A panhole is a depression or basin eroded into flat or gently sloping cohesive rock.Twidale, C.R., and Bourne, J.A., 2018
''Rock basins (gnammas) revisited. ''
Géomorphologie: Relief, Processus, Environnement, Articles sous presse, Varia, mis en ligne le 08 janvier 2018, consulté le 01 juillet 2018. Similar terms for this feature are gnamma or rock holes (Australia), armchair hollows, weathering pans (or pits) and solution pans (or pits).Hughes, Kebbi A., 2012
''Bacterial Communities and their Influence on the Formation and Development of Potholes in Sandstone Surfaces of the Semi-Arid Colorado Plateau''
University of Western Ontario - Electronic Thesis and Dissertation Repository. Paper 543. Some authors refer to panholes also as

pothole A pothole is a pot-shaped depression in a road surface, usually asphalt pavement, where traffic has removed broken pieces of the pavement. It is usually the result of water in the underlying soil structure and traffic passing over the affecte ...

s, which is a term typically used for similarly shaped riverine landforms. In fluvial

geomorphology Geomorphology () is the scientific study of the origin and evolution of topographic and bathymetric features generated by physical, chemical or biological processes operating at or near Earth's surface. Geomorphologists seek to understand wh ...

, the term ''pothole'' is typically used for a smooth, bowl-shaped or cylindrical hollow, generally deeper than wide, found developed in the rocky bed of a stream. This type of feature is created by the grinding action either of a stone or stones or of coarse sediment whirled around and kept in motion by eddies or the force of the stream current in a given spot.Neuendorf, K.K.E., J.P. Mehl, Jr., and J.A. Jackson, eds., 2011. ''Glossary of Geology'' (5th ed.). Alexandria, Virginia, American Geological Institute. 800 pp.

Description

Panholes are erosional or destructional features that are developed in a variety of climatic environments and in a wide range of rock types. These shallow basins, or closed depressions, are quite commonly well developed in surfaces of

granitic A granitoid is a broad term referring to a diverse group of coarse-grained igneous rocks that are widely distributed across the globe, covering a significant portion of the Earth's exposed surface and constituting a large part of the continental ...

rocks and

sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...

. They are generally characterized by flat bottoms and sometimes by overhanging sides. The initial form may be a closed hollow created by a patch of

humus In classical soil science, humus is the dark organic matter in soil that is formed by the decomposition of plant and animal matter. It is a kind of soil organic matter. It is rich in nutrients and retains moisture in the soil. Humus is the Lati ...

. Diameters are rarely greater than .Paradise, T. R., 2013
Tafoni and Other Rock Basins
in ''Treatise on Geomorphology'', V.4, 111-126 Some panholes were at one time thought to be man-made because their roundness was so perfect they were argued not be natural and must have been shaped by humans. Panholes are most commonly found in desert environments such as the

Colorado Plateau The Colorado Plateau is a physiographic and desert region of the Intermontane Plateaus, roughly centered on the Four Corners region of the Southwestern United States. This plateau covers an area of 336,700 km2 (130,000 mi2) within w ...

. A few well-known panholes are found developed in sandstone surfaces in

Canyonlands National Park Canyonlands National Park is a national park of the United States located in southeastern Utah near the town of Moab. The park preserves a colorful landscape eroded into numerous canyons, mesas, and buttes by the Colorado River, the Green Rive ...

Capitol Reef National Park Capitol Reef National Park is a List of national parks of the United States, national park of the United States in south-central Utah. The park is approximately long on its northsouth axis and just wide on average. The park was established in ...

, and Moab. Panholes are capable of collecting water when it rains, freezing over when the weather gets cold, dry out in hotter weather, and can even contain some species of bacteria,

lichen A lichen ( , ) is a hybrid colony (biology), colony of algae or cyanobacteria living symbiotically among hypha, filaments of multiple fungus species, along with yeasts and bacteria embedded in the cortex or "skin", in a mutualism (biology), m ...

s, mosses, and

blue-green algae Cyanobacteria ( ) are a group of autotrophic gram-negative bacteria that can obtain biological energy via oxygenic photosynthesis. The name "cyanobacteria" () refers to their bluish green (cyan) color, which forms the basis of cyanobacteria' ...

. Panholes range in size from a few centimeters to many meters in diameter. The cavities can be shallow or more than deep, containing hundreds of liters of water. The

Australian Aboriginal Aboriginal Australians are the various indigenous peoples of the Mainland Australia, Australian mainland and many of its islands, excluding the ethnically distinct people of the Torres Strait Islands. Humans first migrated to Australia (co ...

term ''gnamma'', in particular, implies a depression capable of holding water in arid areas, forming an important water resource for Aboriginal people that needed to be carefully maintained. Within the panholes is a varied eco-system that contains bacteria such as cyanobacteria, fungi, and algae which can be referred to as

biofilm A biofilm is a Syntrophy, syntrophic Microbial consortium, community of microorganisms in which cell (biology), cells cell adhesion, stick to each other and often also to a surface. These adherent cells become embedded within a slimy ext ...

. Panholes do not contain predators like fish or aquatic insects.Davis, Jim, 2013
"What are “Potholes” and how are organisms able to live in them?."
Utah Geological Survey. N.p., n.d. Web. 9 Oct 2013. The biofilm breaks down some of the siliceous minerals in the panhole for nutrients resulting in furthering the weathering the panhole. The organisms that live in the panholes have to tolerate rapid change in water temperature, pH, oxygen, carbon dioxide concentration, and ion concentration.

Origin

In the Sierra Nevada, California these features were termed weathering pits by

François E. Matthes François Émile Matthes ( – ) was a geologist and an expert in topographic mapping, glaciers, and climate change. He mapped remote areas of the American West for the United States Geological Survey (USGS). His maps coincided with the devel ...

, where they are thought to indicate rock surfaces that are unglaciated or escaped more recent glaciations. In Sierra Nevada granitic rocks, these features have a characteristic shape such that they expand more rapidly in width than they grow in depth. One explanation for their conformation is because the most active environment for weathering is the zone of alternate wetting and drying along the margins of the pools that collect in the pits, the margins tend to deepen and enlarge until all points of the bottom are equally wet or dry at the same time, thus producing their characteristic shape. Panholes can expand from weathering and erosion but the main activity of how panholes expand is from biological weathering. At one time the belief was that the only agents involved with the expansion of panholes were physical weathering.

Terminology

Australia

In Australia, the terms "gnamma" and "rock hole" (or "rockhole") are used. Gnamma is an anglicization of a

Nyoongar language Noongar (), also Nyungar (), is an Australian Aboriginal language or dialect continuum, spoken by some members of the Noongar community and others. It is taught actively in Australia, including at schools, universities and through public broa ...

word, used by that Aboriginal people of

Western Australia Western Australia (WA) is the westernmost state of Australia. It is bounded by the Indian Ocean to the north and west, the Southern Ocean to the south, the Northern Territory to the north-east, and South Australia to the south-east. Western Aust ...

to describe a naturally formed rock hole as well as its retained rainwater. The term "gnamma hole" is also widely used, but is incorrect, being a tautology.

Locations

Australia

Hiltaba Nature Reserve Hiltaba Nature Reserve is located in the north of the Eyre Peninsula on the western edge of the Gawler Ranges, South Australia. It is situated on a former pastoral lease known as Hiltaba, or Hiltaba Station, that had operated as a sheep statio ...

United States

*Beam Rocks,

Forbes State Forest Forbes State Forest is a List of Pennsylvania state forests, Pennsylvania state forest in Pennsylvania Bureau of Forestry District #4. The main offices are located in Ligonier Township, Pennsylvania, Laughlintown in Westmoreland County, Penns ...

, Pennsylvania *

, Utah *

Glacier Point Glacier Point is a viewpoint above Yosemite Valley in the U.S. state of California. It is located on the south wall of Yosemite Valley at an elevation of , above Curry Village. The point offers a superb view of several of Yosemite National Par ...

, Yosemite National Park, California *

Navajo National Monument Navajo National Monument is a national monument located within the northwest portion of the Navajo Nation territory in northern Arizona, which was established to preserve three well-preserved cliff dwellings of the Ancestral Puebloan people: Kee ...

, Arizona *

Shenandoah National Park Shenandoah National Park (often ) is a List of national parks of the United States, national park of the United States that encompasses part of the Blue Ridge Mountains in Virginia. The park is long and narrow, with the Shenandoah River and its ...

, Virginia *

Stone Mountain Stone Mountain is a quartz monzonite dome Inselberg, monadnock and the site of Stone Mountain Park, east of Atlanta, Georgia. Outside the park is the city of Stone Mountain, Georgia. The park is the most visited tourist site in the state of Ge ...

, Georgia *

Stone Mountain (North Carolina) Stone Mountain is the centerpiece of Stone Mountain State Park in North Carolina. It is a dome of exposed granite (specifically a quartz diorite to granodiorite) of Devonian The Devonian ( ) is a period (geology), geologic period and sy ...

, North Carolina

References

{{reflist Landforms Depressions (geology)