Érin is a small village on the south side of the small river, the Ternoise which flows through St.Pol-sur-Ternoise and joins the River Canche at Hesdin. The geology of the area is like that of the Somme valley to its south. The Ternoise valley is the most northerly flowing in the syncline to the south of the Variscan front. The London-Brabant Massif underlies the land to the north.