Beecher Rock shows up as a silhouette on the skyline in this image. An intrusion of pegmatite into Precambrian schist is west of the road. 1.4 Drive north on Highway 89. Directly to Needles turnaround within Custer State Park. 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.