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

Abstract

Research Article| September 01, 1965 Rockfall-Avalanche and Rockslide-Avalanche Deposits at Sawtooth Ridge, Montana M. R MUDGE M. R MUDGE U. S. Geological Survey, Denver, Colorado Search for other works by this author on: GSW Google Scholar Author and Article Information M. R MUDGE U. S. Geological Survey, Denver, Colorado Publisher: Geological Society of America Received: 22 Jun 1964 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Copyright © 1965, The Geological Society of America, Inc. Copyright is not claimed on any material prepared by U.S. government employees within the scope of their employment. GSA Bulletin (1965) 76 (9): 1003–1014. https://doi.org/10.1130/0016-7606(1965)76[1003:RARDAS]2.0.CO;2 Article history Received: 22 Jun 1964 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation M. R MUDGE; Rockfall-Avalanche and Rockslide-Avalanche Deposits at Sawtooth Ridge, Montana. GSA Bulletin 1965;; 76 (9): 1003–1014. doi: https://doi.org/10.1130/0016-7606(1965)76[1003:RARDAS]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search 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. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.