Geo-pic of the Week: Potholes

This is a picture of a pothole.  Not the kind that you curse at as you run over it with your car.  This kind of pothole is an erosional feature common in creeks with rocky bottoms. They form by an elegantly simple process.

As a creek flows it carries sediment, such as sand and gravel, downstream.  If there’s a slight pit in the rocky channel bottom, a piece of gravel will sometimes get stuck in the pit.  As water flows over the pit it causes the trapped gravel to roll around.  The tumbling gravel erodes an ever larger pit, which is increasingly better at trapping gravel; the process snowballs.  Eventually, what was a slight pit becomes a smooth, bowl-shaped depression that may be up to several feet deep.

This pothole is in the creek in Duggan Hollow just north of Greer’s Ferry Lake in north Arkansas.

Geo-pic of the week: “Painted rocks”

Pictured above is a bluff of St. Peter Sandstone exhibiting some spectacular black staining.  The bluff is exposed near the confluence of Sylamore Creek and the White River north of Mountain View, Arkansas.  Bluffs with this staining are referred to as “painted” because it looks like paint has been poured over the face of the rock.

The stains, which are manganese oxide, were deposited by groundwater as it seeped from the sandstone.  The St. Peter Sandstone contains a minute amount of manganese that gets picked up by water as it flows through the rock.  When the groundwater flows out of the sandstone, some of it evaporates leaving the manganese behind.  Over time, a coating of manganese builds up on the bluff face.

The St. Peter Sandstone is also found along certain reaches of the Buffalo National River.  The “Painted Bluff” – as it is known locally to river folk – is another great  example of manganese staining.