At Falls Park in Sioux Falls, SD, this beautiful example of fossilized sand ripples can be seen on a massive slab of pink quartzite along the Big Sioux River, just north of the Falls.
The pink quartzite here is among the oldest rock in North America. It can be seen as outcrops around SE South Dakota, SW Minnesota and NW Iowa.
It was an extremely durable and plentiful material that supplied much of the matrix for the earliest fine homes and public buildings in Sioux Falls. It eventually became a popular source of street pavers from South Dakota to Illinois and Iowa.