Ancient Quartzite Boulders in the Red Valley Area of the Northern Black Hills, South Dakota and Wyoming

Large orthoquartzite boulders mantle parts of the Red valley, the lowland underlain by the Spearfish Formation which separates the Black Hills from the Cretaceous hogback. The boulders are concentrated near the top of small hills where they retard the erosion of the underlying shale. The boulders are believed to represent relict positions of outcrops of the Lakota Formation, and to have been let down as much as 300m in elevation during the later part of the Cenozoic era.