SIOUX FALLS, SOUTH DAKOTA
Debris from the Penokee Mountains
1.8 billion years ago
The Penokee Mountains once ran at least from New York to South Dakota and, perhaps, as far as Arizona. Weathering and erosion broke down these mountains. Of the original minerals, mostly highly durable quartz was left, which, as grains of sand, was moved by rivers and spread across a broad costal plain. The grains became tarnished with a film of iron oxide that colored the normally white quartz to various shades of pink and red. The grains accumulated into thick deposits, which were compressed by shallow burial, then exhumed, exposed today as a hard quartz-rich rock known as the Sioux Quartzite. It is this hard rock that forms the series of waterfalls at Sioux Falls in South Dakota.