Algarve - New Find - Grindstone Blank

My wife and I came across this interesting find whilst on a search for rock art near Vale fuzeiros in the Algarve. although it is not rock art and is likely to be of no great age, it is nevertheless an interesting and unexpected find. It is an incised circle on a smooth rock surface, almost level with the ground. It is the blank for a grinding stone, possibly intended as a millstone. There are no visible ruined mill towers in the locality as far as I know though. The incised circle is 0.8m in diameter. The NW quadrant of the stone has been shaped up to the circumferance but it then appears to have been abandoned for whatever reason. GPS : N 37.246061 W -8.360305 Scale bar length is 30cm