Sill
Sills are intrusions parallel to the layering of the geological column. This makes them much more difficult to recognize. Sills appear due to pressure, stress and deformation in an area nearby. In order to recognize a sill more detailed investigation often needs to be made.
For both dikes and sills it applies that they often create columnar joints, due to the cooling process.
Photo showing columnar basalts on the Faroe Islands. (http://www.panoramio.com/photo/55215166)
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