sentence: Sills are fed by dykes as they form from a lower magma source.
sentences_before: The sill does not cut across preexisting rocks, unlike dykes.
sentences_after: 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.
concept_name: sill
headings: Sill (geology)
categories: Igneous rocks