sills

Sill is a term used in geology and architecture to describe a flat, sheet-like feature that lies parallel to surrounding layers or serves as a horizontal supporting element.

In geology, a sill is a tabular igneous intrusion that is concordant with the structure of the host rock, meaning it runs parallel to bedding planes rather than cutting across them. Sills form when magma exploits weaknesses between strata and spreads between layers, often cooling and crystallizing to form solid rock. They can vary greatly in thickness—from centimeters to hundreds of meters—and can extend for many kilometers. Common sill compositions range from mafic basalts to felsic rhyolites, with granitic sills also present in continental crust. Features such as chilled margins, contact metamorphism along the margins, and occasionally columnar jointing may be observed. Sills are distinct from dikes, which cut across rock layers.

In architecture, a sill (or window sill) refers to the horizontal surface at the bottom of a window opening or a door threshold. Window sills are designed to shed water, protect interior surfaces, and provide a finishing ledge. They may be made from stone, concrete, wood, metal, or composite materials and are often sloped slightly outward. Interior sills can serve as a decorative or functional ledge for placing objects.

Etymology derives from older forms of the word related to supporting and lying beneath. The term is used across disciplines to denote a flat, bed-like element that interacts with surrounding materials or structures.