Ok, I think I know right where you guy's were , that really looks familiar, man that garbage is moving on over the rocks, that sucks!
Did you see the old steep steps section? Me and Brian used to ride up and down those on the Sherco's.
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The Recovery Sock
I don't know if you meant the little drain pipe or the big stone structure, the big one is a drainage tunnel under a railroad line! Nice stone work though, but it is decaying rapidly, too bad as it was hand cut stone with no motar, really old world craftsmanship we will prolly never see again in the day and age of reinforced concrete.