Question: That title is probably way too specific, but essentially I'm trying to develop my book's setting. Currently, my characters (a team of six adventurers) awaken as amnesiacs who were once human but now have been transformed into a motley menagerie, within a grassland. This hilly grassland has a road running through the center of it, and at one point, there lies a small mountain next to the road.

This mountain is rather important, as carved into its interior is a town named Rockhaven, where a plot-important SC (supporting character) awaits the team. What I don't know is what would cause a solitary mountain to form in the aforementioned biome-a grassland with rolling hills and plains-akin to the Kansan prairie. So, as you can see, my question is **What Could Explain A Solitary Mountain in a Grassland?**

Answer: 
In ancient times, before the birth of America, even before the people we know as Native Americans populated the plains and rolling hills of the North American continent, there were a mysterious society of technologically advanced people known as the Mound Builders. To this day, people are still discovering their massive works hidden in plain sight, having been mistaken for centuries as naturally occurring hills and mountains. Some of these enormous man-made mountains were used as grave yards for the elite VIPs of their culture, others seem to have been designed to be sanctuaries in the event of sudden, catastrophic flooding, but most of these "mounds" were built for reasons that still evade explanation. Iowa, Kansas, Nebraska, Illinois, Missouri and Minnesota were favored by these people for mountain building, as the miles upon miles of flat land emphasized their work's looming presence on the landscape.
Just to give an alternative to (dead) volcanoes or volcanic intrusions:

**[Salt tectonics](https://en.wikipedia.org/wiki/Salt_tectonics)**

A salt dome may push a solitary rock or hill upwards, examples are the [Segeberger Kalkberg](https://en.wikipedia.org/wiki/Segeberger_Kalkberg) in Bad Segeberg, Germany, or the off-shore island [Heligoland](https://en.wikipedia.org/wiki/Heligoland) (imagine a lower sea level to place it in a grass plain).