The territory is the watershed of the Authie, a coastal river marking the boundary between the departments of Pas-de-Calais and Somme. The Authie is a river parallel to the Canche and the lower bed of the Somme.
The valley is long (about 100 km) and not very wide (from 9 to 23 km). Its geological formation explains this configuration. Oriented on a west/north-west axis, its sides are asymmetrical: on the right bank, the slope is gently sloping; on the left bank, the slope is short and steep.
The Authie has its source in Coigneux, about 100 m above sea level. After receiving the waters of its 3 tributaries (Kilienne, Grouche and Gézaincourtoise), it flows into the English Channel between Berck and Fort-Mahon via the Bay of Authie, a network of complex channels granted the particular landscape of the low fields.
The terms high, middle, low valleys and low fields are often used to find your way around the valley; the upper valley is between the source and Occoche/Outrevois; then follow the middle valley to Dompierre/Douriez, and the lower valley to the Falaise Morte, near Colline-Beaumont. Finally, the low fields join the sea, encompassing the Bay of Authie.