Carpet Rock: Types, and How Did Carpet Rock Form

Carpet rock is a rare and unique type of sandstone formation that is characterized by its intricate geometric patterns. These patterns are created by the differential weathering of different layers of sandstone, which have been cemented together by quartz. Carpet rock is typically found in arid regions, such as the southwestern United States and Australia.

Carpet Rock

Carpet Rock is formed when sandstone is weathered and eroded, leaving behind the harder quartz minerals. The quartz minerals then form a hard crust on the surface of the rock, which protects the underlying softer sandstone from further erosion. Over time, the softer sandstone erodes away, leaving behind the hard quartz crust with its distinctive polygonal patterns.

Carpet Rock is often found in areas that have a long history of arid conditions. This is because arid conditions promote the weathering and erosion of sandstone. Carpet Rock is also often found in areas that have been subjected to tectonic uplift. This is because tectonic uplift can expose sandstone that has been buried deep underground, where it has been protected from erosion.

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 a type of rock weathering that results in the formation of a honeycomb pattern of interlocking ridges and grooves. It is typically found in sandstone and limestone formations. Boxwork is formed when the rock is exposed to acidic groundwater, which dissolves the rock along certain planes. The resulting pattern is similar to the boxwork of a beehive.

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.

Turtle Rocks

Turtle Rocks at Petit Jean State Park
Photo: Courtney Van Stolk

The exact processes that create “turtle rocks” are poorly understood. One explanation involves spheroidal weathering. This process occurs when water percolating through cracks and between individual grains in the rock loosens and separates layers of the rock. The weathering acts more rapidly on the corners and edges of the rock producing a rounded shape. Another theory concerns the amount of calcite present in the matrix of the rock holding the grains together along with the size of the grains that allow for this type of weathering. Either way the weathering of the rocks is strongly influenced by the polygonal joint pattern seen in all “turtle rocks”.