Monthly Archives: February 2016

Geo-pic of the Week: Potholes

Pothole

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”

manganese staining

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.

Geo-pic of the week: Olistoliths

olistolith 3

Pictured above is what geologists refer to as an olistolith.  The name comes from the Greek olistomai – to slide, and lithos – rock.  Olitstoliths are basically the geologic record of an ancient landslide.

The mass of rock at the base of the outcrop broke free from where it formed and slid downhill to this location.  After sliding (or tumbling) to its new location, sediment accumulated around the olistolith.  Eventually, it and the sediment became a new rock .  That’s what we’re looking at in this picture.

This olistolith is located on the north side of highway 412, west of Springdale, Arkansas.  It was deposited about 350 million years ago when that area was a gently sloping ocean shelf. 

Geopic of the week: Waterfalls

petit jean 005edited

Everyone knows that a waterfall is a place where a river or creek flows over a vertical drop-off, but did you know that there is a geologic reason why they form?  A waterfall, like Cedar Falls pictured above, forms where a hard, resistant rock such as sandstone overlies a soft, easily eroded rock like shale.  The difference in the rate each rock type weathers is what creates the waterfall.

When a stream passes over a single rock type, it erodes it evenly, carving a channel with a gradual slope.  However, when a stream’s course passes from a hard to a soft bedrock, it scours the soft rock at a faster rate.  As the supporting soft rock is eroded, the overlying harder rock progressively collapses, creating a vertical bluff over which the stream flows.  As this process continues an ever taller waterfall develops, and the location of the waterfall gradually migrates upstream.

Because we know how landforms such as waterfalls form, geologists can use tools, like aerial photographs and satellite images, to predict what kind of rock will be in an area before ever going there.