Burnard describes the photograph as "bottom of supposed millstone at Riddipit".
The remains at Riddipit (Crossing calls it 'Riddy Pit') are evidence of the tin mining industry which operated on Dartmoor until around 1939.
The round stone in the foreground may be a crazing mill stone. They were used for grinding the tin ore between two flat-faced circular stones. The stones were rotated by a waterwheel driven by a moorland stream.
Crazing mills could only grind the alluvial gravels and they were replaced by stamping mills when coarser ores started being mined.