I found this one recently. The Glen finglas Reservoir is at its lowest ever level, exposing a landscape that has been submerged since 1965. The stones are probably protruding bedrock and are covered in transverse grooves. Undoubtably man-made and probably tool-sharpening surface but could be plough marks (although they could be very precise ones!). I’ve no idea what the tools would be but these were near to a ring-shaped structure and a small group of cairns - all usually deep underwater but now exposed on the dry reservoir banks.