Sill (geology)

A sill is a concordant intrusive sheet, meaning that it does not cut across preexisting rock beds.

Sills are fed by dikes,[3] except in unusual locations where they form in nearly vertical beds attached directly to a magma source.

Sills run parallel to beds (layers) and foliations in the surrounding country rock.

In addition, lava flows will typically show evidence of vesicles (bubbles) where gases escaped into the atmosphere.

The geometry of large sill complexes in sedimentary basins has become clearer with the availability of 3D seismic reflection data.

Illustration showing the difference between a dike and a sill.
Salisbury Crags in Edinburgh , Scotland , a sill partially exposed during the Quaternary glaciation
Mid- Carboniferous dolerite sill cutting Lower Carboniferous shales and sandstones , Horton Bluff, Minas Basin South Shore, Nova Scotia
Seismic expression of basaltic sills from the Rockall Trough. Data courtesy UK OGA.