Tag Archives: karst

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

Geopic of the week: Travertine falls

Travertine Fall over St. Peter ss, Searcy County

Pictured above is a travertine falls.  It looks like a waterfall except that, rather than being water, it’s composed of solid rock.

Travertine is made of calcite which also forms stalactites and stalagmites.  Like those familiar cave features, travertine falls form by precipitation from water; the water is flowing in a creek, over a ledge instead of dripping from a cave ceiling.  As the travertine precipitates in layer upon layer, it begins to take on the appearance of flowing rock.

Dripstone features like these only form in areas where the groundwater carries a high load of dissolved carbonate minerals.  This one was photographed in Searcy County, Arkansas, not far from the Buffalo National River, near the contact between the St. Peter and Plattin Formations.

For another view of this travertine falls click here

Geopic of the week: Disappearing stream, Izard County, Arkansas

Disappearling stream

The above picture shows what can happen when a sinkhole forms in the channel of a creek.  This photo is taken facing in what use to be the upstream direction.  If you look carefully, you can tell that the water is actually flowing into the fracture; the sinkhole has captured the flow of the creek and caused a strech of it to flow backwards. 

Sinkholes form in a variety of ways, but this one evidently was the result of sudden collapse of bedrock into a subterranean cave that was forming below the land surface.  You can’t see the sinkhole itself because it’s just inside of the dark opening in the rock.

If the photographer were to turn around and snap a picture in the downstream direction, you would see that the stream bed is dry just downstream.  In geologic terms this is called a disappearing stream.