Overlie

Overlie is a verb meaning to lie on top of something or to be placed above another object. In geology and related fields, overlie describes a stratigraphic relationship in which one rock unit lies atop and contacts another. In this context the unit that overlies the other is younger, in accordance with the principle of superposition. The past participle forms are overlain and overlaid.

In practical use, a sentence such as “the sandstone overlie the shale” communicates that the sandstone rests above the shale. The contact between units can be conformable, indicating continuous deposition, or unconformable, indicating a gap in the geologic record and possible erosion or non-deposition during a time interval. Geologists use the term to help interpret depositional environments, tectonic history, and relative ages of rock sequences.

Beyond geology, overlie can be used in general English to mean to lie above or rest on top of something. The related noun overlay refers to something that covers or sits on top of another, while the adjective overlying describes the unit that lies above another. The term is often contrasted with underlie, which denotes the unit beneath.