Sills, dykes and lava flows. Image from Golden Gate Highlands National Park SA

dyke

laccolith

lava flow

trap

vent

volcanic neck

Holyrood Park geology

Hutton's Section at the base of the teschenite sill of Salisbury Crags. The section was on eof many that convinced James Hutton and others that the sill rock had been injected between the layers of cementstone as molten magma. The section shows chilling of the sill margin and baking of the cementstone, together with crumpling and partial incorporation of the underlying sediments.

Sill

Definition: a minor intrusion of igneous rock between layers of sedimentary rock

Salisbury Crags, Edinburgh. Teschenite sill intruded within the Carboniferous Cementstones.

Sills are minor intrusions which are generally concordant with the bedding planes of the surrounding rock. Most sills vary greatly in thickness from a metre or so to ~10 m but the maximum thickness of the Whin Sill as it crosses south-east Scotland and north-east England is just 60 m.

Usually sills are harder than the intruded sedimentary rock. In these circumstances the sills act as cap rocks to form a scarp and dip slope. The most marked effects on the terrain occur when the sills and the rocks that contain them lie near to the horizontal. The effect of sills on relief is not greatly different from that of stacked lava flows; both give trap terrain.