A pebble is a clast of rock with a

particle size Particle size is a notion introduced for comparing dimensions of solid particles ('' flecks''), liquid particles ('' droplets''), or gaseous particles ('' bubbles''). The notion of particle size applies to particles in colloids, in ecology, in ...

of based on the Udden-Wentworth scale of

sedimentology Sedimentology encompasses the study of modern sediments such as sand, silt, and clay, and the processes that result in their formation (erosion and weathering), transport, deposition and diagenesis. Sedimentologists apply their understanding of m ...

. Pebbles are generally considered larger than granules ( in diameter) and smaller than cobbles ( in diameter). A rock made predominantly of pebbles is termed a conglomerate. Pebble tools are among the earliest known man-made artifacts, dating from the

Palaeolithic The Paleolithic or Palaeolithic ( years ago) ( ), also called the Old Stone Age (), is a period in human prehistory that is distinguished by the original development of stone tools, and which represents almost the entire period of human prehist ...

period of human history. A beach composed chiefly of surface pebbles is commonly termed a

shingle beach A shingle beach, also known as either a cobble beach or gravel beach, is a commonly narrow beach that is composed of coarse, loose, well-rounded, and waterworn gravel, called ''shingle''. The gravel (shingle) typically consists of smooth, sphero ...

. This type of beach has armoring characteristics with respect to wave

erosion Erosion is the action of surface processes (such as Surface runoff, water flow or wind) that removes soil, Rock (geology), rock, or dissolved material from one location on the Earth's crust#Crust, Earth's crust and then sediment transport, tran ...

, as well as ecological niches that provide

habitat In ecology, habitat refers to the array of resources, biotic factors that are present in an area, such as to support the survival and reproduction of a particular species. A species' habitat can be seen as the physical manifestation of its ...

for animals and plants. Inshore banks of shingle (large quantities of pebbles) exist in some locations, such as the entrance to the River Ore, England, where the moving banks of shingle give notable navigational challenges. Pebbles come in various colors and textures and can have streaks, known as

vein Veins () are blood vessels in the circulatory system of humans and most other animals that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are those of the pulmonary and feta ...

s, of

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...

or other

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid substance with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2011): Mi ...

s. Pebbles are mostly smooth but, dependent on how frequently they come in contact with the sea, they can have marks of contact with other rocks or other pebbles. Pebbles left above the high water mark may have growths of organisms such as

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 ...

on them, signifying the lack of contact with

seawater Seawater, or sea water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has approximat ...

Location

Pebbles on Earth exist in two types of locations – on the beaches of various oceans and seas, and inland where ancient seas used to cover the land. Then, when the seas retreated, the rocks became landlocked. Here, they entered lakes and ponds, and form in

river A river is a natural stream of fresh water that flows on land or inside Subterranean river, caves towards another body of water at a lower elevation, such as an ocean, lake, or another river. A river may run dry before reaching the end of ...

s, travelling into

estuaries An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environm ...

where the smoothing continues in the sea. Beach pebbles and river pebbles (also known as river rock) are distinct in their geological formation and appearance. Manufactured Pebbles-These are made from natural stones such as marble, granite, and sandstone. Manufactured Pebbles are designed to specified sizes and forms, making them flexible for many purposes.

Beach

Beach pebbles form gradually over time as the ocean water washes over loose rock particles. The result is a smooth, rounded appearance. The typical size range is from 2 mm to 50 mm. The colors range from translucent white to black, and include shades of yellow, brown, red and green. Some of the more plentiful pebble beaches are along the coast of the

Pacific Ocean The Pacific Ocean is the largest and deepest of Earth's five Borders of the oceans, oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean, or, depending on the definition, to Antarctica in the south, and is ...

, beginning in Canada and extending down to the tip of South America in Argentina. Other pebble beaches are in northern Europe (particularly on the beaches of the Norwegian Sea), along the coast of the U.K. and Ireland, on the shores of Australia, and around the islands of Indonesia and Japan.

Inland

Inland pebbles (river pebbles of river rock) are usually found along the shores of large rivers and lakes. These pebbles form as the flowing water washes over rock particles on the bottom and along the shores of the

. The smoothness and color of river pebbles depends on several factors, such as the composition of the

soil Soil, also commonly referred to as earth, is a mixture of organic matter, minerals, gases, water, and organisms that together support the life of plants and soil organisms. Some scientific definitions distinguish dirt from ''soil'' by re ...

of the river banks, the chemical characteristics of the water, and the speed of the current. Because river current is gentler than the

ocean waves In fluid dynamics, a wind wave, or wind-generated water wave, is a surface wave that occurs on the free surface of Body of water, bodies of water as a result of the wind blowing over the water's surface. The contact distance in the wind directi ...

, river pebbles are usually not as smooth as beach pebbles. The most common colors of river rock are black, grey, green, brown and white.

Human use

Beach pebbles and river pebbles are used for a variety of purposes, both outdoors and indoors. They can be sorted by colour and size, and they can also be polished to improve the texture and colour. Outdoors, beach pebbles are often used for

landscaping Landscaping refers to any activity that modifies the visible features of an area of land, including the following: # Living elements, such as flora or fauna; or what is commonly called gardening, the art and craft of growing plants with a goal ...

construction Construction are processes involved in delivering buildings, infrastructure, industrial facilities, and associated activities through to the end of their life. It typically starts with planning, financing, and design that continues until the a ...

and as decorative elements. Beach pebbles are often used to cover walkways and driveways, around pools, in and around plant containers, on patios and decks. Beach and river pebbles are also used to create water-smart gardens in areas where water is scarce. Small pebbles are also used to create living spaces and gardens on the rooftops of buildings. Indoors, pebbles can be used as bookends and

paperweight A paperweight is a small solid object heavy enough, when placed on top of papers, to keep them from blowing away in a breeze or from moving under the strokes of a painting brush (as with Chinese calligraphy). While any object, such as a stone, ...

s. Large pebbles are also used to create " pet rocks" for children.

Mars

Mars Mars is the fourth planet from the Sun. It is also known as the "Red Planet", because of its orange-red appearance. Mars is a desert-like rocky planet with a tenuous carbon dioxide () atmosphere. At the average surface level the atmosph ...

, slabs of pebbly conglomerate rock have been found and have been interpreted by scientists as having formed in an ancient streambed. The gravels, which were discovered by

NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...

's Mars rover

Curiosity Curiosity (from Latin , from "careful, diligent, curious", akin to "care") is a quality related to inquisitive thinking, such as exploration, investigation, and learning, evident in humans and other animals. Curiosity helps Developmental psyc ...

, range from the size of sand particles to the size of golf balls. Analysis has shown that the pebbles were deposited by a stream that flowed at walking pace and was ankle- to hip-deep.

Gallery

File:Wave Retreating from Pebbles.jpg , Pebbles given a rounded shape by wave action File:Halite Dead Sea Beach Pebbles.JPG, Beach pebbles made of

halite Halite ( ), commonly known as rock salt, is a type of salt, the mineral (natural) form of sodium chloride ( Na Cl). Halite forms isometric crystals. The mineral is typically colorless or white, but may also be light blue, dark blue, purple, pi ...

; western

Dead Sea The Dead Sea (; or ; ), also known by #Names, other names, is a landlocked salt lake bordered by Jordan to the east, the Israeli-occupied West Bank to the west and Israel to the southwest. It lies in the endorheic basin of the Jordan Rift Valle ...

coast,

Israel Israel, officially the State of Israel, is a country in West Asia. It Borders of Israel, shares borders with Lebanon to the north, Syria to the north-east, Jordan to the east, Egypt to the south-west, and the Mediterranean Sea to the west. Isr ...

File:Pebbles on beach at Broulee -NSW -Australia-2Jan2009.jpg, Pebbles on a beach at Broulee, Australia

References

External links

{{Authority control Stone (material) Sedimentology Granularity of materials Building stone Natural materials