AND

ABSTRACT. The general concepts generated by modern studies of geomorphological processes are examined in terms of their utility for models of long-term landform evolution. The work is summarized by four fundamental propositions of landform genesis. These include the idea that each set of environments is represented by constant processes and characteristic landforms which tend to persist over time. 'Geomorphological ' time is divided into the time taken to attain this characteristic state and the time over which it persists. The systems and forms are subject, over Io2-Io5 years, to perturbations caused by high magnitude-low frequency events, environmental change and internal structural instabilities which initiate change. The responses to these impulses are complex and include damped, sustained and reinforcing changes taking place by ubiquitous, linear or diffusive propagation which reflect the sensitivity of the landscape to change. This sensitivity is dependent on the path density of the process and the strength of the coupling between the system components and has two end members, mobile-sensitive systems and slowly responding-insensitive areas. Some of the results include the concepts of (i) relief and pattern persistence; (2) stagnancy of development and the hypothesis of unequal activity; (3) convergence of form; (4) the concept of transient forms; (5) stability-instability phases and episodic landscape evolution, which together form a coherent framework for long-term landform evolution. Davis's great mistake was the assumption that we know the processes involved in the development of land forms. We don't; and until we do we shall be ignorant of the general course of their development. LEIGHLY, I94