abstract angles backdrop background barrier bolders boulders cemented coated concrete constructed cover covered erosion-prevention fitted hill mortar perspectives random randomly-coursed Rendered retaining-rocks retaining-wall rock rocks rocky rough rugged secure security slope sloping solid stone stone-walled stones stonework strength strong surface surfaces texture textures uneven variations variety wall walled well-built appearance backgrounds blocks colors colours contrast contrasts cracks discoloration foundation foundational growth height high large limestone linear lines moss old perspective plants retaining architecture calm dam Elan Valley lake mountain power reflection reservoir retain wales water welsh arches hold landmark landscape tower hills buttresses road