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, I940 INTRODUCTIONONE of the most appealing features of the contemporary fashion for process studies is that we are beginning to understand the mechanics of process and associated landform changes in 'mansized' areas and 'human' time scales. It is worth pausing, however, to consider Stoddart's (I978) complaint that we are 'increasingly encapsulated in (our) small drainage basins and pollen profiles' and to recognize that the challenge of extrapolating the short-term record of measurable processes to the relatively unknown time span of, say, Ioo-io ooo years and beyond still remains.There are many approaches to this challenge. There has been an impressive increase in detailed information about past changes in climate, sea-level, rates of uplift and continental movements, which enables more accurate reconstructions of fluctuations in environmental controls. There has been an accumulation of data on the rates of operation of geomorphological processes. The description of landforms has become more objective and precise and the use of statistical techniques has improved the description of relationships between processes and between forms and processes. There is a better understanding of the dynamic basis of geomorphology which has led to the development of deterministic and stochastic modelling techniques and the adoption of the method...

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Deep towed 30 kHz sidescan sonar data from the Saharan Debris Flow deposit, west of the Canary Islands, show spectacular backscatter patterns which are interpreted in terms of flow banding, longitudinal shears, lateral ridges (levees) and transported blocks. Identification of these features is based on high resolution seismic profiles and on a comparison with similar structures seen in better known environments including other marine debris flows and slides, subaerial sediment failures (particularly rock fall avalanches), glaciers and lava flows. Flow banding in the debris flow, picked out by bands of differing backscatter intensity, is on a scale of tens to hundreds of metres. It is considered to result from flow streaming of clasts, with variation in clast size between bands. This primary fabric is cut by a series of distinct flow‐parallel longitudinal shears. Broad, high backscatter longitudinal bands along the edge of and within the debris flow are interpreted as lateral ridges associated with multiple flow pulses; the high backscatter possibly reflects either a concentration of coarse grained material or chaotic sediments deposited from a turbulent flow. Coherent, low backscatter patches are interpreted as rafted blocks, although streamlined haloes of high backscatter material around some blocks indicates differential movement between block and flow, possibly during the waning stages of the flow. A non‐turbulent debris flow model is preferred, in which a raft of more or less coherent material is carried along by a base undergoing laminar flow. Speculatively, the lack of turbulent mixing preserves original sedimentological heterogeneity from the debris flow source area, possibly in the form of clast size distributions. These heterogeneous sediments are drawn out into a flow‐parallel banding which is imaged as the flow‐parallel backscatter intensity banding. The upper raft of material responds to cross‐flow velocity differences, and perhaps to variations in the timing of flow movement, primarily by longitudinal shearing. More complex deformation of the flow banding occurs at the flow margins and around obstacles in the flow, where lateral velocity shear would be expected to be highest.

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'Gilbert was primari1,v concerned with the manner in which eyurlihriurn landforms became adjusted to geomorphic processes, andan interesf in the progress toward such adjustment and the changes to which such adjustment is susceprihle through time' ( Chorely, 1965) ABSTRACT The concept of dynamic equilibrium has provided geomorphologists with a challenging paradigm for studying landform evolution but quantitative evidence for its existence has proved illusive, particularly for complex geomorphological systems. The authors believe that the principle has now been verified through the application of the 'archival photogrammetric technique' to a sequence of historical photographs spanning 50 years of process at the Black Ven mudslide complex in Dorset, U.K. The principles and limitations of the archival photogrammetric technique are described. The method is applied to oblique and vertical aerial photographs of Black Ven at five epochs, commencing in 1946, continuing at approximately 10 year intervals until 1988. The technique is used to generate plans/contours/sections and a dense and accurate digital elevation model (DEM) of the whole site at each epoch. This is used to generate 'DEMs of difference' and a 'distribution of slope angle' which suggest that the mudslides are in equilibrium despite the removal of 200000m3 of sediment between 1958 and 1988. Extrapolation of the slope distribution through time suggests that the frequency of an episodic landform change model at Black Ven may be approximately 60 years.