You’ve heard the term “old as the hills”? Well, the Black Hills are pretty old. Some of the rock is estimated to be more than 2 billion years old. The granite spires west of Keystone (including Mount Rushmore) are relatively young, a mere 1.8 billion years.
"The geology of the Black Hills is complex,” according to Wikipedia. An understatement.
Picture a nice, flat plain, minding its own business, building up sandy sediment over millions of years. Each layer presses down on the layer below, until a thick blanket of sandstone and limestone forms.
Then suddenly (geologically speaking) a big, hot bulge of magma creeps up from the mantle, pushing the sandstone and other sedimentary rock up above it. Eventually the magma cools, becoming granite. (It's still below the surface at this point.) The uplifted sedimentary rock breaks apart, erodes away and slowly reveals the granite below.
Today, the strange, beautiful granite spires you see while driving the Peter Norbeck Scenic Byway are the eroded remnants of that granite bubble. That rock forms the core of the Black Hills. It’s surrounded by metamorphic and sedimentary rock formations. In some areas, you can see the layers of rock upset by the bubble.
This very visible variety of rock formations is one reason you see a van loads of university students in the Black Hills every year. Instead of looking at photos, geology students can actually see the results of all different kinds of geologic processes.