The degraded linear dunefield of north west Ngamiland, Botswana occurs in a seismically active area, lying to the northwest of the Okavango graben, widely considered to be the tail end of the East African Rift system. To assess the effects of neotectonism on the dunes, an area was selected for examination close to the Gumare fault, which bounds the graben on the northwest side. Digital SRTM data were converted to light shaded representation of the topography. it was found that dune forms only occur along the margin of the rift and on both sides of incising valleys, graded to the graben floor. A selection of long profiles showed dune crests standing some 25 in above the straats at the edge of the rift. The relative relief pinches Out away from the fault, towards interfluves that do not depict dune morphology. Landsat satellite imagery shows linear features continuous across the flat interfluves, inviting the incorrect inference from the vegetation that the linear dune forms occur there. A model is proposed that an original linear dune field formed under arid conditions, was entirely flattened under wetter conditions, crest material being washed into the straats, thus obliterating the original dune morphology. Upon rifting and steam incision, these forms ire being replicated, an example of equifinality. Replication is suggested to be by the action of infiltrating water, controlled by the groundwater gradients. These observations Suggest that the active dunefield significantly predates the tectonic processes. With the additional time required for degradation and replication it would Suggest that the dunes are of considerable antiquity. it has already been recognised that the dunefield has a complex history of construction and destruction, to which must now he added a process of base level-controlled replication.

The long morphodynamic history of the linear dune system of northwest Ngamiland comprises both construction and degradation processes. Here we describe features resulting from the latter.Crest lowering during wet periods is associated with lateral transfer of sand into the straws. Weathering and leaching have mobilised iron to the base of the dune profiles and have corroded the quartz grains. Satellite image interpretation indicates that dune sand has been diluted by bioturbation which transferred Damaran saprolite from below the dunes to the surface. Flooding and waterlogging have modified and in places completely destroyed the original crisply linear morphology. Small linear collapse features across the linear dunes are associated with open fractures or faults where water infiltration is facilitated.The implications for luminescence dating of dune sands, to identify periods of aridity, are discussed. The dunes cannot simply be regarded as providing a stacked record of such periods. They are very old and much of the record is missing as a result or crest lowering. The chronostratigraphy includes degradational components which must be distinguished from those of construction. Bioturbation cannot be disregarded as a complication which overprints the luminescence signals. (c) 2004 Elsevier Ltd and INQUA. All rights reserved.

Earth Surface Processes and LandformsVolume 22, Issue 8 p. 797-798 Book Review Book Review: Geomorphology in the Tropics — A Study of Weathering and Denudation in Low Latitudes by Michael F. Thomas, John Wiley & Sons Ltd, Chichester, 1994. No. of pages: xix+460. Price: £75.00. ISBN 0471930350. M. J. McFarlane, Corresponding Author M. J. McFarlane Department of Earth Sciences, University of BotswanaDepartment of Earth Sciences, University of BotswanaSearch for more papers by this author M. J. McFarlane, Corresponding Author M. J. McFarlane Department of Earth Sciences, University of BotswanaDepartment of Earth Sciences, University of BotswanaSearch for more papers by this author First published: 04 December 1998 https://doi.org/10.1002/(SICI)1096-9837(199708)22:8<797::AID-ESP774>3.0.CO;2-2AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume22, Issue8August 1997Pages 797-798 RelatedInformation

The effect of NH4NO3 on the expression of take-all in wheat and on the ability of fluorescent pseudomonad bacteria to decrease the growth of the pathogen and the symptoms of the disease was examined in a nitrogen-deficient acidic soil from Western Australia. Application of NH4NO3 increased the fresh weight of shoots, decreased root weight and length, and offset some of the deleterious effects of infection. Inoculation with Gaeumannomyces graminis var. triciti reduced shoot weight and root weight and length at all but the highest level of applied nitrogen. Fluorescent pseudomonads applied to wheat seed (at approximately 108 colony-forming units/seed) reduced seed germination and failed to reduce the severity of take-all expressed as weight or length of shoots or N content. Although the density of black runner hyphae of G. graminis var. triciti was reduced at 40 days and the proportion of root colonized by the pathogen was lower at 28 days with bacterization under severe nitrogen deficiency, no effect of the bacteria was observed if N was added. Key words: Gaeumannomyces graminis var. triciti, fluorescent pseudomonads, wheat, nitrogen.