Physical Geography

I

Introduction

Physical Geography, branch of systematic geography concerned with the physical environment. It encompasses several subject areas that have close links with other environmental academic disciplines. The subject areas include geomorphology (linked with geology), biogeography (linked with ecology), climatology (linked with meteorology), hydrology (linked with civil engineering), and pedology (the study of soils). Aspects of these subjects studied by geographers have traditionally emphasized the spatial aspects of the disciplines, that is the relationships between phenomena in space. All these areas have developed their own distinct approaches but geomorphology, the study of landform development, has taken a leading role in the development of a theoretical underpinning for the subject, with the other branches more closely reflecting developments in their linked disciplines.

II

Early Development

Although an understanding of the processes of landform development can be found in the writings of Greek and Roman scholars, the foundations for modern theories of landscape development were laid by the geologist James Hutton who, in 1795, suggested that the surface of the Earth was shaped by slow and perpetually operating processes. Hutton’s ideas were formalized in 1830 by Charles Lyell in his principle of uniformitarianism, the basic notion of which is “the present is the key to the past”. Put another way, we should be able to account for landforms that have developed over many years by observing slow, steady, and gradual processes creating landforms at the present time. Lyell also believed that the surface of the Earth will never be reduced by erosion to a flat surface but will pass through endless repeating cycles of change. This theme was to be developed by later generations of geomorphologists.

Lyell’s ideas met with considerable opposition at the time they were published. This was partly due to the fact that large areas of the temperate parts of the world were covered by a material that could not be explained by processes operating in these areas at the present time. This material, a mixture of clay, sand, and boulders known as “drift”, was used by adherents to an alternative school of thought, catastrophism, to argue that some landscape features might be explained by rapidly occurring and very powerful events rather than by progressive slow change. Indeed, to some, the biblical Flood seemed a plausible explanation of such material. However, Louis Agassiz, observing the effects of glaciers in his native Switzerland, was able to explain the origin of drift, which became known as boulder clay or till, as material deposited by moving ice. Agassiz also realized that glaciers had not always been confined to mountains but had once formed huge ice sheets covering much of the temperate continental areas of the Earth, and thus the notion of ice ages was born. However, the idea of catastrophism, in a slightly modified form in which rare but large-magnitude events are significant in the creation of landforms, has continued to be important in geomorphological thinking to the present day.

Two American geologists were very influential in the development of physical geography during the 19th century. John Wesley Powell, who worked in the semi-arid terrain of the western United States, emphasized the relationship between landform and underlying geology. He attempted to classify mountains and landforms and introduced the very important notion of base level, the lowest level to which a river can erode, normally assumed to be sea level. Grove Karl Gilbert was one of the first geomorphologists to ask the scientific question—how? He investigated the relationships between erosion forces and the resistance of rocks and soils to these forces. He constructed a series of laws of landscape development that were based on the very important concept of dynamic equilibrium, which in turn emphasizes the close links between processes and landforms. To appreciate Gilbert’s interpretation of this concept, imagine a flooding river which causes, through erosion, a small lowering of the bed, and so a change in form of the river channel. When the river returns to normal flow conditions it will begin to deposit sediments on its bed to restore the original elevation. In other words, although change is occurring it is around an average condition. Geomorphologists today have a modified view of this concept of dynamic equilibrium (see below) and would refer to this condition as steady-state equilibrium.

III

The Cycle of Erosion

Gilbert was perhaps ahead of his time and his innovative “modern” approach was to some extent obscured by the development, again in the United States, of the first successful attempt to develop a theoretical model of landscape development through time. This model, the cycle of erosion, was developed by William Morris Davis in the early years of the 20th century and grew out of the pioneering work of Lyell and, much later, that of Powell and his colleagues. Davis suggested landforms could be explained in terms of structure (all aspects of the underlying geology), process (all the agents of erosion and weathering, for example, water, wind, and ice), and time (the period during which processes have been able to operate). During the cycle of erosion, landscapes were considered to pass through three stages: youth, maturity, and old age. This method of the study of landforms is called the historical or evolutionary approach.

The cycle of erosion begins with a rather simplified assumption that an area is uplifted and erosion begins on a pristine and relatively even surface. Rivers, called consequent streams, would develop on this surface with an irregular profile that eventually, through erosion, will become progressively smoother. The drainage network extends through subsequent streams that develop along lines of structural weakness (for example, softer rocks, faults, etc.) in the underlying rocks. The extending drainage network and adjustments in the gradient of the river profile would eventually permit the efficient removal of water and debris from the land with little energy left in the system for further vertical erosion. This condition was called grade. The whole landscape would progress towards a graded condition as the drainage network continued to develop through river capture. Once grade was achieved the rivers would cease rapid vertical erosion and meanders would create lateral erosion and the development of a flood plain. Slopes on valley sides were considered to develop a similar graded form that would progressively flatten.

During the stage of youth, relief in the landscape increases as rivers cut down to achieve grade. During the stage of maturity, meandering rivers, adjusted to the underlying structure, have reached grade and relief diminishes as the initial land-surface is progressively removed. During the stage of old age, the relief becomes further subdued and an almost flat surface develops called a peneplain.

IV

A Quantitative Approach

This model of landscape development was enthusiastically adopted by both American and British geographers. However, European geomorphologists considered that the concept of the cycle was too simplistic and did not permit a full appreciation of the role of climate as a major influence on landform development. Also, from the mid-1940s onward, American geomorphologists began questioning the poor understanding of the actual relationships between form and processes and suggested a need for a more empirical, as opposed to theoretical, approach to the study of the change of landforms through time. This led to a much more quantitative approach to the subject in which two main branches emerged: morphometry, or the quantitative study of the shape or morphology of landforms; and detailed field and laboratory measurement and modelling of the basic physical principles and relationships between form and process, which drew to some extent on the work of civil engineers.

In the 1960s and 1970s this increasingly quantitative approach was paralleled by the application of systems analysis to the study of landforms in which a system is a set of objects which are related to one another and operate together as a complex whole, for example, a drainage basin. Three types of system were envisaged: morphological systems, representing the relationships (often statistical) between morphological (shape) components of landforms; cascading systems, representing flows of energy and mass through the landscape (processes); and process-response systems, representing the adjustments between form and processes. Various conditions of adjustment between form and process are envisaged depending on the length of time over which the landforms are being considered. In the short term a steady-state equilibrium exists in which, for example, an individual flood event in a river causes erosion but subsequent deposition of sediment leads to a return to the conditions prior to the flood (Gilbert’s dynamic equilibrium). The modern concept of dynamic equilibrium applies to the slightly longer term, say over thousands of years, when many floods would eventually lead to progressive erosion or lowering of the channel floor and the channel gradient. Over the long term, say millions of years, decay equilibrium exists in which the land surface can be reduced to a surface of low relief—an erosion or planation surface (almost the peneplain of Davis).

The emphasis on the relationship between form and process is the core of most modern geomorphological study and is known as the functional approach. The essentially scientific nature of this approach means that models can be developed (both physical and mathematical) that allow for prediction of short-term landform changes. Because such predictions are increasingly reliable, a new branch of the subject, applied geomorphology (with similar developments in biogeography and climatology), has emerged which allows physical geographers to advise civil engineers, planners, government departments, environmental agencies, and other interested parties on the nature and magnitudes of possible impacts on the environment of actual and potential human activities (for example, road building, dam construction, intensive agriculture, and flood-control schemes).