The peasants in the department Eure used to mine marl (mudstone which contains clays and aragonite) to fertilize their fields with it. The fertilizing effect of marl isn’t really great, but it was cheap to get. They dug holes in the ground of 1,5-2 meters diameter, when they reached the layer of marl, they dug a horizontal gallery. When they filled the holes later, they often only filled the hole from the surface, and not the gallery. Thus the marl hole can open again, especially during heavy rain. When the infill moves in the gallery.
We have 3 filled marnières on the meadow. Chef made a fence around one of them.
There were 16000 marnières in Eure in 2005.
I hereby release this article (text and images) into Public Domain.