Xerus, do you 'need' to have rocks? Why not just go with dirt, it would likely more commonly be found directly underneath grass anyway, and you could probably accomplish a smoother, cleaner, and flatter look with the dirt, because the grass on the side looks like it is pushing on the screen, like a clean cut cross section.
I don't think the rocks are cutting it, although you might want to experiment with tile variation if you're not fond of the repeating patterns, if you haven't already.