Sand Dunes

Dunes formed by either water or wind have some common characteristics. Both are likely to have ripple marks developed on their upper surface, especially on their 'windward' side. More importantly, the dune form has a relatively shallow slope facing the source of the energy (up-wind or up-wave) and a relatively steeper slope on the leeward or down-wave side. The slope of the leeward side is determined by the angle of repose of the material composing the dune. The angle of repose is in turn determined by the composition, size, and shape of the particles making up the sediment and the viscosity of the medium (air or water) overlying the sediment.

Both kinds of dunes are preserved in the rock record - how can we tell them apart? The easiest way to distinguish these very different paleo-environments is by examining the scale of the crossbeds making up the dunes. Wind-blown sand dunes tend to be much larger than their water-lain cousins. A classic example is the Navajo Sandstone in the SW part of the United States where crossbed sets are 10's of feet in height. The cross beds preserved in the Pinnacles are consistent with a wind-blown origin.

Here spectacular differential erosion emphasizes the crossbeds in the upper portion of this pinnacle. The differential erosion is controlled by the relative degree of cementation of the various layers of sand.