When I first read about "wagon wheel ruts" I thought I was going to find the imprint of a wagon wheel sunk in the mud hardened to stone.

That is not even close to reality. What you really find is a path worn deep into the surrounding praire by thousands of wagon teams and people trekking along an unhardened dirt road. Over time the grasses have grown back over the trail. It is the same as a well used mountain trail worn deep by hundreds of people using it.

If you look close you can see the dip with both sides higher. It is lots easier to see in real life than it is in the picture. The light was not right to be able to see contour relief.