Bt. Werner et E. Merino, CONCAVE SAND GRAINS IN EOLIAN ENVIRONMENTS - EVIDENCE, MECHANISM, ANDMODELING, Journal of sedimentary research, 67(5), 1997, pp. 754-762
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.