A LONG-RUNOUT LANDSLIDE IN SOUTHEASTERN CALIFORNIA: BLOCK GLIDE OR ROCK AVALANCHE? - A SUGGESTED MODIFICATION IN THE DEFINITION OF A ROCK AVALANCHE

The American Geological Institute Glossary of Geology (1997, 4th ed.) defines a "block glide" as "a translational landslide in which the slide mass remains essentially intact, moving outward and downward as a unit...". A "rock avalanche", on the other hand, is "the very rapid downslope flowage of rock fragments...characteristic features include chaotic distribution of large blocks, flow morphology and internal structure, relative thinness in comparison to large aerial extent, high porosity, angularity of fragments, and lobate form." From these definitions, a critical distinction between block glides and rock avalanches is that block glides consist of intact source material, whereas rock avalanches are fragmented. Furthermore, rock avalanches contain morphology and structure derived from "flowage", whereas block glides are translational.

The structure of a long-runout landslide interbedded in the terrestrial Shadow Valley basin, southeastern California has characteristics that fit both definitions. An estimated 75% of the landslide's volume consists of essentially intact rock that was emplaced essentially by translational movement, suggesting that the deposit is best classified as a block glide. On the the other hand, the remaining 25% of the deposit is fragmented (brecciated) and contains flow-like internal features. In addition, the deposit is relatively thin compared to its aerial extent and initially had a high porosity where brecciated. Based on all of the above characteristics, the deposit can be (and has been) classified either as a block glide or as a rock avalanche.

I suggest that the deposit is best classified as a rock avalanche because of its thinness compared to aerial extent and because the breccia within the deposit has the same characteristics as the breccia of classic rock avalanches (such as the Blackhawk landslide in southern California). From this, I further suggest that the definition of a rock avalanche be modified to permit large proportions of the deposit to consist of intact source rock. Also, flow morphology and structure throughout the deposit should not be a requisite.