Carpet Rock: How Did Carpet Rock Form?

Carpet Rock

The carpet rock formations are rare structures. Some people are convinced that aliens are the culprit behind the weird formations but there's a geological explanation as well. Fractured sandstone filled with quartz and slowly eroded away leaving the carpet-like pattern.

Those structures typically develop in siliceous coarse-grained sedimentary (sandstone) rocks. The formation of this kind of rock is thought to begin with the fracturing of sandstone and the filling of the resulting cracks with quartz cement. Subsequent weathering erodes the relatively soft sandstone more than the quartz. Eventually the quartz stands out from the sandstone in a carpet-like pattern.

Carpet Rock at Chattanooga, Tennessee. Photo: Beth Kendall on Facebook.

These patterns are abundant in sandstone from this area and are formed when Iron minerals such as Hematite or Pyrite in the sandstone oxidize because water has permeated the sandstone dissolving the iron minerals into a solution and subsequently erosion has exposed the iron mineral solution to oxygen in the atmosphere.

The oxidized solution precipitates between the layers of sandstone, finding tiny crevices where joints exist and form the different colour bands within the rock giving the patterns, often in polygonal shapes, which lead to the name "Carpet Rock”.

Carpet Rock at Chattanooga, Tennessee. Photo: Beth Kendall on Facebook.

This effect occurs when Quartz forms harder zones in sandstone which resist erosion such as the sandstone from Petit Jean Set Park in Conway County, Arkansas USA.
  

The Carpet Rock in Petit Jean State Park, Conway County, Arkansas Photo: Jonathan Ball

Boxwork

Boxwork is defined as a honeycomb-like structure that can form in some fractured or jointed sedimentary rocks. If the fractures in the host rock are mineralized, they can become more resistant to weathering than the surrounding rock, and subsequent erosion can produce boxwork structures. 
 

Boxwork on a cave’s ceiling at Wind Cave National Park in South Dakota Photo: YellowstonePark.com

In cave geology, Boxwork is commonly composed of thin blades of the mineral calcite that project from cave walls or ceilings that intersect one another at various angles, forming a box-like or honeycomb pattern. The boxwork fins once filled cracks in the rock before the host cave formed. As the walls of the cave began to dissolve away, the more resistant vein and crack fillings did not, or at least dissolved at a slower rate than the surrounding rock, leaving the calcite fins projecting from the cave surfaces.

Box-shaped and triangular patterns are abundant in the sandstones on top of Petit Jean Mountain. These patterns form when iron present in the rock is oxidized. Iron exists as the minerals siderite, magnetite, hematite and some clay minerals that are present in the Hartshorne Sandstone. At some point in geologic history water filled the pore spaces of the rock formation and came into contact with minerals made up of iron. This caused the iron to go into solution. If the rock becomes exposed to air then oxygen is added to the solution and causes the iron to oxidize and precipitate out along exposed joints in the rock formation.

Sometimes color bands result from the different oxidation states of iron. These bands are also referred to as Liesegang banding or box work by the scientific community.

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