Dike and Sill

Igneous rock intrudes sedimentary rock.

The light-colored layers are sedimentary rocks, and the dark mass is diorite (an igneous rock). The lower mass of diorite is a sill, because it is parallel to the sedimentary layers. A section branches from the top of the sill and cuts across the layers, forming a dike.

Header photo is from St. John, U.S. Virgin Islands.