maerl
/ˈmɑːəːl/ /mɛːl/(also maërl)
noun
The remains of calcareous red seaweeds (coralline rhodophytes), especially those occurring in deposits off the coast of Brittany, used as a source of lime by farmers; any of these seaweeds, belonging chiefly to the genus Lithothamnion.
Origin
Early 20th century; earliest use found in Encyclopaedia Britannica. From French maërl or its etymon Breton maerl, merl from Old French marle.