Comment on "Lightning as a geomorphic agent on mountain summits : evidence from southern Africa" by Knight and Grab (2014)

12 May 2014

Insights into the possible effect of lightning strikes on rock breakdown are presented by Knight and Grab (2014) froma summit area in the Lesotho highlands. Based on their findings, the authors challenge the association of angular debris with frost shattering and use this as a platform for directing critique against palaeo-geomorphic studies.While the lightning strike data are not questioned directly here, the palaeo-environmental contextwithin which the paper is set, the portrayal of former findings and the assumptions regarding weathering mechanisms in Lesotho are commented on. Frost shattering is the centre of Knight and Grab's weathering critique but, contrary to that stated in their text, none of the cited authors invoke this process in Lesotho. Other weathering processes that are speculated upon are not specific to cold climates either and thus cannot be used in support of their argument. In terms of debris and block distribution, lightning will not account for the preferential location of relict blocks and debris below the summits on south-facing slopes, or for the extensive valley floor accumulations that are documented in Lesotho. Knight and Grab also falsely portray former studies by implying that palaeo-environmental inferences in the area are drawn fromblock origin or morphology alonewhen the climatic signatures were derived from integrated assessments. In a palaeo-context, the relative contribution of lightning to debris production under dryer and colder conditions, when convective thunderstorm activity in the highlands was likely reduced, is also questioned. The weathering context, as well as the critique that Knight and Grab direct at other studies on relict landforms, is thus shown as inappropriate.