In prehistoric times this area was all land. But the scouring of the sea over eons has removed the softer rocks leaving this hunk of granite.The natural shape of the islet served several different peoples as guardian fortresss over the centuries. Following the departure of the Romans in 460 AD, the Mont was held by the Amoricans, a Romano-Breton culture which extended into Britain. But it was conquered by the Franks.
The origin of the monastery on the peak of the Mont is told in a very colorful legend. Supposedly the Archangel Michael appeared to St. Aubert, bishop of nearby Avranches in 708 AD, telling him to build a church on the islet. St. Aubert ignored him several times until reportedly the Archangel burned a hole in his skull with his finger. The church was built and eventually did become a monastery.