Fig. i. Heddon river basin The drainage basin of the river Heddon in north Devon (O.S. i : 25,000, sheet 21/64) is an excellent example of what is co ventionally termed "poly-cyclic landscape." The higher parts of the basin are made up of a gently sloping surface which contrasts strikingly with the steep-sided valley which forms the lower part of the stream system (Fig. 1). This separation of the surface into distinct slope elements is also reflected in the profile of the river Heddon (Fig. 2) which exhibits two "graded" reaches, with a possible third in the most headward part, separated by a marked break of slope at about 520 feet. The curve of the second graded reach has been extended until it attains zero