously. Then, smaller segments of the cliff calved off individually, controlled by more closely spaced, steeply dipping strike joints. Finally, frost wedging of closely spaced joints released small blocks that accumulated as talus. Rockslide avalanches on the west side of Sawtooth Ridge descended a dip slope and formed two dams across Home Gulch; the largest has a volume of about 10 million cubic yards.