One of the Mastcam raw images contains a number of apparently sculpted stones with some much smaller rounded pebbles. Here's a full color version of the image.
What does the scene tell us? The larger stones could be ventifacts formed by wind blown particles through a process known as saltation. Another explanation might be dislodged particles accelerated by gravity as they move downslope. Ignoring momentum lost by collisions the terminal velocity is a function of the angle of the slope, θ. The following plot gives the terminal velocity as a function of the slope for a small sphere 1cm in radius and a density of 3gm/cm3 for average conditions on Mars.