The Obed: how flat rock makes a gorge

The Obed Wild and Scenic River protects 45 miles of free-flowing streams on
the Cumberland Plateau in Tennessee — 5,073 acres of gorge, whitewater and
still pools. It was authorised on 12 October 1976 for, in the designation's
words, outstandingly remarkable aesthetics, wildlife, fish, recreation,
culture, ecology, geology and water. The scenery is much as it was when
American Indians and then the first settlers walked its banks.

Flat rock, cut deep

The rock is Pennsylvanian, 323 to 300 million years old, and it lies
nearly flat. It is mostly sandstone and shale, with siltstone,
conglomerate and coal between them.

Flat-lying layers of hard and soft rock are what make this landscape. The
river cut down through them into narrow V-shaped gorges; the softer beds
weather back faster than the hard ones above, and what is left standing is an
array of forms — arches, mesas, chimneys, cracks and rock shelters. Boulders
the size of rooms have come off the cliff faces and lie in the valleys.

The three units you see most along the river:

The streams have a dendritic pattern — branching like a tree — which is
what water does on rock with no strong grain to follow.

Where it fits

The Obed is part of the
Appalachian Plateaus physiographic province,
and its history continues well beyond the park.

Sources

Written from the National Park Service's Geodiversity Atlas entry for the Obed
Wild and Scenic River, a work of the United States government in the public
domain, drawing on Anderson (2017).