If you look closely at the rock wall, you'll see two separate gradients. The easiest one to see is at the top, between the wet rock and the dry rock - there's a slow transition between thoroughly wet and completely dry. That's the natural distribution of the splashed drops – the higher you go, the less likely that any drop will reach there, so the less wet it is. That gradient looks like a linear density change to my eyeball, but I could easily be wrong about that.
The second gradient is hard to see in the photo (though it wasn't hard at all standing next to the wall). It's a dirt gradient. :) Near the bottom of the wall, there's quite a bit of mud splashed up on the wall, darkening the rock more than just plain water could. As you go higher on the wall, there's less and less mud. Again, it looks like a linear density change to me. I'm making an educated guess that because a splashed droplet containing mud has a higher specific gravity than plain water, it weighs more and therefore can't fly as high.
I'm sure this dual gradient has been here ever since my barn was built, but somehow I never noticed it before. I'm planning to put gutters up there this year, so after this year I don't think I'll see it again...
