Blackhawk Slide

The Blackhawk Slide is a huge prehistoric landslide some 17,000-20,000 years old. A large piece of rock worked loose from the steep north slope of the San Bernardino mountains and came rushing down the side of the mountain nearly in one piece, apparently riding on a cushion of air. Imagine an air hockey puck some five miles long and two miles wide, roughly fifty feet thick, moving about 270 miles per hour for eighty seconds.

The slide is easy to see from the road (highway 247 ten miles east of Lucerne Valley, CA) and there are numerous dirt roads leading in to the slide to get a closer view. (Some of these roads lead into private property, so watch for "No Trespassing" signs; there are other roads which don't require trespassing.)

The rock comprising the slide is mostly marble breccia and alluvium, but there are formations visible up on top which look like basalt (not mentioned in any of the references I've found on the slide). The sedimentary layers are more or less preserved in the same order they show up in the mountains where the slide originated, which was one of the early clues that something unusual happened here.

We inspected the layers of the slide where they'd been exposed by drainage cutting a wash into the alluvium, then drove up onto the slide to view the top surface and the hill at the slide's southeast edge, a small granitic hill which was overridden by the slide, and the second granitic hill just north of it which was apparently far enough to survive without being covered. We tried looking at the two quarries at the north end, mentioned in "Geology Underfoot", but they were marked NO TRESPASSING so we stayed out, having gotten a good enough look elsewhere.
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Strange Formations East of the Slide

Then we drove back to the highway and took a dirt road off to the east of the slide to get a look at that end. That was our big surprise of the day: we found some fascinating rock formations just beyond the edge of the slide which I haven't found mentioned in any book. Jagged fingers of layered sandstone reach like twisted branches into the air. You'd swear they were sun-baked wood, but they're not: they're rock.

Under the sandstone is a layer of granite, bright with mica flakes (the sandstone, too, is full of glittery fragments). At the contact between the granite and the sandstone, the granite becomes crumbly and soft, and the sandstone becomes hard and slate-black: presumably contact metamorphism.
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