400 BOOK REVIEWS

the field including, for example, Greenland, Quirk, Hillel, Monteith and Edwards. These scene-setting reviews are excellent introductions to the state of our knowledge in the specified area. In fact, for students of soil physics and tropical agronomy they could well prove to be invaluable. There is as yet no useful text on the physics of tropical soils, but an amalgam of these intro- ductory essays goes a long way towards filling the gap. To illustrate this by one example, the paper by Hillel on soil-water relations emphasizes the fundamental change which has taken place in our understanding of soil- plant-water relationships. In this field the use of a dynamic systems approach has superceded traditional availability concepts and their use of soil-water constants such as field capacity, wilting point, capillary water and gravitational water.

In a book of this size and diversity it is, of course, impossible in a short review to do justice to the details of the individual papers presented. Suffice it to say that this is going to be an invaluable reference volume for the book shelves of soil scientists, agronomists and crop ecologists. A wider audience of geomorphologists, hydrologists and earth scientists will be more concerned about the relevance of the papers for their own parti- cular field of interest. This group of readers will not be disappointed as there are several studies of relevance. D. J. Greenland in his paper on soil structure presents a useful summary of the many systems extant for classify- ing soil pores. In suggesting a functional rather than a morphological scheme, he i s surely compiling a system which is more akin to hydrological reality than previous classifications. P. M. Ahn, in a study of microaggregation in tropical soils, reminds us that the familiar sin of viewing tropical soils through temperate zone spectacles is still leading us to rnisappreciations of the tropical environment. Microaggregation in tropical soils is affected by anhydrous sesquioxides which impart a high degree of stability to microaggregates. Thus many tro- pical soils feel like loams and sandy loams in the field, and indeed behave as such from the point of view of properties such as moisture characteristics and erodi-

bility for example. However, laboratory mechanical analyses reveal high clay contents and thus we are dealing with clay soils which in terms of behaviour are ‘pseudosands’. H. Ghadiri and D. Payne give details of a dramatic study of raindrop impact and soil splash by means of cine-photography. Stress on a soil aggregate due to dro impact reaches a maximum value of 200- 400 kN/m lasting for less than 1 millisecond. This initial pressure is rapidly followed by a complex system of lower pressures and surface shearing stresses which tend to detach and lift soil particles. J. L. Monteith in his study of soil temperature and crop growth in the tropics reminds us of the paradox that this definitive aspect of the tropical environment is in fact one of the most neglected in terms of research. He points out that tropi- cal soil temperatures commonly reach 40°C and can exceed 50°C. Although he is mainly concerned with the effects of high temperatures on plant physiology, it is equally clear that there are strong implications for wea- thering rates, soil chemical processes, and the responses of soil microbiology. K. A. Edwards attempts an assessment of soil erosion rates in Kenya from data on sediment transport in rivers. Regional rates of soil erosion are discussed as also are the staggering rates of reservoir siltation in Kenya.

Overall, then, one’s judgement must be that this will be an invaluable work for soil scientists and agronomists, but will also be consulted by geomorphologists, hydro- logists and earth scientists in general.

P

KEN ATKINSON School of Geography

University of Leeds

REFERENCES

Greenland, D. J. (1978). The responsibilities of soil science. Transactions 11 th Congress, International Society of Soil Science, Edmonton, 2 , pp. 341-358.

Greenland, D. J. and Lal, R. (Eds) (1977). Soil Conservation and Management in the Humid Tropics. Chichester, Wiley.

PROCESS IN GEOMORPHOLOGY, Embleton and Thornes (Eds), Edward Arnold. Price: f8.95.

Thirty years ago the general geomorphology could still provide an overview at advanced undergraduate level of all that had then been published. Then, starting with Leopold, Wolman and Miller in 1964, came the specialized texts on each branch: rivers, slopes, glaciers, the periglacial zone, deserts, coasts and weathering. Each of these treated both processes and landforms, using the one to explain the other.

Now a group of six authors have attempted a synthesis

of a different kind, that of taking the process elements from each of these branches and putting them together. In the middle of the book are 8 chapters on the processes of, or associated with, weathering, mass movements, rivers, ‘sub-surface processes’ (which turns out to be groundwater and ground ice), glaciers, snow, wind and coasts; this otherwise orderly arrangement breaks down when it comes to the flow of water on slopes, which is treated in three scattered sections. To summarize marine processes at this level in 25 pages is attempting the impossible and this would have been better omitted. Preceding these are two chapters intended to provide a

BOOK REVIEWS 401

physical basis, on energy, forces and resistances and on fluid motion; and making the book into a sandwich, two brief discussions of general concepts, by the same author and with no strong rationale for what comes first and what last.

The main customers are undoubtedly undergraduates taking courses in advanced geomorphology, and for all the carping which follows it is undoubtedly good value for their money. But since (curiously) reviews of text- books directed at undergraduates are rarely written by undergraduates we must make do with one by another if less numerous class of readers, those whose under- graduate geomorphology came out of Thornbury, devoted their research to one branch, and seize avidly on a text which looks as if it might bring their all-round knowledge up to the level of the present-day graduate.

Given a basic requirement of the scientific competence of the authors, which is amply fulfilled, two questions may be asked of such a book. Is the aim, that of making a synthesis based on process, a laudable one? And if so, does it come off?

There is a range of levels of explanation in geomor- phology. First come the basic laws of physics-energy, momentum, etc.-which are expressed in very short equations but which only explain behaviour under highly artificial controlled conditions. Next, what may be called laws of applied physics, which are derived from the preceding but add the properties of the phenomena concerned, such as strength of materials or flow in channels; these are often embodied in equations which look complicated but their significance can be grasped by puzzling out which terms come on top and which beneath. Then there are empirical equations, shorter again, derived from statistical treatment of experimental data, useful for forecasting whether the cutting will collapse or channel will be stable but not pretending to explain in fundamental terms. Finally, we come to the observations on process behaviour and landforms that we are trying to explain: that landslides happen when a storm follows a wet spell, meander wavelength is related to discharge, and so on.

Historically, geomorphology has worked backwards through these levels of explanation, digging ever deeper and (like the psychiatrist as compared with the psy- chologist) coming up dirtier, that is, with more and more equations appearing on the page. This is as it should be if the subject is to advance. But for the mathematics to be meangingful or, indeed, worth bringing in at all we must be shown its significance in real terms. Rather too often

in this book, if we plough through a tough passage we are left with a feeling of ‘so what’, of being none the wiser for the effort. There seem to be two reasons. First, through deliberately being about the processes only, there is not space to describe the phenomena that are to be explained. This is an inevitable consequence of the concept of the book. Secondly, some of the authors lack the highest skill of scientific writing, that of making the difficult seem easy. Too often they have not even cast off the standard fault of the thesis, that of showing off how clever the writer is, either by omitting to follow some complex concept with an explanation in easier terms (e.g. ‘One clear result . . . ’ p. 236) introduces an unfathomable sentence) or by throwing in unifor- matative references. To give two examples among far too many, ‘another way in which overland flow may be generated according to Hewlett (1961)’ (p. 223) and ‘Ellison (1947) defined four subprocesses in wash erosion on slopes’ (p. 260) should read ‘. . . generated.’ and ‘There are four . . . ’ respectively, for in each case this is received scientific fact. The two chapters not only easiest to read but most informative are those by Embleton on glacial and nival processes; and besides the fact that he is summarizing what he has written before, significantly there are only three equations.

I am far from arguing against the use of physically- based reasoning, expressed in mathematics that some- times needs must be complex, as a means of geomor- phological explanation. But we must be shown what such explanation means in real terms. It is only worth delving down the levels of explanation if this provides enlightenment.

So was the aim laudable? In these days of process- orientated geomorphology courses undeniably yes, even if the severing of processes from landforms leads to an inevitable loss of excitement. Does it come off? Regret- tably, no. By now there does exist a large body of process-based geomorphology founded on a rigorous physical basis; but to select the essentials and bring them together into a coherent and meaningful account calls for clearer and more considered writing skill than is found here. Like, I suspect, will be the case with many undergraduates, I sought enlightment and failed to find it.

ANTHONY YOUNG School of Environmental Sciences

University of East Anglia