Imagine being an archaeologist in the 1930s and finding, etched into a fossilized chunk of sandstone, a perfect circle. Far too perfect to have been drawn by human hands. Or imagine being a Viking coming ashore onto Iceland for the first time, and finding perfect circles in the sand of the same sort you'd seen on the coast back in Sweden. This is what they would've looked like, and it might've taken you a moment to realize where they came from:
Scratch Circles or Scharrkreise, as they're known, have popped up everywhere from Iceland to Sweden to Finland to the shore of Lake Michigan as photographed above, and even in the fossil record. At least one gent cited in this paper [PDF] was studying them as early as 1935.
The way that they form is simple, as explained by photographer David Marvin:
Etched by windblown, dried dune grasses, the circles take shape when the wind causes a bent stalk of grass to pivot around on its axis, scratching out an arc or full circle in the sand.
Scratch Circles range in diameter from roughly four inches to sixteen inches (10cm to 40cm), and are occasionally half-circles or partial arcs. And apparently some folks use them to predict the weather: Sometime in the mid-20th-century the nature writer, marine biologist and environmental pioneer Rachel Carson wrote that "Arcs, especially on the southeast side of the grass, mean unsettled weather, so they say; whole circles foretell fair weather because they show the wind to be blowing alternately from different quarters." It makes some degree of sense, and prior to Weather.com I guess you could do a lot worse.
It's safe to assume ancient people with even basic observation skills could deduce how they were formed (unless they were looking at a fossilized sample, which must've had them stumped if the vegetable matter had decayed). I wonder if Scratch Circles were what inspired the first compasses.