Sills: form when magma intrudes between the rock layers, forming a horizontal or gently-dipping sheet of igneous rock.

Where do sills form?

sill, also called sheet, flat intrusion of igneous rock that forms between preexisting layers of rock. Sills occur in parallel to the bedding of the other rocks that enclose them, and, though they may have vertical to horizontal orientations, nearly horizontal sills are the most common.

Why are sills formed?

Sills form as rising magma encounters vertical resistance from host rock. The upwelling magma then spreads out in the horizontal plane into area of lower resistance to form sheet-like layers of rock. Sill texture is a function of the time it takes for the magma to cool and solidify.