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A sill is a sheet of igneous rock, often almost horizontal, formed when molten rock material or magma forced its way between two layers of previously existing rock, and cooled and solidified there. The magma that produces sills is usually rather fluid, so sill rocks are usually basic in composition. A sill's thickness may be a few inches or several hundred feet, but its horizontal extent is always great in comparison with its thickness. Unlike a laccolith, a sill usually does not noticeably deform the invaded beds. A sill often protects softer rocks beneath, and owing to unequal erosion may lead to the formation of a ridge or a flat-topped mountain. A sill may have several dikes associated with it.
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