Sentence:  The rock was one of those tremendously solid brown, or rather black, rocks which emerge from the sand like something primitive. <WSD>Rough</WSD> with crinkled limpet shells and sparsely strewn with locks of dry seaweed, a small boy has to stretch his legs far apart, and indeed to feel rather heroic, before he gets to the top. 
Meaning of the word marked with <WSD> tags:  Not smooth; uneven.