Acobble (sometimes acobblestone) is aclast of rock defined on theUdden–Wentworth scale as having aparticle size of 64–256 millimeters (2.5–10.1 in), larger than apebble and smaller than aboulder. Other scales define a cobble's size differently. A rock made predominantly of cobbles is termed aconglomerate.Cobblestone is a building material based on cobbles.
Cobbles, also calledcobblestones, derive their name from the wordcob, meaning a rounded lump. The term is further related to the GermanKopf, meaninghead.[1] Chester Wentworth referred to cobbles ascobble bowlders [sic] in his 1922 paper that would become the basis for the Udden–Wentworth scale.[2]
Within the widely usedKrumbein phi scale ofgrain sizes, cobbles are defined asclasts 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 64–256 millimeters (2.5–10.1 in). On this scale, cobbles are larger than pebbles which measure 4–64 millimeters (0.16–2.52 in) in diameter and smaller than boulders, whose diameters range from 256–4,096 millimeters (10.1–161.3 in). On the Udden–Wentworth scale, anunlithified fraction of cobbles is classified asgravel while a lithified sample primarily composed of cobbles is aconglomerate.[2] The Committee on Sedimentation of the USNational Research Council has recommended thatin situ cobbles be identified by their process of origination, if possible (e.g.,cobbles by disintegration,byexfoliation, etc.).[3]
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 2 millimeters (0.079 in) together asgravel orstones. Other scales have defined the size of a cobble slightly differently than the Udden–Wentworth; theBritish Standards Institution denotes a cobble as any clast ranging in diameter from 60–200 millimeters (2.4–7.9 in) while theUnited States Department of Agriculture's definition suggests a range of 75–250 millimeters (3.0–9.8 in) and theISO standard14688 names cobbles as ranging from 63–200 millimeters (2.5–7.9 in) in diameter.[4]
Various attempts have been made to refine the Udden–Wentworth scale, including its definition of cobbles.[4] In 1968, D. J. Doeglas proposed subdividing thecobble designation into two fractions,small cobbles (for particles with diameters from 64–125 millimeters [2.5–4.9 in]) andlarge cobbles (for particles with diameters from 125–250 millimeters [4.9–9.8 in]).[5] 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 definingfine cobbles as those with diameters from 64–128 millimeters (2.5–5.0 in) (−6 to −7 φ) andcoarse cobbles as those with diameters from 128–256 millimeters (5.0–10.1 in) (−7 to −8 φ).[2] In 2012, Simon J. Blott and Kenneth Pye suggested that the cobble designation be eliminated altogether, replaced byvery small boulder andsmall boulder designations equivalent in size to Blair and McPherson's fine and coarse cobbles, respectively.[4]
When occurring in streams, cobbles are likely to be found in mountain valleystreambeds that are moderately steep.[6] Cobbles are also transported byglaciers and deposited as with other grades of sediment astill. 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 termshingle beach refers to abeach 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 diagonalstriations on lateral facets.[7]
Cobble conglomerates may bealluvial in origin or the product of "stone avalanches", a type ofdebris flow resulting from unconsolidated cobbles and gravel.[8] In such stone avalanches, well-rounded cobbles may travel the farthest on account of their low rolling friction.[9] When the product of alluvial processes, the cobble conglomerate'smatrix consists of gravel and coarsesand. In contrast, the matrices of flow-deposited conglomerates are primarilymud.[8]
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