I was guessing you'd probably know it if you're on the Plateau as you'd be noticeably higher in elevation. This is a little unfortunate, as there are sooooooooo many good riding areas up and down the Plateau, but gotta draw the line somewhere, right? I think those counties more or less sum it up. But I don't think they evenly divide the Plateau from the valley. The Plateau is quite visible. How 'bout say once you start climbing up to it, you're out of bounds. This will make a nice visual cue to anyone without having to try and find county lines.