the cotentin peninsula is part of the armorican massif (with the exception of the plain lying in the paris basin) and lies between the estuary of the vire river and mont saint-michel bay. it is divided into three areas: the headland of cap de la hague, the cotentin pass (the plain), and the valley of the saire river (val de saire). it forms the bulk of the department of manche. its southern part, known as "le marais" (the marshlands), crosses from east to west from just north west of saint lo and east of lessay and marks a natural border with the rest of manche.