Eolian environments occur where prevailing or seasonal winds blow across loose materials not stabilized by the ground water table or vegetation (Figs. 9.1, 9.2, and 9.3). Where loose sand is in abundant supply, winds may accumulate large, coalescing dune masses over a broad area, designated as a sand sea environment. Where sand supply is less, or winds more variable, smaller eolian bodies occur as individual dunes or patch dune areas surrounded by interdunal areas of erosion, nondeposition, or deposition associated with intermittent streams or standing water (ponds or lakes). Evaluation of vertically and laterally associated sedimentary features and lithologies is essential to the interpretation of large sets of well sorted, cross-stratified sandstone as eolian in origin.

In large dune fields (sand seas), individual genetic units of deposition are large compound sets, tens of feet in thickness, of high-angle cross-stratified, well sorted and rounded, fine-grained sand resulting from different types of dune migration. Smaller cross-stratified sets are commonly interbedded with the thick sets, as are thin interdunal deposits of small areal extent. Principal sedimentary structures and features of large dune fields are: 1) large sets of high-angle cross strata; 2) deflation (blowout) surfaces, frequently exhibiting topographic relief; 3) coarser grain lags scattered over deflation surfaces; 4) very low-relief cross-wind ripples on avalanche slopes with axes oriented parallel to dip direction of major dune sets; 5) occasional animal tracks, insect burrows and plant root traces; and 6) avalanche scars and disrupted and deformed layers associated with steeply dipping cross strata (Figs. 9.4, 9.5).