"clayey soil used for fertilizer, mixture of clay and carbonate of lime," mid-14c. (late 13c. in place-names), from Old French marle (Modern French marne) and directly from Medieval Latin margila, diminutive of Latin marga "marl," which is said by Pliny to be a Gaulish word, but modern Celtic cognates are considered to be borrowed from English or French.
As a verb, "to manure with marl," by late 14c. (marlen, from Old French marler and Medieval Latin marlare). Medieval Latin margila also is the source of Dutch mergel, German Mergel.