Roundness is the degree of smoothing due to abrasion

Abrasion (geology)

Abrasion is the mechanical scraping of a rock surface by friction between rocks and moving particles during their transport by wind, glacier, waves, gravity, running water or erosion. After friction, the moving particles dislodge loose and weak debris from the side of the rock...

 of sedimentary particles. It is expressed as the radius of the average radius of curvature of the edges or corners to the radius of curvature of the maximum inscribed sphere.

Measure of roundness

Rounding, roundness or angularity are terms used to describe the shape of the corners on a particle (or clast) of sediment

Sediment

Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....

. Such a particle may be a grain of sand

Sand

Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...

, a pebble

Pebble

A pebble is a clast of rock with a particle size of 4 to 64 millimetres based on the Krumbein phi scale of sedimentology. Pebbles are generally considered to be larger than granules and smaller than cobbles . A rock made predominantly of pebbles is termed a conglomerate...

, cobble

Cobble (geology)

A cobble is a clast of rock with a particle size of to based on the Krumbein phi scale of sedimentology. Cobbles are generally considered to be larger than pebbles and smaller than boulders . A rock made predominantly of cobbles is termed a conglomerate....

 or boulder

Boulder

In geology, a boulder is a rock with grain size of usually no less than 256 mm diameter. While a boulder may be small enough to move or roll manually, others are extremely massive....

. Although roundness can be numerically quantified, for practical reasons geologist

Geologist

A geologist is a scientist who studies the solid and liquid matter that constitutes the Earth as well as the processes and history that has shaped it. Geologists usually engage in studying geology. Geologists, studying more of an applied science than a theoretical one, must approach Geology using...

s typically use a simple visual chart with up to six categories of roundness:

• Very angular: corners sharp and jagged
• Angular
• Sub-angular
• Sub-rounded
• Rounded
• Well-rounded: corners completely rounded

Rounding of sediment particles can indicate the distance and time involved in the transportation of the sediment from the source area to where it is deposited.

Speed of rounding will depend on composition, hardness and mineral cleavage

Cleavage (crystal)

Cleavage, in mineralogy, is the tendency of crystalline materials to split along definite crystallographic structural planes. These planes of relative weakness are a result of the regular locations of atoms and ions in the crystal, which create smooth repeating surfaces that are visible both in the...

. For example, a soft claystone

Claystone

Claystone is a geological term used to describe a clastic sedimentary rock that is composed primarily of clay-sized particles ....

 pebble will obviously round much faster, and over a shorter distance of transport, than a more resistant quartz

Quartz

Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...

 pebble. The rate of rounding is also affected by the grain size and energy conditions.

Abrasion

Abrasion occurs in natural environments such as beach

Beach

A beach is a geological landform along the shoreline of an ocean, sea, lake or river. It usually consists of loose particles which are often composed of rock, such as sand, gravel, shingle, pebbles or cobblestones...

es, sand dunes, river

River

A river is a natural watercourse, usually freshwater, flowing towards an ocean, a lake, a sea, or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. Small rivers may also be called by several other names, including...

 or stream beds by the action of current flow, wave impact, glacial action, wind, gravitational creep

Downhill creep

Downhill creep, or commonly just creep, is the slow downward progression of rock and soil down a low grade slope; it can also refer to slow deformation of such materials as a result of prolonged pressure and stress. Creep may appear to an observer to be continuous, but it really is the sum of...

 and other erosive agents

Erosion

Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...

.

Paleogeographic value of determining the degree of roundness of clastic material

Roundness is an important indicator of the genetic affiliation of a clastic rock. The degree of roundness points to the range and mode of transport of clastic material, and can also serve as a search criterion in mineral exploration, especially for placer

Placer

Placer may refer to one of the following:*Placer deposit*Placer sheep*Placer mining*Placer, rugby league football role.Geographical names:* Placer, Masbate, Philippines* Placer, Surigao del Norte, Philippines...

 deposits.

Alluvial debris

Alluvium

Alluvium is loose, unconsolidated soil or sediments, eroded, deposited, and reshaped by water in some form in a non-marine setting. Alluvium is typically made up of a variety of materials, including fine particles of silt and clay and larger particles of sand and gravel...

 in major rivers tend to exhibit a high degree of roundness. Alluvium from small rivers is less rounded. Deposits of ephemeral streams exhibit little rounding with angular clasts.

Clast rounding in non-sedimentary environments

Pebble dikes are dikelike bodies found in intrusive environments which contain variably rounded fragments. The clasts originate in deeper formations and have been brought up by diatreme

Diatreme

A diatreme is a breccia-filled volcanic pipe that was formed by a gaseous explosion. Diatremes often breach the surface and produce a tuff cone, a filled relatively shallow crater known as a maar, or other volcanic pipes.- Word origin :...

 or intrusive breccia

Breccia

Breccia is a rock composed of broken fragments of minerals or rock cemented together by a fine-grained matrix, that can be either similar to or different from the composition of the fragments....

s. The fragments have been rounded due to either milling action or corrosion by hydrothermal fluids. The ore deposits of Tintic mining district, Utah

Utah

Utah is a state in the Western United States. It was the 45th state to join the Union, on January 4, 1896. Approximately 80% of Utah's 2,763,885 people live along the Wasatch Front, centering on Salt Lake City. This leaves vast expanses of the state nearly uninhabited, making the population the...

; Ouray, Colorado

Ouray, Colorado

The historic City of Ouray is a Home Rule Municipality that is the county seat and the most populous city of Ouray County, Colorado, United States. The city population was 813 at the U.S. Census 2000 and 1,000 as of the U.S. Census 2010...

; Silver Bell; and Bisbee

Bisbee

Bisbee may refer to:* Bisbee, Arizona* Bisbee, North Dakota* Bisbee Blue, turquoise from Bisbee, Arizona.* Bisbee Deportation, the illegal expulsion of 1,300 miners from Bisbee, Arizona...

, Arizona

Arizona

Arizona ; is a state located in the southwestern region of the United States. It is also part of the western United States and the mountain west. The capital and largest city is Phoenix...

 and the Kiruna

Kiruna

Kiruna is the northernmost city in Sweden, situated in Lapland province, with 18,154 inhabitants in 2005. It is the seat of Kiruna Municipality Kiruna (Northern Sami: Giron, Finnish: Kiiruna) is the northernmost city in Sweden, situated in Lapland province, with 18,154 inhabitants in 2005. It is...

 iron deposit in Sweden

Sweden

Sweden , officially the Kingdom of Sweden , is a Nordic country on the Scandinavian Peninsula in Northern Europe. Sweden borders with Norway and Finland and is connected to Denmark by a bridge-tunnel across the Öresund....

contain these pebble dikes.