The Rock That Is One Point Six Billion Years Old
Near Sioux Falls, South Dakota
Stand at the edge of the falls and look down at the riverbed. What you are looking at is not just rock. It is one point six billion years old, among the oldest exposed surface stone in the Northern Plains, formed long before the first fish swam in any ocean, long before there was anything alive on land.
The stone is Sioux quartzite. About one point seven billion years ago, a shallow continental sea covered this region. Wave after wave rolled across the floor of that sea for millions of years, rounding sand grains and laying them down in thin, precise layers. You can still read those layers in the stone, geologists call them laminations, and if you get close enough you can also see ripple marks, the frozen record of currents that moved through water before the Cambrian period had even begun. Then tectonic pressure buried the sandstone under miles of overlying rock, subjecting it to heat and weight that transformed loose sand into something harder than most concrete. That transformation is called metamorphism.
What came out of it was quartzite: silica-cemented quartz fused into a matrix so dense that water barely penetrates it. The pink and red tones come from iron oxide, essentially rust, locked inside the grains. Across this region it outcrops over roughly six thousand square miles, from the James River eastward into Minnesota and Iowa, all of it the same ancient sea floor.
The Big Sioux River has been cutting across this formation for roughly fourteen thousand years, since the last ice age and meltwater found its current channel. The river did not wear the quartzite down easily. What it could not dissolve or carry away, it stepped around and over, producing the cascade you are looking at now. About seven thousand four hundred gallons of water drop one hundred feet over the course of these falls every second.
The town that grew around those falls named itself after the people who had lived beside the river for centuries, and it built its first permanent buildings from the quartzite it found in the riverbed.
