sentences:  The oldest layer, the Potomac Formation also contains some scattered lignite.As recently as five million years ago, much of present-day Delaware was submerged as the floor of a shallow sea, the Salisbury Embayment; only with the retreat of global sea levels in recent geologic times did the coastal plain areas of the state emerge as dry land.Delaware has a 25-mile (40 km) coastline that includes the communities of Rehoboth Beach, Dewey Beach, Bethany Beach, South Bethany, and Fenwick Island.