Crestline Soaring Society

Food for thought...speed of 170 mph!
longplay landslide
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BLACKHAWK LANDSLIDE
DESCRIPTION
External features:
Lobate shape, 4.5 miles long, 1.5 miles average width 30 - 50 feet thick,
Hummocky surface with longitudinal furrows and small depressions with dry lake deposits.
Mantled with alluvium from adjacent mountains on upper 3 miles.
No indication of water run-out at the base.
Has a sharp edge, i.e., no rocks were thrown out onto the surrounding desert floor.
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Internal features:
Angular fragments of marble, brownish sandstone, gneiss, quartzite and granite.
Fragments range in size from powder to 10 inches, with a few between 3 and 10 feet.
In places the fragments in the slide lie nestled together and layering can be traced from fragment to fragment.
Various lithologies not mixed, but occur in sequence. Marble fragments make up the upper part and most of the slide. Below the marble fragments are scattered masses of brownish sandstone, and below that are pieces of
gneiss, quartzite and granite.
In the mountains above the slide a thrust fault has moved the Furnace Marble (Paleozoic) so that it lies on top of the Old Woman brown sandstone (Tertiary) which, in turn, lies on top of metamorphics and granite. This is the same sequence of lithologies found in the slide.
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ANALYSIS
Calculations indicate a launch speed of 170 mph that could have accelerated to 270 mph. The entire journey would have taken 80 seconds. As the slide came down and moved across a north dipping slope of 2 - 3 degrees, it had to ride over a hill 250 feet high, yet the stratigraphic sequence of fragments and continuous layering from fragment to fragment suggest no internal mixing. This suggests that the slide moved as a sheet with little friction between it and the valley floor over which it rode.
Shreve (1968) proposed that it trapped about a one foot layer of air beneath it.
Hsu (1975) suggested that all of these features were produced by flowage.
Stout (1975) found through radiocarbon dating of fresh water mollusk shells that a dry lake bed deposit on the slide is 17,400 + /550 years old. This puts the slide into a time of heavy
precipitation (Tioga glaciation) and thus probable saturated ground conditions.
References: Sharp and Glazner, 1993, p.147 - 158.; Stout, 1975