The nature, distribution and formation of pans in arid zones

Abstract

Pans, closed depressions, are a widespread feature of many of the world'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. Their distribution is controlled to a great extent by the availability of susceptible surfaces. They also develop in certain particular environmental 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 combine to create and maintain pans, and these can be considered in a general model which has certain key elements.The first of these is that the area should not be one where fluvial processes are fully integrated. It should also not be one where aeolian accumulation is such as to infill any irregularities in the land's surface. If these two predisposing conditions are fulfilled then under dryland conditions, if susceptible surfaces are present, there are various circumstances that may lead to hollow development and enlargement. Although in some cases such processes as solution, suffosion and animal activities may play a role, we believe that the predominant reason why pans have the characteristics that they do (including their shapes, lunettes, alignments, etc.) is that they result from the operation of the twin processes of salt weathering and aeolian deflation.