Sentence:  The upper layer of the soil is, therefore, less solid than in other trampled habitats and the optimum development of the community can only take place in the broader joints between paving stones in more <WSD>untrodden</WSD> places, often in strips of pavement at the foots of walls where the shifting sand accumulates and turbulent air currents may play an important role. 
Meaning of the word marked with <WSD> tags:  That has never been trod upon; unexplored, unspoiled.