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: 
Volcanos. An example is the Sutter Buttes in California's Central Valley: <https://en.wikipedia.org/wiki/Sutter_Buttes> It's the eroded remains of a volcano, surrounded by flat land, and was almost certainly a larger mountain in the past.

A somewhat similar example is Mount Shasta. Although some of the surroundings are mountainous (though much lower), some of the surrounding area, especially to the north is fairly flat grassland. Shasta also has a set of legends which describe it as housing a city of advanced beings: <https://en.wikipedia.org/wiki/Mount_Shasta> See also the Heinlein story "Lost Legacy".
Since there is a town in the interior of the mountain, perhaps the "mountan" is acutally a gigantic building built to contain a totally self enclosed habitat, including the town, reached by a few airlocks leading to the outside world.

And maybe the outside of the building has slowly weathered and assumed a more rugged and natural shape, although it still looks very artifical compared to most real mountains.