Thanks, Dune.
The compute normal node certainly helps. I also just discovered that the problem seems to be that I had the hard layer steepness set too high in the strata and outcrops shader. I had it at 10. Setting it to 1 yields better results. But at that setting, the hard layers aren't forced quite as vertical, which makes the slope constraint a bit less selective, as my mountain, even between hard layers, is pretty steep.
Anyway, I am getting roughly the effect I was after now. Thanks for the suggestions!