Sill (geology)

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. 
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.