On a gusty coast anywhere in the world, don’t be surprised to stumble upon a perfect circle, or concentric circles, etched in the sand. As Kottke points out, these aren’t alien landing pads—they’re scratch circles.
Scratch circles, or Scharrkreise (pdf) in German (literally “scrape circles”) are formed when plants on sandy shores are flattened by persistent wind. These plants, typically grasses with long stems, are pushed around and around by the wind; because their roots are a fixed point, they end up working like a compass to create arcs.
Typically, scratch circles appear on beaches. However, scientists have also found these circles in deep-sea sediment (paywall), or in fossils (pdf).
Image of the first scratch circle used with permission from David Marvin on Flickr. Image of the second and third scratch circle by Flickr user James St. John used under Creative Commons 2.0 license.