Drought in Ireland Reveals Ancient Henge

[BillTre]

The holes in the ground that once held the bases of posts decayed away and left soil there more resistant to desiccation that the surrounding soil, making their location visible from the air.
Picture and little article here.

 

[Bystander]

Lignins? What's the cultivation history? Have there been any soil accumulation rates measured? That's an impressively persistent "deep" scarring/marking/tattooing of the soil.