What is a sill in geological terms?

A sill in geological terms refers to a horizontal sheet of rock that intrudes between layers of existing strata, usually formed from magma that solidified. It occurs when magma rises into the crust but does not reach the surface, spreading out laterally and filling cracks or spaces in the surrounding rock.

This horizontal position distinguishes a sill from other igneous formations; for example, a dike, which is a vertical intrusion of magma. Understanding the formation and characteristics of sills is essential in geology, particularly when studying the behavior of igneous rocks and tectonic processes. The other options do not accurately describe the nature of a sill in geology, as none convey the concept of a horizontal intrusion effectively.