Norway's deepest coastal inlets were carved by ice, not by the sea

Fjords form when glaciers grind valleys far below sea level as they slowly flow toward the coast; once the ice retreats, the ocean floods in to fill the steep-walled trench left behind. That's why fjords like Norway's Sognefjord are so narrow, deep and sheer-sided compared with ordinary river-cut coastal inlets.

— Glacial geomorphology scholarship, Fjord — standard glacial-valley process