Geomorphology in the tropics

Geomorphology in the tropics

82 Book Rr~:iews method described to quantitatively interpret paleofluxes; Abrantes and colleagues describe 350 kyr records of diatom accumulation r...

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82

Book Rr~:iews

method described to quantitatively interpret paleofluxes; Abrantes and colleagues describe 350 kyr records of diatom accumulation rates (DAR) and export production P,,,> based on Corg accumulation rates in the eastern equatorial Atlantic; comparison with comparable records from the N.E. Atlantic indicate general correspondence between the two records indicating that both proxies primarily reflect export production. Various proxy records (benthic foram abundances, accumulation rates) indicate that glacial productivity off Western Australia was higher than Holocene values and support the suggestion that upwelling and enhanced primary productivity occurred off Western Australia during the Last Glacial Maximum caused by a north-flowing West Australian Current. (McCorkle and colleagues). Herguera indicates that a 250 kyr paleoproductivity record from the western equatorial Pacific based on nutrient, mixing and export indices shows a strong coherence with the ice record (6’*0 of benthic forams). Two particularly well written papers present a broad overview of the use of dinoflagellate cysts (Dale and Fjellsa) and benthic forams (Schnitker) as paleoproductivity proxies and water mass tracers, respectively. The former group is particularly useful in coastal/ neritic zones where nearly 50% of primary productivity is produced; the latter group is widely used as a tracer of bottom water masses and as an indicator of productivity where productivity is high; however fauna1 composition does not yet allow an unequivocal differentiation between productivity and water mass changes, requiring dependence upon an independent proxy for productivity. This volume will serve as an excellent standard reference on the present status of studies on oceanic carbon cycling during glacial periods. It should form basic reading of any scientist interested in the problem and uncertainties of global climatic fluctuations facing the world in the years ahead. It has been well edited and typographical errors are few (I detected less than half a dozen) and the quality of writing style is generally high considering the diversity of authorship. It is only a pity that the NATO volumes are produced in such a monotonous and unimaginative format:

white covers, black type, uninspired white dot matrix “decoration”.

blue and

William A. Berggren, Woods Hole, Mass.

SSDI 0012-8252(94)00068-9

Geomorphology M.F. Thomas, 1994. Geomorphology in the Tropics. Wiley, Chichester, U.K. Hardcover XIX + 460 pp. Price: f85.00. ISBN 0-471-93035-O. This is a difficult book to review for though it contains much of value, it lacks a clear identity. It is not a text book concerned exclusively with the landforms of humid tropical regions, though such a focus can be read into the title, part of the back cover blurb, and part of the Preface. On the other hand Thomas concerns himself with processes and events which have from time to time affected what are now the tropics (which is legitimate) and also with landscapes far beyond the confines of the tropical lands (which is an indulgence). But this slight lack of definition ought not to be allowed to distract from the considerable merits of this book, which the author describes as providing a tropical perspective on geomorphology and that is the best way of approaching it. The author is primarily concerned with the selvas and with monsoon lands, though he frequently wanders into the dry savannas. The book is divided into four parts, in which are discussed various aspects of weathering, denudation processes, Quaternary environmental change, and the evolution of landscapes, each in the tropical context. In each, regional case studies are effectively interspersed with systematic treatments. The chapters dealing with weathering are very good, with penetrating discussions on, for example, hydrothermal alteration, and lateritisation processes. The treatment of silcrete is less satisfactory, and illustrates a general feature of the

Book Reviews

book, namely some inappropriate, not to say curious, referencing; for surely in respect of silcrete the work of Milnes, Thiry and Hutton ought to have been mentioned and the published work of Milnes and Hutton would have been a useful supplement to the brief discussion of calcrete. The chapters on denudation processes are less comprehensive. Several old misconceptions about tropical rivers are firmly dispelled but the mechanics of river behaviour receive less than their due. How tropical rivers compare with their temperate and arid land counterparts in respect, for example, to channel characteristics and geometry is neglected. Accelerated soil erosion and mass movements are analysed and discussed in some detail, as befits their importance to mankind. The review of evidence on Late Cainozoic climatic change is comprehensive and critical, and is linked with landform developments. The treatment of pediments and alluvial fans is rather thin; and again the referencing is rather curious. The discussion of such comparatively recent landscapes is followed by several chapters concerned with longer term landscape evolution; and this is both appropriate and refreshing, for in the literature and in the landscape old palaeoforms and surfaces loom large in tropical lands. But whether this is entirely due to the latter regions having escaped the direct impacts of Late Cainozoic glaciations, or whether it is also a question of dating the forms and surfaces is debatable. There are after all many accounts of exhumed, etch and epigene forms of considerable antiquity from high latitude regions. Etch planation is discussed informatively and at length, and Thomas’ particular interests are beneficially indulged here as well as in the lengthy and detailed discussions of azonal granitic and karst forms. Naturally there are various statements that can be regarded as contentious. For example, it is surely specious to compare Wahrhaftig’s stepped topography with stepped land surfaces associated with faulting (p. 296). And surely not all two stage forms are related to climatic change (p. 327)? The term etch or etching used in the geomorphological context is due to Wayland and Willis, but the concept goes back to Falconer [whose key statement Thomas cites earlier (p. 1211, and to

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Jutson, with respect to etchplains, and back to the late Seventeenth Century with regard to boulders and other minor forms. On p. 8 Thomas gives the impression that in semiarid Australia the duricrusted regolith has been stripped, which is at odds with the landscape with its many laterite- or silcrete-capped plateaux. In addition to some odd referencing, and various differences of opinion concerning points of substance, however, my main difficulties are with wording and terminology. Several of the figure and plate captions are ambiguous or otherwise misleading; for example, the weathering front depicted in PI 2-l is described as “arched”, which term can carry an active overtone, as in arched strata in an upwarp or anticline; surely “convex upward”, and a nascent bomhardt? Similarly the author commonly, though not quite consistently (core boulders on p. 24, and tor monoliths on p. 261, uses the term “to? for boulders, but in places uses it for nubbins or knolls and for castle koppies. And with reference to the subtitle of the book, is not weathering a component of denudation? But these are minor quibbles. Overall Thomas is to be congratulated on his book. It provides a scholarly review of landforms and associated processes in a complex and difficult environment. Whether the humid tropics constitute a geomorphologically distinctive region remains an open question, but Thomas has drawn our attention to the fascinating forms and processes of these relatively neglected regions. The line drawings are distinctively good, the book is splendidly illustrated [though a better example of a “tafone” (Fig. lo-251 could have been found] and there are few vulgar errors, though Fig. 5-15 (p. 161) is upside down. The colour plates add to the attractiveness of what is a welcome addition to the literature.

CR. Twidale, Adelaide

SSDZ 0012-8252(94)00068-9