Why deposits of longitudinal dunes are rarely recognized in the geologic record

Dunes that are morphologically of linear type, many of which are probably of longitudinal type in a morphodynamic sense, are common in modern deserts, but their deposits are rarely identified in aeolian sandstones. One reason for non‐recognition of such dunes is that they can migrate laterally when they are not exactly parallel to the long‐term sand‐transport direction, thereby depositing cross‐strata that have unimodal cross‐bed dip directions and consequently resemble deposits of transverse dunes. Dune‐parallel components of sand transport can be recognized in ancient aeolian sands by examining compound cross‐bedding formed by small dunes that migrated across the lee slopes of large dunes and documenting that the small dunes migrated with a component in a preferred along‐crest direction over the large dunes.