People in rural areas are not connoisseurs of art nowadays, and in earlier, much hungrier times were probably not interested in creating masterpieces anymore than rural farmers from Kansas are today.
Some people milled with a two-handed method using vertical strokes, others used big pestles and rotated. The two handed method would explain the "heart shape" (right). 3000 years ago Socrates wasn't even born, and he was the smartest guy in the world up until then... even he thought that the liver was where our intelligence resided. But these starving rural farmers probably had it all figured out and were trying to communicate it to us...
But I'm not really sure. Perhaps you should all send me money so I could study this further... a million or two should do...
When I'm done with this, I'll study crop-circles.
I don't know how you can be sure it was rye, but milling with stone pestles seems to me the most probable explanation. These stories always amuse me because they talk about "metal tools" being used and don't identify what kind of rock is involved. There are a few very soft types of rock that can be carved with metal but most stone is much harder than metal, especially any metals available long ago. Many people today think steel is harder than rock. Try a steel chisel on seriously hard rock like basalt or granite and get back to me with your report. And remember that steel is a relatively recent invention.
Is it true that most folks from Kansas believe that The Wizard of Oz is a documentary?