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 ...

sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand ...

that is unstratified and structureless and contains

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 ...

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. kaolin, Al2 Si2 O5( OH)4). Clays develop plasticity when wet, due to a molecular film of water surrounding the clay pa ...

or rock flour. Boulder clay is also know as either known as drift clay; till; unstratified drift, geschiebelehm (German); argile á blocaux (French); and keileem (Dutch).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 infreqently used for gravelly

sedimentary Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...

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 diamictons associated with the Guangxi

karst Karst is a topography formed from the dissolution of soluble rocks such as limestone, Dolomite (rock), dolomite, and gypsum. It is characterized by underground drainage systems with sinkholes and caves. It has also been documented for more weathe ...

, China.Waltham, T., 2010. ''Guangxi karst: The fenglin and fengcong karst of Guilin and Yangshuo.'' In: Migon, 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 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 predomina ...

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 and composition of these gravels vary according to the distance through which they have travelled and the hardness, structure, and compoistion 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 be ...

source from which they have been plucked. The

cobbles 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 f ...

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 ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms w ...

clasts quite commonly exhibit 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 criss-cross each other. The overall texture of boulder clay varies with its source. The glacial erosion of

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicat ...

s often produce loose and

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 ...

y boulder clays. The glacial erosion of

granite Granite () is a coarse-grained ( phaneritic) 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 cools and solidifies un ...

gneiss Gneiss ( ) 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. Gneiss forms at higher temperatures a ...

es and

quartzose Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...

schist Schist ( ) is a medium-grained metamorphic rock showing pronounced schistosity. This means that the rock is composed of mineral grains easily seen with a low-power hand lens, oriented in such a way that the rock is easily split into thin flakes ...

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 minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especiall ...

results in clayey glacial deposists typical 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 Europe, off the north-western coast of the continental mainland. It comprises England, Scotland, Wales and ...

, 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 period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carboniferou ...

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 elements, chiefly hydrogen, sulfur, oxygen, and nitrogen. Coal is formed when ...

, and red boulder clays are derived from eroded

Old Red Sandstone The Old Red Sandstone 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 northeastern seaboard of North America. It also exte ...

New Red Sandstone The New Red Sandstone, chiefly in British geology, is composed of beds of red sandstone and associated rocks laid down throughout the Permian (300 million years ago) to the end of the Triassic (about 200 million years ago), that under ...

. 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 overconsolidated. These physical propoerties 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