Cross-Bedding: How Sandstone Records Moving Dunes
Rock layers do not always lie neatly on top of one another. In some sandstone, groups of thin lines slope across the larger layers. This pattern is called cross-bedding.
How the sloping layers form
Wind or flowing water moves sand into ripples and dunes. As a dune moves, grains collect on its sloping surface. New layers build over older ones, leaving a record of that movement.
If the sand is later buried and becomes rock, parts of those sloping layers can survive. A cliff or road cutting may expose them in cross-section.
An ancient desert example
The Navajo Sandstone at Zion National Park preserves large groups of cross-beds formed in ancient windblown dunes. Their orientation helps geologists work out the directions in which the dunes moved.
Does every cross-bed mean desert?
No. Water can also create cross-bedding. A geologist considers the grain size, the shape and scale of the layers, and other nearby features before deciding how the sand was deposited.
A useful way to look at an outcrop is to find the main layer boundaries first. Then look for smaller, sloping layers inside them. Photograph both a wide view and a close view, with a scale if possible.
A few diagonal lines can preserve movement from a landscape that disappeared long ago.
Sources: NPS: Navajo Sandstone; NPS: Cross-stratified Sand.
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