What you could do (what I would do) is keep the stones much smaller and gathered in patches. So, make a PF (perlin) with white dots kind of where you want patches of stones, like deposit areas. Use that white to displace the terrain down a few decimeters, and as mask for complete coverage of those 'holes' with a mix of small and somewhat larger stones. Coverage of very small pebbles = 2, larger stones less, until the largest maybe at 0.1 coverage. Three or four sizes of stones mixed by merge shader set to highest raise.
