The first time I visited the famous Cedar Falls at Petit Jean State Park, my jaw dropped … right there in front of everyone was a great big glaring Unconformity on full-display – and it’s been going on for eons!
But this time it isn’t Hippies or the latest thing your kids have come up with to get your goat – it’s a Geologic Unconformity, and usually represents a missing gap of time in the geologic record. Can you see it?
The Sandstones of this area were originally mud and silt on the bottom of a warm, shallow sea – and eventually hardened into flat, even layers. At sometime in the past either the seas dried up or the land was thrust above sea-level long enough for this process to stop – and erosion to begin instead. So rather than being laid-down in even “plates”, wind and water started carving and shaping the exposed sandstone long enough to create an uneven surface across it…
… then either sea-levels rose or the land sank and the process of sandstone building up in layers started again – filling in the uneven surface of the older, eroded sandstone as it did.
This older, lower-level of sandstone is called the Atoka Formation, and it is said to lie in “uncomfortable contact” with the upper (and newer) Desmoinesian Formation. That uneven line represents not only the erosion of the older surface – but also an uncertain period of time when rocks were exposed and the sandstone-making process stopped.
I’m calling this an “Unconformity” in the general sense – specifically, it could be a Disconformity … you’ll have to get a real geologist to tell you which. I just know enough to spot one of the things. See the photo below for my illustration of the divide between the two formations.
So how old is the Atoka? Since it’s believed to have formed in the early Phanerozoic – maybe 545 million years ago? I’ll note that most of the rocks you see at Petit Jean are somewhat more recent than that…