User blog comment:Lord Loss/Monster Hunter Theory... Blog/@comment-5043481-20120817113112/@comment-1011548-20120826145336

Smooth-ish. Yeah, it's covered in rocks, but all those rocks are worn down quite smooth. It they were lumpy and uneven Uragaan would be crashing all over the place whenever it tried to roll. Plus the friction from rolling probably helps wear them down evenly.