The vertical granite spires known as the Needles were formed by weathering along joints in the rock. Joints are fractures in rock that occur with regular spacing. This rock has a well-developed set of vertical joints. Water seeping into these fractures splits the rock by frost-wedging , then further breaks down the rock by chemical weathering. Rain water, especially if it is slightly acid, encourages chemical reactions that cause feldspars to turn to clay.
Ferromagnesian minerals break down to iron oxides, staining the rock red.
These weathering products are much softer than the original granite and they are easily eroded by rain and snowmelt. What is left behind is the rock between the joints, these pinnacles of granite. 9.0 Back to Sylvan Lake. Ranger Station at Custer State Park. 1.2 Harney Peak, the highest point in the Black Hills and the highest elevation in North America east of the Rockies is visible in the distance.