Some sandstones have very obvious sweeping curved patterns within their layers.
This sandstone dates from the Permian period, and is in south Devon.
This pattern is called cross-bedding.
It is formed when loose sand is moved, sometimes by water and sometimes by wind.
This larger sized cross-bedding is more often linked to wind movement, but to be sure geologists look closely at the sand grains.
The grains are mostly rounded off, not sharp, which shows they were blown by the wind.
This suggests that the sandstone was produced in a desert.
The cross-bedding forms as sand flows down the front of sand dunes.
Another well-known cross-bedded sandstone is the Coconino Sandstone, seen in the Grand Canyon.
It is another Permian rock formed in a desert.