A principal reference section for the Nepean Formation in Eastern Ontario is proposed. This section is located west of Ottawa, Ontario in Nepean Township and south of the original Nepean quarries which supplied much of the stone used in the construction of the Parliament Buildings. These quarries are now filled in and little outcrop is visible.The Nepean sandstones are considered to be an offshore deposit of a shallow sea. The sandstone beds low in the section show foreset bedding, oscillation ripple marked surfaces, desiccation cracks, and occasional burrowing. Bioturbation is a dominant feature in the upper beds of the sequence.Throughout southeastern Ontario, the Nepean Formation unconformabîy overlies the Precambriam, and is overlain with a sharp, possibly disconformable contact, by the March Formation of Tremadocian age. The Nepean Formation is considered to be younger than the Potsdam Formation of Potsdam County, New York State, and to be Lower Tremadocian.