Badlands in marl lithologies: a field guide to soil dispersion, sub-surface erosion and piping-origin gullies

Scientists studying badland processes in Mediterranean and Semi-arid climates require assurances that the material in which gullies are presented is not dispersive. A dispersive context means; first, infiltration rates may be radically changing in very short periods due to swelling and deflocculation of clays; second, surface crusts could be the result of translocation of sodium into subsurface positions; third, rills may be formed or at least exacerbated by shallow subsurface erosion; fourth, large gullies with substantial up-channel headcuts, including so-called ‘bank gullies’, may have formed because subsurface pipes have collapsed; and fifth, that network connectivity and evolution may be principally internal, being effected by subsurface pipe capture network integration; and most importantly, the bulk of the sediment moving around in the landscape is not being lost from the surface. This paper presents a decision-support tool to assist the effective diagnosis of a landscape's principal genetic process suite. The soil's behaviour in response to its geochemistry in marls with high exchangeable sodium percentages (ESPs) is outlined in simple terms with minimum use of laboratory or field chemical investigations. Using examples the paper then presents a simple set of form indicators that can be used in the field to diagnose the possibility that subsurface process are dominating landscape erosion. Surface crust character, ephemeral rills, and large subsurface tunnel settings are explained and classified. In a final section, the geomorphological implications of piping in gullied landscapes are explored by reference to the literature on connectivity.