Page 24 - Fundamentals of Geomorphology

WHAT IS GEOMORPHOLOGY? 7

( ) Youth
a

( ) Maturity
b

() Old age
c

Figure 1.2 William Morris Davis’s idealized ‘geographical cycle’ in which a landscape evolves through ‘life-stages’ to
produce a peneplain. (a) Youth: a few ‘consequent’ streams (p. 135), V-shaped valley cross-sections, limited floodplain
formation, large areas of poorly drained terrain between streams with lakes and marshes, waterfalls and rapids common
where streams cross more resistant beds, stream divides broad and ill-defined, some meanders on the original surface.
(b) Maturity: well-integrated drainage system, some streams exploiting lines of weak rocks, master streams have attained
grade (p. 229), waterfalls, rapids, lakes, and marshes largely eliminated, floodplains common on valley floors and bearing
meandering rivers, valley no wider than the width of meander belts, relief (difference in elevation between highest and
lowest points) is at a maximum, hillslopes and valley sides dominate the landscape. (c) Old age: trunk streams more
important again, very broad and gently sloping valleys, floodplains extensive and carrying rivers with broadly meandering
courses, valleys much wider than the width of meander belts, areas between streams reduced in height and stream divides
not so sharp as in the maturity stage, lakes, swamps, and marshes lie on the floodplains, mass-wasting dominates fluvial
processes, stream adjustments to rock types now vague, extensive areas lie at or near the base level of erosion.
Source: Adapted from Holmes (1965, 473)