That's very good, especially the variation in surface texture of the rocks—looks like a strongly foliated type, like gneiss.
Suggestions:
Depending on scale, stipple some color underneath the mud/grime layer—green, pink, yellow, white, blue, or dark red, etc. Not much, just a little in a fine spatter. Or you might try doing this with pastel chalks or dry brushing.
If the rocks are not all the same, paint each one a different color, brown, dark gray, dark red, buff, yellow white, pale green, medium to dark green.
Vary the width of the joints between the rocks, and may your roadway crack edges a little more irregular.