Boulder clay is an unsorted agglomeration of

clastic 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 by ...

sediment Sediment is a solid material that is transported to a new location where it is deposited. It occurs naturally and, through the processes of weathering and erosion, is broken down and subsequently sediment transport, transported by the action of ...

that is unstratified and structureless and contains

gravel Gravel () is a loose aggregation of rock fragments. Gravel occurs naturally on Earth as a result of sedimentation, sedimentary and erosion, erosive geological processes; it is also produced in large quantities commercially as crushed stone. Gr ...

of various sizes, shapes, and compositions distributed at random in a fine-grained matrix. The fine-grained matrix consists of stiff, hard, pulverized

clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolinite, ). Most pure clay minerals are white or light-coloured, but natural clays show a variety of colours from impuriti ...

or rock flour. Boulder clay is also known as drift clay;

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 ...

; and unstratified drift.Charlesworth, J. K., 1957. ''Chap 18, Boulder Clay''. In ''The Quaternary Era, with Special Reference to its Glaciation''. v. 1, London, United Kingdom, Edward Arnold, p. 376-388.Neuendorf, K.K.E., J.P. Mehl, Jr., and J.A. Jackson, eds. (2005) ''Glossary of Geology'' (5th ed.). Alexandria, Virginia, American Geological Institute. 779 pp. The term ''boulder clay'' is infrequently used for gravelly

sedimentary Sedimentary rocks are types of rock formed by the cementation of sediments—i.e. particles made of minerals (geological detritus) or organic matter (biological detritus)—that have been accumulated or deposited at Earth's surface. Sedime ...

deposits of nonglacial origin. These deposits include submarine slump and slide deposits along continental margins,Reineck, H.E. and Singh, I.B., 1982. ''Depositional sedimentary environments: with reference to terrigenous clastics.'' New York, New York, Springer Science & Business Media. 551 pp. lacustrine debris flow deposits consisting of pebbly mudstones,Jiang, X., 2018. ''Sedimentary Dynamics of Windfield-Source-Basin System New Concept for Interpretation and Prediction.'' Beijing, China, Science Press, Springer Geology. 341 pp. and coarse, poorly sorted, cobbly

diamicton Diamicton (also diamict) (from Greek ''δια'' (dia-): through and ''μεικτός'' (meiktós): mixed) is a terrigenous sediment (a sediment resulting from dry-land erosion) that is unsorted to poorly sorted and contains particles ranging in siz ...

s associated with the Guangxi

karst Karst () is a topography formed from the dissolution of soluble carbonate rocks such as limestone and Dolomite (rock), dolomite. It is characterized by features like poljes above and drainage systems with sinkholes and caves underground. Ther ...

, China.Waltham, T., 2010. ''Guangxi karst: The fenglin and fengcong karst of Guilin and Yangshuo.'' In: Migoń, P. (Ed.), pp. 293–302. ''Geomorphological Landscapes of the World.'' Germany, Dordrecht, Springer. 371 pp.

Lithology

The gravel content of boulder clay varies from 1 to 99 percent and ranges in size from

pebble A pebble is a clastic rocks, clast of rock (geology), rock with a grain size, particle size of based on the Particle size (grain size), Udden-Wentworth scale of sedimentology. Pebbles are generally considered larger than Granule (geology), gra ...

s to

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 ...

s of large dimensions. The size, shape, and composition of these gravels vary according to the distance through which they have travelled and the hardness, structure, and composition of the

bedrock In geology, bedrock is solid rock that lies under loose material ( regolith) within the crust of Earth or another terrestrial planet. Definition Bedrock is the solid rock that underlies looser surface material. An exposed portion of bed ...

source from which they have been plucked. The cobbles and boulders found in boulder clay often have a distinctive shape, especially in the harder rock types. Their shapes are either inherited from the structure of bedrock source or how it was transported, or combination of both. Cobbles and boulders that have been transported over long distances by glaciers are often roughly triangular or pentagonal in shape with rounded and smoothed edges and smooth, flat soles. Faceted cobbles and boulders are common in boulder clays that accumulated beneath the centers of ice sheets and are relatively rare in boulder clays deposited near the periphery of the ice-sheets. Cobbles and boulders that occur within boulder clays commonly exhibit striations like those on found on bedrock surfaces eroded and polished by glacial ice.

Limestone Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...

clasts quite commonly exhibits striations. On a cobble or boulder, striations may be confined to one or more facets. Also, they may ring an entire clast. On particularly large boulders, striations lie typically parallel to its long axis. If the cobble or boulder shifted during transport, striations will crisscross each other. The overall texture of boulder clay varies with its source. The glacial erosion of

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 ...

s often produces loose and

sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is usually defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural ...

y boulder clays. The glacial erosion of

granite Granite ( ) is a coarse-grained (phanerite, phaneritic) intrusive rock, intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly coo ...

gneiss Gneiss (pronounced ) is a common and widely distributed type of metamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. This rock is formed under p ...

es and quartzose

schist Schist ( ) is a medium-grained metamorphic rock generally derived from fine-grained sedimentary rock, like shale. It shows pronounced ''schistosity'' (named for the rock). This means that the rock is composed of mineral grains easily seen with a l ...

s often results in the formation of stony, coarse and gravelly boulder clay that, in some instances, can be hardly distinguishable from in situ decayed rock. In case of both of these boulder clays, the term ''clay'' is misleading and inappropriate. Finally, glacial erosion of limestone,

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of Clay mineral, clay minerals (hydrous aluminium phyllosilicates, e.g., Kaolinite, kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4) and tiny f ...

results in clayey glacial deposits typical of boulder-clay. The color of boulder clays varies with the type of bedrock from which it was eroded. For example, within the

United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Northwestern Europe, off the coast of European mainland, the continental mainland. It comprises England, Scotlan ...

, dull grey boulder clays are derived from eroded

; blue boulder clays are derived from eroded Oxford Clay, dark blue or nearly black boulder clays are derived from eroded

Carboniferous The Carboniferous ( ) is a Geologic time scale, geologic period and System (stratigraphy), system of the Paleozoic era (geology), era that spans 60 million years, from the end of the Devonian Period Ma (million years ago) to the beginning of the ...

clay and

coal Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other Chemical element, elements, chiefly hydrogen, sulfur, oxygen, and nitrogen. Coal i ...

, and red boulder clays are derived from eroded

Old Red Sandstone Old Red Sandstone, abbreviated ORS, is an assemblage of rocks in the North Atlantic region largely of Devonian age. It extends in the east across Great Britain, Ireland and Norway, and in the west along the eastern seaboard of North America. It ...

or New Red Sandstone. Like its texture and color, the composition of a boulder clay varies with the composition of the bedrock from which it was eroded. Boulder-clay is typically tough, compact, tenacious, and over consolidated. These physical properties result from a combination of the fine rock-flour it contains and the weight of the overlying ice while the boulder clay accumulated.

References

{{reflist Sediments Glaciology

Lithology

See also

References