Why would a half submerged monument indicate that it was built before the last ice age? We've dozens of upon dozens of submerged Greek and Roman monuments. Venice will be before long.
20k years ago, when Scottland was being pressed down by the weight of the ice, Brittany, far to the south, was being pushed up just like when everyone sits at one end of a boat. As the North of Britain now bobs up, the South subsides. 5k years ago when the Er Lannic stones were set, and well after the return to fairly modern sea levels, the land beneath them was high and dry. Half of it still is. The South of England and Brittany are still subsiding. What you need is a stone circle in the middle of the North Sea.
You are now a million miles away from where you were in space-time when you started reading this sentence.
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