Should you be wandering around in / through Kansas, and in a mood for a geological point of interest, your options are a bit limited. But at the very least, Mushroom Rock State Park should definitely be on your list.
This is a 5 acre (2 ha) park that was donated to the state by the Ellsworth County Historical Society in 1965, and contains a nice cluster of formations. These were formed when beach sand in a few places was rendered a bit more durable (vs. the neighboring rock) by some natural cementing agent. Then after tens of millions of years, the harder areas survived erosion while the softer parts gradually faded away. The result is a set of mushroom-shaped rock formations.
The park is just a few miles off of I-70, so it’s a nice place to pull over for a break should you be driving through Kansas on the interstate.