have roundish/smoothish stones, which barely stay put even on level
ground. You might do better with the kind of gravel they use for roads,
which is much more angular; the pieces lock together and stay put.

No, they are large, rough stones similar to driveway stones, but
bigger and a little whiter. I'd say on average they are about 3" to 4"
across. They don't tumble particularly easily, but the slope is such
that if you remove what is underneath the one above, they tumble.

I like the idea that some of you had about pavers & such. One of my
friends had the idea of putting in additional landscape timbers, but
that wouldn't have looked so good. (unless perhaps painted to match the
stone, I guess)

I could probably use some long concrete sections, which would be a
similar color to the stone. That would help hold the rocks a lot.