lobes. According to the contrast in density and viscosity of the specific layers, the wavelength produced by the instability varies considerably with values generally between a few millimeters and 10 centimeters. Extreme examples have been reported with wavelengths up to 10 meters. Normally the lobes/pockets and the fingers/diapirs are relatively symmetrical about the vertical, but can become asymmetrical in some places. They lean then into a consistent direction, usually interpreted as the direction of the paleocurrent. Asymmetrical load casts are called squamiform or flow casts. It is important to note that in load casts the flame-like fingers never completely pierce the upper layer, whereas in flame structures they do. Occurrence Load casts appear in very different depositional environments. They are most common in turbidites, but can also occur in fluvial and shallow-marine settings. Occasionally they appear in lake sediments. They have even been found in layered igneous and pyroclastic successions. Good examples come from the Borrowdale Volcanic Series in the English Lake District and from the Carboniferous Bude Formation of southwestern England. Formation Essential for the formation of load casts is an inverted density layering, which is unstable under gravity, i.e. the potential energy of the layered system is not a minimum. Load casts are an example of the instability of an interface in a gravitationally unstable arrangement of layered sediments. The instability involved is called a Rayleigh-Taylor instability, whose driving forces are due to buoyancy. The instability is, however, only latent because it is dependent on liquefaction to become real. The process of liquefaction implies a considerable to almost complete loss of yield strength of the layer involved. This important prerequisite has been appreciated since Sorby in 1908 (and later on by Shrock in 1948), who recognized the hydroplastic condition of the lower layer. On the basis that liquefaction is linked to shock(s), Sims was able in 1975 to correlate the formation of load casts in modern lake deposits with historical earthquakes which had liquefied the sediments. References Literature Allen JRL (1984). Sedimentary structures. Their character and physical basis. Elsevier. Allen JRL (1985). Principles of Physical Sedimentology. Chapman & Hall. Reineck HE & Singh IB (1980). Depositional Sedimentary Environments. Springer-Verlag. Sedimentology Sedimentary structures