Ball-and-pillow structures are masses of

clastic sediment 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 ...

that take the form of isolated pillows or protruding ball structures. These soft-sediment deformations are usually found at the base 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 silicates) ...

beds that are interbedded with

mudstone Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from '' shale'' by its lack of fissility (parallel layering).Blatt, H., and R.J. Tracy, 1996, ''Petrology. ...

. It is also possible to find ball-and-pillows in

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 whe ...

beds that overlie

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, especial ...

, but it's less common. They are normally hemispherical or kidney shaped, and range in size from a few inches to several feet. Merry Beach Ball And Pillow Structure

Development and preservation

Ball-and-pillow deformations are a result of a physical shock that has been applied to unconsolidated sediment. This shock causes rupturing to occur in the sedimentary rock layer, which induces instability. Individual lobes caused by this unstable state break off and move downward, settling into the underlying layers. There may also be a repeated lobe detachment, causing a greater downward movement into the sediments below. The "pillows" that have now been produced typically retain their original bedding compositions. Both Kuenen (1958) and Owen (1996) have duplicated ball-and-pillow structures in the laboratory, which was done by applying a shock to multilayered

strata In geology and related fields, a stratum ( : strata) is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as ei ...

. These experiments further conclude that

earthquake An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, from ...

s, erupting

volcano A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often found where tectonic plates are ...

es, or meteoric impacts can create these formations.

History

The first record of these structures was in 1916, when Smith wrote a paper called "Ball or Pillow-form Structures in Sandstones." Here he referred to these deformations as pillow-forms or ball structures. The actual term "ball-and-pillow" came from Potter and Pettijohn (1963), when they cited Smith's work incorrectly.Middleton, Gerard. Encyclopedia of sediments and sedimentary rocks. Dordrecht: Kluwer Academic Publishers, 2003. 39-40.

Development and preservation

History

References

See also