Sill (geology)

Salisbury Crags in Edinburgh, a sill partly exposed during the ice ages

In geology, a sill is a flat sheet-like intrusion. As molten magma, it pushed between older layers of rock. The older rock may be sedimentary rock, beds of volcanic lava or tuff, or metamorphic rock.

Mid-Carboniferous sill between Lower Carboniferous shales and sandstones: Horton Bluff, Nova Scotia

The sill does not cut across preexisting rocks, unlike dykes. Sills are fed by dykes as they form from a lower magma source. The existing rocks must split to create the planes along which the magma moves in. These planes or weakened areas allow the intrusion of a thin sheet-like body of magma paralleling the existing strata. When it cools and crystallises, it is then a sill.

This article includes text from the public domain 1911 Encyclopaedia Britannica.

Sill (geology) Media

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  Illustration showing the difference between a dike and a sill.

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  Seismic expression of basaltic sills from the Rockall Trough. Data courtesy UK OGA.