Folkestone is located where the southern edge of the North Downs, escarpment meets the sea. In contrast to the white cliffs at Dover further to the east, the cliffs at Folkestone are composed of Greensand and Gault Clay. A small stream, Pent Brook, cuts through the cliffs at this point, and provided the original haven for fishermen and cross-channel boats. The cliffs are constantly under attack from the sea: the original headlands, which once protected the port, ceased to do so, and artificial protection, in the form of breakwaters and piers have been necessary since the 17th century.