Slides of Rock and Ice

Gros Ventre Slide of 1925
looking south across Gros Ventre River

I visited a curious part in Wyoming yesterday, so unlike the rugged Wind River and Teton Ranges, yet near them—east of the Tetons and north of the Winds.  The area was known for centuries as having minor earth tremors and small landslides, and was considered by some to be spooky.  

Slide of 1925, 95 years after,
huge rocks still showing

But in 1925 it drew national attention for a massive landslide along the Gros Ventre River, pronounced "grow-vont." Within fifteen minutes, a mass of rocks, soil and trees slid from the south canyon wall, rumbled across the river and didn’t stop before going three hundred feet up the other side.  The slide was a mile long, half a mile wide, and left a dam across the river 225 feet high.  Geologists think that a shale layer became slippery and allowed everything above it to slide. 

Tensleep Sandstone in the rubble

Tensleep Sandstone in the rubble

In my rummaging through the rubble, I found pieces of the hard gray Tensleep Sandstone which lay above the shale.  But I did not find the Amsden Shale, which they say became like grease and caused the problem.  Records show that the sedimentary layers dip down to the north at about fifteen degrees, and that two very wet years preceded the slide.  And those earth tremors certainly did not help to prevent it from happening. 

End of the rubble 300 feet up the far bank

A slide of this magnitude did not occur in previous millennia, probably because the Gros Ventre River had not cut through the Amsden Shale, removing the one holding force against a slide.  

Slide Lake, looking upstream

Slide Lake, looking downstream,
Slide scarp in background

The lake created upstream by the slide, Slide Lake, is still here and is still held in place by material from the 1925 slide.  But it was not always so.  Two years after the slide, part of the dam was over-topped sending a tremendous flood rushing three miles down the canyon, wiping out the small town of Kelly, killing six people.  But in the past ninety-three years the dam has held. 

On another day, I stood at the foot of another kind of slide.  I imagined a gigantic wall of ice that was the leading edge of a glacier.  Perhaps it  moved to where I stood sometime in a past ice age, then hesitated and retreated, only to move downward again as climate changed.  In one of its retreats it delivered this boulder, as part of its accumulated baggage, brought from far upstream, as if delivering it to me for my perusal and pleasure.  The glacier might have slid forward and melted backward several times before finally depositing this particular boulder, but eventually its retreat became a final motion, and the glacier was finished.  The rock slide remains.  

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Michael Angerman is making a map of nightly locations, as he has done for many of my trips.  Please see  Michael's Map