Sunday, September 4, 2016

Volcanos, bones, geology and the land I farm

A couple of years ago we had a major disaster just up the valley from me - the oso landslide, and I've got one of the most active and dangerous volcanoes in the united states up the valley not too far - glacier peak.
The landslide, and the volcano, were on my mind when I was looking at river bank erosion on a patch of the bank on my property.
Click for a larger version
 Near the top of the bank, the most geologically recent, there's a clear record of pretty stable conditions.  what's nearest the top is what appears to be lake deposits; very fine silt and mud that probably settled out of still water.  That's what I farm in; it's 4 to 6' deep, and rock-free.  The only rocks in that layer are ones that people added.  Below that is some stream deposits; small rounded stones and gravel, put down horizontally.  Below that is another settling pond of that I think is glacial sand and gravel - it's got less very fine silt in it, and it's a few feet deep.
Below that, about 8' below the surface, is where things get interesting.  When the oso landslide happened the trees in the area just got ripped apart; splinters.  Even big trees got split into small pieces.  And that's whats in the mud layer below the 2nd lake deposit.  A layer of mud and tree debris, with the trees showing every evidence of getting ripped to pieces.  Below that is river cobble, and below that is another lake deposit.

It's hard for me to tell the difference between lahar deposits and landslide deposits, given that they're both pretty much composed of the same materials.  If it didn't come down the valley when the volcano erupted last, it came down when the landslide pushed it down later, maybe thousands of years later.  

The trees in the debris layer might be 10-12,000 years old; old enough that there might be elephant bones in there if any had the bad luck to be in the valley when the event happened.  I regularly scan the river bank for artifacts, bones or fossils exposed due to erosion, and I've found a few bones.  One gave me pause - it looked a lot like a human fibula

I'll leave it to you to give your best guess.

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