In
geology, a
sill is a tabular
sheet intrusion that has
intruded between older
layers of
sedimentary rock, beds of
volcanic lava or
tuff, or even along the direction of
foliation in
metamorphic rock. The term
sill is synonymous with
concordant intrusive sheet. This means that the sill does not cut across preexisting rocks, in contrast to
dikes, discordant intrusive sheets which do cut across older rocks. Sills are fed by dikes, except in unusual locations where they form in nearly vertical beds attached directly to a magma source. The rocks must be brittle and fracture to create the planes along which the magma intrudes the parent rock bodies, whether this occurs along preexisting planes between sedimentary or volcanic beds or weakened planes related to foliation in metamorphic rock. These planes or weakened areas allow the intrusion of a thin sheet-like body of magma paralleling the existing bedding planes, concordant fracture zone, or foliations.