How to build an anchor in Shale/Slate? upstate New York

[jdamin]

Does anyone have ideas/experience for building an anchor in a Shale/Slate canyon with no trees and no rocks, boulders, or even pebbles?

While scouting some canyons in upstate New York, I got a really good look from above a waterfall that was roughly 80ft high. The problem was, I couldn't figure out how the heck I would build an anchor on top of it. Peering down, I saw a long narrow chute with no rocks, pebbles, sand or vegetation in sight. The rock is mostly shale and some slate. The walls of the canyon were pretty straight and the canyon floor was flat rock that looked as smooth as glass, so no chance of digging and there wasn't any kind of lip or depression in the group for about 60ft behind the waterfall (that's as far as I could see)

Bolts are out of the question. The rock is to crumbly/frail and this region passes the freeze/thaw zone dozens of times a year so any bolt (even if you managed to get good placement) wouldn't last more than a few months and would leave an ever growing hole in it's place.

I have a few potshots and came across a video of a modified sandtrap that you could fill with water? http://vimeo.com/53952364
For obvious reasons, they place it behind the lip/edge for added friction. Does anyone have experience using these on flat ground? Is it even feasible?? and does anyone have any ideas? Thanks!

[ilipichicuma]

Apparently it's the unsolvable puzzle. Maybe you could carry rocks in for a deadman? Would there be anywhere to put it?

[Bootboy]

Use a piece of 2x4 wedged underwater across the pothole just upstream of the drop. Come back and retrieve it later, or if the water is flowing slowly enough, it would float up and come downstream.