I'm building a gravel patio inside my U-shaped gravel driveway. I will be using two different kinds of gravel, so I need a separator at the border to prevent the mixing of the two types of gravel. If I use some kind of edging, wouldn't it become a trip hazard if it sticks above the surface? And if it's below the surface, wouldn't it be ineffective at preventing mixing? Any way out of this conundrum?