Ball-and-pillow structures are masses of clastic sediment that take the form of isolated pillows or protruding ball structures. These soft-sediment deformations are usually found at the base of sandstone beds that are interbedded with mudstone.Ailsa Allaby and Michael Allaby. "ball and pillow structure." A Dictionary of Earth Sciences. 1999. Encyclopedia.com. 1 Nov. 2010 It is also possible to find ball-and-pillows in limestone beds that overlie shale, but it's less common.Boggs, S., 2006, Principles of Sedimentology and Stratigraphy (4th ed.), Pearson Prentice Hall, Upper Saddle River, NJ, p.94-97 They are normally hemispherical or kidney shaped, and range in size from a few inches to several feet.
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.P. Van Rensbergen, R. R. Hillis, A. J. Maltman, and C. K. Morley. Subsurface Sediment Mobilization. London: Geological Society, 2003. Print. 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. These experiments further conclude that earthquakes, erupting volcanoes, 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.
References
See also Load cast Flame structure