This site is quite close to the back road from Downholme to Richmond. Beckensall & Laurie describe the site in their 1998 book Prehistoric Rock Art of County Durham, Swaledale and Wensleydale.
Apparently there are three carved rocks here centred on Thorn Hill. I spent a good hour scouring the area, I found bullet cases, smoke bombs and weird parachute looking things (this is army land) but only found the one boulder. The other two carved rocks are earthfasts so I presume the've been buried.
The rock pictured is a large boulder surrounded by sedges. Beckensall and Laurie describe it as the most remarkable of the three carvings and as " a glacial sandstone boulder, which has a triangular or heart shaped groove pecked into it: unique in this region. The boulder is isolated and obverlooked by rising ground. The upper surface has been defaced in antiquity, possibly to remove a more prominent carving".