Detail of stones in the western arc.
Image credit: A. Brookes/C. Bickerton (17.11.2012)
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This is the stone that looks like it hasn't been exposed for too long .The top is very angular and unworn , no lichen ,unlike some of the others ,at the top too .
The jagged top may suggest a recent-ish break, particularly given the effects of freezing/thawing in such a waterlogged place. The stone is slender and only a few inches thick - I could imagine it being broken off as easily as newly erected. The sides are smooth in comparison.
How does lichen fare in serious amounts of standing water? The circle is basically in the little lake shown on the 1/25000 map - which suggests that the water levels fluctuate and that when the survey was carried out the water was deeper than it is now. I think the circle may have been entirely submerged from time to time - none of the stones is taller than 0.5m and most are leaning.
The close ups seem to show lichen at the top on most of the others . Except the stone Pic 36 ,dunno if there is another ,but it also has the look and reddish lichens seen when stones are recently exposed e.g. beside ditches and tracks , although in this case it may have been under water .