Monthly Archives: August 2016

Geo-pic of the week: Castles

Castlerock

According to the American Geosciences institute, a castle, in the geologic sense, is a natural rock formation bearing a fancied resemblance to a castle – sophisticated science, I know!  The limestone boulder pictured above, which is from north central Arkansas, is one such castle.   Rocks like this one owe their appearance to their solubility in weak acid.

Most rain water is actually slightly acidic, due to the CO2  it absorbs from the atmosphere and soil it passes through.  Over time, this acidic water is capable of dissolving limestone bedrock into features such as caves, sinkholes, and, in this case, castles.  The boulder pictured here has been flipped over by the creek’s current; they typically form with the castle side down.

below is an example of a castle that is still forming.  The base of the rock dissolves faster than the upper part, because it is under the water more often.  This differential weathering is what gives the boulder its characteristic castle shape.

 

Castle still forming

Photos by Richard Hutto

Geo-pic of the week: Loess Doll

loessdollCrowleysridgeedited

Pictured above is a Loess doll, collected from Crowley’s Ridge in the Mississippi River Valley, eastern Arkansas.  A loess doll is a type of concretion, or chemically precipitated mineral mass.  They grow in deposits of wind-blown sediment, known as loess, by a process of leaching and concentration of minerals within a soil.  They typically form by nucleating around a particle in the loess, such as a fragment of a fossil.

They were given the name loess doll due to the whimsical shapes these concretions take, which sometimes resemble people or animals; the one above looks like a lamb to me.  Aside from their charm, they are important to geologists because they’re associated with buried soils, or fossilized soils, which are useful markers for correlating land surfaces across large areas.

 

To see more loess dolls click here

Geo-pic of the week: Key beds, and strike and dip

Kirby quarry edited

 

In this photo we are looking at rock beds, tilted till they are nearly vertical, and exposed in three levels in a quarry near Kirby, Arkansas, Ouachita Mountains.  Like a humongous 3-step staircase, each ascending level of the outcrop provides a deeper view into the rock formation.  An outcrop like this one illuminates a couple of basic but important concepts of geology: key beds, and strike and dip.

The key beds or beds that can be traced across the outcrop, such as the one marked with red dots above, appear to shift to the right as your eyes ascend the steps.  These are not faults!  It’s an optical illusion.  If our view were aligned parallel with the sides of the beds, they would appear aligned, but our view is actually diagonal to the bedding.  To illustrate this, here is the same picture with a drawing of the key bed as if it were jutting out of the outcrop. 

 

Kirby quarry edited3

 

This is why geologists measure the orientation of rock beds – known as the bed’s strike and dip.  Knowing how a bed is oriented in one place can help you to predict where it will be in another, perhaps inaccessible, place such as deep in the subsurface.  If that bed is full of oil, gas, or other precious commodity, predicting where it is becomes very important.