Hallelujah! Thanks Dune...
I've been experimenting with a fake stones clip called 'Fake Stones 0.01m-1m', which has four groups of stone sizes split into two sets, each with a Merge shader combing them. Then, those two merge shaders are combined using a third "Final Merge shader". Somewhere in my hours and hours of infinite tweaks, I'd left the merge shader on the larger stones set to "Choose by altitude: ALL"...("choose by altitude: Highest (raise)" made me think that might display only the highest stones; I thought 'ALL' sounded like I'd get it all.) Obviously that wasn't right.
Still, I had colored the stones bright solid colors so they would be very distinct and I could see the complete effect. But when 'choose by altitude' is set to "ALL", each stone color fades by about 50% and the stones themselves seem to sink half way into the ground. I definitely like having that kind of control, but "ALL" doesn't seem to accurately convey that kind of result (though I'm pretty sure it's me who doesn't yet understand what 'choose by altitude' means in this context.)
Fwiw, the '?' icon in the shader window, which takes you the online Wiki, only shows a picture of the shader window and describes it's for merging shaders. Just my opinion , but it might help new users if the Wiki included a clear explanation of what all those options actually do and what the labels are meant to convey, perhaps with images to show the result (an idea that perhaps all us users could contribute to piece by piece.)
But I'm making progress..so thanks for the help (again)..