CONCAVE SAND GRAINS IN EOLIAN ENVIRONMENTS - EVIDENCE, MECHANISM, ANDMODELING

Many eolian sand grains have well-rounded concavities, These concaviti es occur in grains of all minerals in a single sample and preferential ly at larger grain sizes, suggesting that they form by mechanical abra sion in the eolian environment rather than by chemical dissolution, Th e concavities develop through abrasion at contacts between near-surfac e grains in the bed, The energy for this abrasion is provided by colli sions of saltating grains with the sand bed. As a grain with a concavi ty cycles through a range of depths in a ripple, agitation of the bed driven by grain-bed impacts promotes the formation of a grain-grain co ntact between that concavity and a protuberance on a neighboring grain , This in turn focuses further wear at the contact, thereby circumvent ing the natural tendency toward grain sphericity that would arise from random grain orientation, A quantitative physical model combining the ories of grain-bed impacts and of granular materials with experimental surface erosion rates establishes the plausibility of this mechanism, Within the model, concavities grow to a limiting size that is a signi ficant fraction (order 10-30%) of a grain diameter, within the range o f observed concavity sizes.