Page 45 - Fundamentals of Geomorphology

28 INTRODUCING LANDFORMS AND LANDSCAPES

100 0
cent) Historical cent)
Historical
explanations
(per explanations (per
explanation explanation

of 50 50 of
component Modern component

Modern
explanations
Modern explanations Historical

0 100
Sediment Bedforms River channel Drainage Major folds Mountain
movement morphology networks and faults ranges
Size and age
Small Medium and Large
and young middle-aged and old
Figure 1.14 The components of historical explanation needed to account for geomorphic events of increasing size and
age. The top right of the diagram contains purely historical explanations, while the bottom left contains purely modern
explanations. The two explanations overlap in the middle zone, the top curve showing the maximum extent of modern
explanations and the lower curve showing the maximum extent of historical explanations.
Source: After Schumm (1985b, 1991, 53)

that those differences greatly influence the interpretation Uniformitarianism was a system of assumptions about
of past processes. So, before the evolution of land plants, Earth history argued by Charles Lyell, the nineteenth-
and especially the grasses, the processes of weathering, century geologist. Lyell articulately advocated three ‘uni-
erosion, and deposition would have occurred in a dif- formities’, as well as the uniformity of law: the uniformity
ferent context, and Palaeozoic deserts, or even Permian of process (actualism), the uniformity of rate (grad-
deserts, may not directly correspond to modern deserts. ualism), and the uniformity of state (steady-statism).
The second substantive claim concerns the rate of Earth Plainly, extended to geomorphology, uniformitarianism,
surface processes, two extreme views being gradualism as introduced by Lyell, is a set of beliefs about Earth
and catastrophism (p. 21). The third substantive claim surface processes and states. Other sets of beliefs are
concerns the changing state of the Earth’s surface, steady- possible. The diametric opposite of Lyell’s uniformitar-
statismarguingforamoreorlessconstantstate,oratleast ian position would be a belief in the non-uniformity
cyclical changes about a comparatively invariant mean of process (non-actualism), the non-uniformity of rate
state, and directionalism arguing in favour of directional (catastrophism), and the non-uniformity of state (direc-
changes. tionalism). All other combinations of assumption are
Uniformitarianism is a widely used, but too often possible and give rise to different ‘systems of Earth his-
loosely used, term in geomorphology. A common mis- tory’ (Huggett 1997a).The various systems may be tested
take is to equate uniformitarianism with actualism. against field evidence. To be sure, directionalism was