 Where a beaten track evolves into a busier "road," constant passage by beasts, sleds and wheeled vehicles suppresses regrowth of vegetation.With the help of rain (and soil creep), a shallow trail can be worn down into the topsoil and smoothed without any purposeful road-making work.The thin soil and rocky subsoil, combined with the natural drainage provided by the slopes on each side, also tended to keep such roads dry.