A "marnière" is the result of the exploitation of marl, soft chalk, which was used to improve farmland.

It was from the 19th century onwards that the exploitation of marnières became intensive. Marling increased the pH of the soil and improved cereal productivity. Mining had to be declared in 1853, but there were no extraction standards. Mining inspectors were responsible for monitoring these operations.

There were two types of mining: open-cast and underground. Underground operations accessible by shaft are by far the most common in Normandy, as the quarries were located close to agricultural areas on the plateaux, where the chalk is on average 25 metres deep.

Generally speaking, quarries are found between 20 and 30 metres below the surface of the natural ground, with a chalk cover of between 5 and 15 metres. The height of the galleries generally corresponds to the height that can be exploited by a worker with a pick, i.e. between 2 and 3 metres. The size and shape of underground workings vary widely. In Seine-Maritime, the average void volume is 250m3.

It was far too complicated to fill in a marnière after it had been mined. For this reason, only the shaft was filled in, either completely or partially. This was sometimes indicated by an isolated tree. Sometimes, the head of the well was built with a dry stone or brick vault.

What is a "marnière" ?

Cavitec's experience shows that there are no real rules for opening a marnière's well. Generally on plateaux, we have also discovered wells in buildings, ditches, at the bottom of talweg, on slopes, near ponds, along roadsides, etc. Mining galleries have also been found within water wells.