We can't possibly tell what's under the turf/water or in an unexcavated barrow but areas like wessex that don't have outcrops of suitable surfaces for carving , do have carvings in monuments like barrows . Considering the amount of unexcavated barrows in the area I believe that the likelihood of finding more carved surfaces is quite high. You could draw an analogy with the Boyne passage graves which have a huge amount of carved surfaces but there are no open air outcrops or boulders with carvings to be found in the area .