(Oshyan): "Are you using population color variation?"
...No, not that I'm aware of (unless it defaults in a node somewhere).
(Yossam): "Or a strata and outcrops shader?"
No (haven't even gotten to try one of those yet..)
Basically, this is a DEM for the terrain, and I've been testing with a couple of grass patches (One 'neuspadrin' sample and one 'Mr. Lamppost' sample.) It's simply a patch selected from the library and brought in as a population. Since this terrain usually also has bushes on it, I may simply have not noticed this before now. But when I disabled the bushes and happened to be testing a ground cover sample, and then just one grass alone, I suddenly noticed the banding pattern in the far background.
I've been testing just now and found something about the 'Object spacing in a,b' that may well be where the issue arises. I noticed that with the earlier samples (above) the a,b values were set to 1,1. I usually set them to different numbers to avoid uniformity, and found just now that setting a,b to 1,2 or even 1,1.5...anything with some variation over 1, seems to make this much less obvious. But when I set a=1 and b=.7...or anything 1 or less, the grass seems to suddenly turn to terraces (see the image below).
It appears to me that it is only the change in the coloration of the grass that makes it appear to be ridges. The grass seems to shift from darker to lighter (or vice versa) front to back, in rows across the terrain. It's back where the change becomes very distinct that the illusion shows as dips in the terrain.
I think that's what I'm seeing. Nevertheless, as long as a,b doesn't go below 1, I think this is a non-issue, or at least can be easily avoided (though it might be a useful trick to know in some circumstances.)
Thanks..