There used to be magma relatively close to the surface.
Explanation:
Laccoliths are formed by massive magma intrusions that invade rock layers above the magma pool. The pressure of the magma pushing upward causes the deformation of the rock strata on top into a dome shape.
The laccoliths are not exposed to the surface until much later, but over geologic time they prove to be more resistant than the surrounding non-igneous rocks and can form gently sloped mountains that resist erosion more than the surrounding landscape.
This is the type of geological scenario that was taking place hundreds of thousands of years ago and more in South Dakota.
