Cobble (geology)
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A cobble (sometimes a cobblestone) is a
clast Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus,Essentials of Geology, 3rd Ed, Stephen Marshak, p. G-3 chunks, and smaller grains of rock broken off other rocks ...
of rock defined on the
Udden–Wentworth scale Grain size (or particle size) is the diameter of individual grains of sediment, or the lithification, lithified particles in clastic rocks. The term may also be applied to other granular materials. This is different from the crystallite si ...
as having 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 , larger than a
pebble A pebble is a clast of rock with a particle size of based on the Udden-Wentworth scale of sedimentology. Pebbles are generally considered larger than granules ( in diameter) and smaller than cobbles ( in diameter). A rock made predominant ...
and smaller than a
boulder In geology, a boulder (or rarely bowlder) is a rock fragment with size greater than in diameter. Smaller pieces are called cobbles and pebbles. While a boulder may be small enough to move or roll manually, others are extremely massive. In c ...
. Other scales define a cobble's size differently. A rock made predominantly of cobbles is termed a conglomerate.
Cobblestone Cobblestone is a natural building material based on cobble-sized stones, and is used for pavement roads, streets, and buildings. Setts, also called Belgian blocks, are often casually referred to as "cobbles", although a sett is distinct fro ...
is a building material based on cobbles.


Etymology

Cobbles, also called
cobblestones Cobblestone is a natural building material based on cobble-sized stones, and is used for pavement roads, streets, and buildings. Setts, also called Belgian blocks, are often casually referred to as "cobbles", although a sett is distinct fr ...
, derive their name from the word cob, meaning a rounded lump. The term is further related to the German ', meaning ''head''. Chester Wentworth referred to cobbles as ''cobble bowlders'' in his 1922 paper that would become the basis for the Udden–Wentworth scale.


Classifications

Within the widely used Krumbein phi scale of
grain size Grain size (or particle size) is the diameter of individual grains of sediment, or the lithified particles in clastic rocks. The term may also be applied to other granular materials. This is different from the crystallite size, which refer ...
s, cobbles are defined as
clast Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus,Essentials of Geology, 3rd Ed, Stephen Marshak, p. G-3 chunks, and smaller grains of rock broken off other rocks ...
s of rock ranging from −6 to −8 φ. This classification corresponds with the Udden–Wentworth size scale which defines cobbles as clasts with diameters from . On this scale, cobbles are larger than pebbles which measure in diameter and smaller than boulders, whose diameters range from . On the Udden–Wentworth scale, an unlithified fraction of cobbles is classified as
gravel Gravel is a loose aggregation of rock fragments. Gravel occurs naturally throughout the world as a result of sedimentary and erosive geologic processes; it is also produced in large quantities commercially as crushed stone. Gravel is classifi ...
while a lithified sample primarily composed of cobbles is a conglomerate. The Committee on Sedimentation of the US
National Research Council National Research Council may refer to: * National Research Council (Canada), sponsoring research and development * National Research Council (Italy), scientific and technological research, Rome * National Research Council (United States), part of ...
has recommended that
in situ ''In situ'' (; often not italicized in English) is a Latin phrase that translates literally to "on site" or "in position." It can mean "locally", "on site", "on the premises", or "in place" to describe where an event takes place and is used in ...
cobbles be identified by their process of origination, if possible (e.g., ''cobbles by disintegration'', ''by exfoliation'', etc.). In the late 1800s and early to mid-1900s, prior to the Udden–Wentworth scale's widespread adoption, size classifications tended to group all particles larger than together as ''gravel'' or ''stones''. Other scales have defined the size of a cobble slightly differently than the Udden–Wentworth; the
British Standards Institution The British Standards Institution (BSI) is the national standards body of the United Kingdom. BSI produces technical standards on a wide range of products and services and also supplies certification and standards-related services to business ...
denotes a cobble as any clast ranging in diameter from while the
United States Department of Agriculture The United States Department of Agriculture (USDA) is the United States federal executive departments, federal executive department responsible for developing and executing federal laws related to farming, forestry, rural economic development, ...
's definition suggests a range of and the
ISO ISO is the most common abbreviation for the International Organization for Standardization. ISO or Iso may also refer to: Business and finance * Iso (supermarket), a chain of Danish supermarkets incorporated into the SuperBest chain in 2007 * Iso ...
standard 14688 names cobbles as ranging from in diameter. Various attempts have been made to refine the Udden–Wentworth scale, including its definition of cobbles. In 1968, D. J. Doeglas proposed subdividing the ''cobble'' designation into two fractions, ''small cobbles'' (for particles with diameters from ) and ''large cobbles'' (for particles with diameters from ). A 1999 paper by Terence C. Blair and John G. McPherson argued that the Udden–Wentworth and Krumbein scales betrayed a historical emphasis on the study of sand grains while ignoring larger gravel grains. They proposed defining ''fine cobbles'' as those with diameters from (−6 to −7 φ) and ''coarse cobbles'' as those with diameters from (−7 to −8 φ). In 2012, Simon J. Blott and Kenneth Pye suggested that the cobble designation be eliminated altogether, replaced by ''very small boulder'' and ''small boulder'' designations equivalent in size to Blair and McPherson's fine and coarse cobbles, respectively.


Settings

When occurring in streams, cobbles are likely to be found in mountain valley
streambed A stream bed or streambed is the bottom of a stream or river (bathymetry) or the physical confine of the normal water flow (channel). The lateral confines or channel margins are known as the stream banks or river banks, during all but flood st ...
s that are moderately steep. Cobbles are also transported by
glacier A glacier (; ) is a persistent body of dense ice that is constantly moving under its own weight. A glacier forms where the accumulation of snow exceeds its Ablation#Glaciology, ablation over many years, often Century, centuries. It acquires dis ...
s and deposited as with other grades of sediment as
till image:Geschiebemergel.JPG, Closeup of glacial till. Note that the larger grains (pebbles and gravel) in the till are completely surrounded by the matrix of finer material (silt and sand), and this characteristic, known as ''matrix support'', is d ...
. If the till is water-laid, finer particles like sand and pebbles may be entirely washed away, leaving a deposit of only boulders and cobbles. The term
shingle beach A shingle beach (also referred to as rocky beach or pebble beach) is a beach which is armoured with pebbles or small- to medium-sized cobbles (as opposed to fine sand). Typically, the stone composition may grade from characteristic sizes ranging ...
refers to a
beach A beach is a landform alongside a body of water which consists of loose particles. The particles composing a beach are typically made from rock, such as sand, gravel, shingle, pebbles, etc., or biological sources, such as mollusc shel ...
covered with small- to medium-sized cobbles or pebbles (as opposed to fine sand). Glacially transported cobbles tend to share several identifying features including a tabular shape and downward diagonal
striations Striations means a series of ridges, furrows or linear marks, and is used in several ways: * Glacial striation * Striation (fatigue), in material * Striation (geology), a ''striation'' as a result of a geological fault * Striation Valley, in ...
on lateral facets. Cobble conglomerates may be
alluvial Alluvium (from Latin ''alluvius'', from ''alluere'' 'to wash against') is loose clay, silt, sand, or gravel that has been deposited by running water in a stream bed, on a floodplain, in an alluvial fan or beach, or in similar settings. Alluv ...
in origin or the product of "stone avalanches", a type of
debris flow Debris flows are geological phenomena in which water-laden masses of soil and fragmented rock rush down mountainsides, funnel into stream channels, entrain objects in their paths, and form thick, muddy deposits on valley floors. They generally ...
resulting from unconsolidated cobbles and gravel. In such stone avalanches, well-rounded cobbles may travel the farthest on account of their low rolling friction. When the product of alluvial processes, the cobble conglomerate's
matrix Matrix most commonly refers to: * ''The Matrix'' (franchise), an American media franchise ** ''The Matrix'', a 1999 science-fiction action film ** "The Matrix", a fictional setting, a virtual reality environment, within ''The Matrix'' (franchis ...
consists of gravel and coarse
sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural class of s ...
. In contrast, the matrices of flow-deposited conglomerates are primarily
mud A MUD (; originally multi-user dungeon, with later variants multi-user dimension and multi-user domain) is a Multiplayer video game, multiplayer Time-keeping systems in games#Real-time, real-time virtual world, usually Text-based game, text-bas ...
.


References


Cited

* * * {{Authority control Stone (material) Sedimentology Granularity of materials