Definition
Sheet-like body of igneous rock which conforms to bedding or other structural planes.
Description
Horizontal or near horizontal igneous intrusions usually comprising of dolerite or diabase.
Why are sills important?
Sills often have low permeabilities and thus form aquicludes or aquitards. Contact zones between the sills and host rocks can be fractured and transmissive.
Source: http://www.npolar.no