Needles Geology in Black Hills of South Dakota

July 6 through 13, 2009

Geology of the "Needles" in the Black Hills of South Dakota.

The Needles Scenic Drive is one of the things you should do when in the Mount Rushmore area. You should also plan to stop by Sylvan Lake, Iron Mountain Road, and Custer State Park when in the Black Hills.

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.

The Needles

This picture was taken from the pullout at the entrance to the famous tunnel through the needles not far from Mt. Rushmore.

You can climb on these rocks to get a better feel of the "texture" of the rock. Igneous rock that cools slowly under pressure creates large grains. You can inspect those "large-grains" while climbing the formations at this pullout.

Igneous rock that formed the "Needles" of the Black Hills

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.

Spires of solid granite on the Needles Highway

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.

Large Granite "needle"

Look at that large "needle" of granite exposed behind the tall pine tree.

Needles Highway

The road is passing between two large granite spires on the Needles highway.

Granite spires visible on the Needles Highway

The "Needles" from a distance on the Needles Highway.

Granite formation that did not exactly make a "spire" or needle

More of the "Needles" formations.

Granite wall visible on Needles Highway

This was taken on the Needles highway.

Spires or Needles the granite formations on the Needles Highway

More "Needles" on the Needles Highway.

Granite formation in the Black Hills of South Dakota

Large granite formations like this are good for artist to use for sculptures.