A "pond stone", a natural geological curiosity once mistaken for a human achievement.
Early archeologists sometimes wrongly concluded that some natural features were the work of megalith builders. This was the case with many "shaking stones" which gave rise to many legends because of their strange shapes.
Many "basins" are the result of biochemical granite corrosion and were not made to collect the blood of victims slaughtered on these so-called "sacrificial stones"!