THE NATURE, DISTRIBUTION AND FORMATION OF PANS IN ARID ZONES

Pans, closed depressions, are a widespread feature of many of the worl d's drylands. By using literature survey, air photographs, topographic maps, orbital photographs and imagery, combined with field work it is possible to describe the major areas where these features occur. Thei r distribution is controlled to a great extent by the availability of susceptible surfaces. They also develop in certain particular environm ental settings: palaeolacustrine basins, palaeodrainages, interdunes, and on coastal plains. Many of the pans have a distinctive morphology while on their lee sides they may have lunette dunes. Many processes c ombine to create and maintain pans, and these can be considered in a g eneral model which has certain key elements. The first of these is tha t the area should not be one where fluvial processes are fully integra ted. It should:also not be one where aeolian accumulation is such as t o infill any irregularities in the land's surface. If these two predis posing conditions are fulfilled then under dryland conditions, if susc eptible surfaces are present, there are various circumstances that may lead to hollow development and enlargement. Although in some cases su ch processes as solution, suffosion and animal activities may play a r ole, we believe that the predominant reason why pans have the characte ristics that they do (including their shapes, lunettes, alignments, et c.) is that they result from the operation of the twin processes of sa lt weathering and aeolian deflation.