Here's why:

* Upwarping: The Black Hills were formed by a process called uplift, where a large area of the Earth's crust was pushed upward by tectonic forces. This upward pressure caused the rock layers to bend and rise, creating a dome-shaped structure.

* Erosion: Over millions of years, erosion by wind and water has worn down the uplifted rocks, creating the distinctive rounded peaks and valleys characteristic of the Black Hills.

Other examples of upwarped mountains include:

* The Adirondack Mountains in New York

* The Wichita Mountains in Oklahoma

* The Ozark Mountains in Missouri and Arkansas

These mountains are all examples of domal uplifts, meaning they were formed by a broad, dome-like uplift of the Earth's crust.