Using a tube sampler might be tough with coarse fragments present.
Sounds like you have shale or mudstone. If it breaks that easily, you might be able to pulverize it with a sheeps foot or by tracking it in with a dozer or front end loader. You would have to apply thin lifts, maybe 6" or so, and crush/compact it evenly before building it up higher.
I'd be more concerned with what you are tying it into in the walls of the valley. Also, if the shale is dark, it could be a source of sulfuric acid. Do plants have any trouble growing on it? Do you see any yellowish spots?
