Thin (< 30-cm-thick) sets of eolian cross-strata with irregular upper bounding surfaces make up 27% of the Lower Jurassic Wingate Sandstone near Moab in southeastern Utah and up to 40% of the Middle Jurassic Entrada Sandstone near Dinosaur National Monument in northeastern Utah. Deformation structures, interpreted as the tracks of large reptiles, are common in windripple laminae within and directly adjacent to these sets. Lithosomes composed primarily of these sets range from < 1 m to 9 m in thickness. Cosets composed of thick, trough cross-stratified sets that are interbedded with cosets composed of the thinner sets contain smooth, nearly horizontal bounding surfaces. The lack of intertonguing with thick sets of cross-strata at coset tops and the absence of any evidence of downlap onto a basal erosional surface make it unlikely that the thin sets of cross-strata could have been deposited in interdune positions between large bedforms. Cosets composed of thick sets of trough cross-strata record episodes when ergs were sand-saturated and erg buildup exceeded the rate of subsidence and water table rise. Interdunes were enclosed and dry. Thin sets with irregular set boundaries accumulated when ergs were unsaturated, as at White Sands dune field in southern New Mexico. Interdunes were flat, extensive, and moist; thin lenses of cross-strata with irregular upper surfaces are the erosional remnants of lowermost portions of dunes that were within the zone of capillary rise. Thin, flat-bedded to massive interdune deposits were also preserved in this setting. Due to low sand supply, upbuilding of the erg surface was limited to the rate of subsidence/water table rise. Like sediments deposited in shallow subaqueous settings, but unlike most colian sands, bed-by-bed accumulation of the thin sets was limited by "short-term" accommodation space. The allogenic interpretation presented above contrasts strongly with the widely-accepted "climbing interdune" (autogenie) interpretation of the Entrada. We question this gradualistic interpretation, not only on the basis of stratal relationships between dune and interdune deposits, but on theoretical grounds as well. To maintain moist/wet interdunes when large bedforms are climbing to preserve thick, tabular sand bodies, deposition must not exceed the rate of groundwater rise. We have seen very few irregular erosion surfaces between thick sets of eolian cross-strata; they are extremely common between thin sets. Widespread moist interdunes in modern ergs and evidence of high water tables in ancient eolian sequences are signals of low sand supply. Thin eolian sets with irregular upper bounding surfaces record the birth and demise of ergs as well as the fluctuations of erg margins. The absence of animal tracks in the thick Jurassic cross-strata and their great abundance in and adjacent to the thin sets may be a result of an erg-margin origin of the thin sets or of ameliorated climatic conditions during their deposition. Because they accumulate in zones (and episodes) of low sand supply, cosets composed of thin sets are akin to deflation surfaces in, for example, the Permian Cedar Mesa Sandstone. The movement of the groundwater table after the initial deflationary phase (slow rise in the Jurassic, rapid(?) drop in the Permian) may reflect the higher amplitude of late Paleozoie sea-level fluctuations.