Ruts is a more-common term than swale. Today we think of two parallel eroded strips left by vehicle tires. But wagons were pulled by animals centered between the wheels--and they also left their mark. The entire wheel-to-wheel distance was scarred in grasslands like these. 200 years of wind, rain, snow and erosion produced the swales we see today.

In the far left distance, a center pivot provides irrigation for crops. This land can be fertile with enough water from aquifiers or underground lakes.