Black Hills of South Dakota Geology

July 6 through 13, 2009

Let's look at some of the geology going on in the Black Hills of South Dakota.

These are a few of the places you will recognize in the Black Hills of South Dakota: Mount Rushmore, Needles Scenic Highway, Needles Tunnels, Iron Mountain Road, and Custer State Park.

Many of these roads are not motorhome friendly.

In fact do not drive your motorhome on either the Iron Mountain Scenic drive or the Needles Highway.

We are camped near Custer State Park and the town of Custer in the southern part of the Black Hills at Custer's Gulch RV-Park. It is an OK commercial campground just like dozens of others in this area.

Igneous rock that cooled deep underground showcases in the Black Hills of South Dakota

This is one of the myriad igneous rock formations visible throughout the Black Hills. Igneous rocks are formed (crystallized) from molten magma. Igneous rocks can crystallize at depth (under pressure and longer cooling period) or upon reaching the surface (little or no pressure and much quicker cooling time). Cooling time and pressure determine what type of rocks are formed. Most, if not all, Igneous rocks in the Black Hills were formed (crystallized) at depth and as such are coarsely granular.

Summit of Iron Mountain in the Black Hills of South Dakota

This is a view from near the top of Iron Mountain not far from Mt. Rushmore.

There is a lot of iron in the ground here. It would not be a good place to be during a thunder storm. Amen!

Outcropping of igneous rock in the Black Hills of South Dakota

The Black Hills expose many different outcroppings of igneous rock like this.

Dike of exposed granite in the Black Hills of South Dakota

The Needles highway winds its way through spires (needles) of granite that crystallized underground as it punched holes through the overlying sedimentary rock

The much softer sedimentary rock has long ago eroded away leaving just this hard mass of once molten rock.

Dikes are created when pressure deep within the earth forces molten rock into cracks in the overlying sedimentary rock. This granite dike was forced into a crack in the overlying sedimentary rock deep underground where it slowly hardened. Over millions of years the sedimentary rock which once surrounded this dike has been eroded away leaving the much harder dike made of granite.

Intrusions of molten rock can leave all manner of shapes in the Black Hills of South Dakota

Some molten rock was injected into big cracks that spread horizontally rather than vertically.

Some sedimentary rock is still visible.

Sedimentary Rock exposed in South Dakota's Custer State Park

This is an exposure of sandstone (sedimentary rock) along French Creek visible from the "wildlife-loop" in Custer State Park. The red is appearing brighter because of the sun setting behind us as this picture was taken. For those of you not familiar with the Black Hills the "wildlife-loop" is located in the southern part of the Black Hills where the dome of molten rock did not push up nearly as far as it did further north around Mt. Rushmore.

This sedimentary rock has not eroded away as much as the sedimentary rock has further north ---- in the Mt. Rushmore, Deadwood area. However, a creek has eroded this formation. You can see the path of that small stream of water outlined by the green trees at the bottom of the cliff. Water & time, lots of time, create cuts like this that expose thousands upon thousands of years in geological time.

Outcropping of igneous rock in the Black Hills of South Dakota

This is nothing more than a representative picture of both the Needles highway and Iron Mountain scenic drive.

Dike of igneous Rock exposed in Black Hills of South Dakota

This is another exposure of granite near Mount Rushmore in the Black Hills of South Dakota.

Black Hills of South Dakota Geology

This is a mixture of sedimentary rock and igneous rock. When the igneous rock was pushing toward the surface it was cracking the sedimentary layers of rock on top. As cracks formed the liquid igneous rock entered cracks in the sedimentary layers. Here you can see layers of both types of rock.

This is where the molten rock flowed horizontally into the cracks and crystallized.