Yellowstone is in the mountains, but is not particularly mountainous, because it comprises mostly a caldera. If I had once known that term, I had forgotten it, but my thirteen year old knew it from last year’s Runkle Geography course (see here).
In the map below, you can see the ring where the caldera is in Yellowstone. Yellowstone was once an enormous mountain and long, long ago, it’s top was blown off scattering ash over most of North America. The caldera marks the spot where the lava erupted, and now after all these years it is a sunken valley with hills at the perimeter, still very close in touch with the thermal activity of it’s past. So here are some pictures of the Yellowstone Caldera activity; it’s so odd to be standing where a whole mountaintop was forcibly removed.