The Outer Shelf Banks were initially bedrock features known as cuestas, typically formed in coastal-plain environments by erosion during early geological periods when they were not submerged. A modern example of a cuesta is Prince Edward Island. Their appearance has been transformed by deposition of glacial till, which has been reworked by the sea to form the present-day surfaces. The banks have moderate relief, generally between 100 and 150 m, and are thus comparable in range to the elevations found on the mainland today. The sandy components of the sand and gravels that are found on the bank tops can be shaped by wave and current activity into a variety of seabed features, including sand ridges, sand waves, ripples, and megaripples.
Sand ridges are the largest of the features and migrate over long periods of time.
Patches of gravel, shell beds, and even boulders occur. Many of the surface features change with each storm or tidal event, and many of the smaller features are erased during intervening periods. These extend onto the bank under the cover of surface sediments and are called tunnel valleys. Sediments moving off the edge of the shelf in these areas contribute material which maintains The Gully, a major submarine canyon and a probable remnant of an early drainage system. Similar movements on the outer edges of the Outer Shelf Banks, particularly during low sea level, have led to the formation of distinctive submarine canyons.