How do fjords form, and why are they deep?
A fjord is a long, narrow, usually steep-sided coastal inlet formed when seawater floods a glacier-carved valley. Not every narrow bay is a fjord; glacial origin is the defining feature.
From glacial valley to marine inlet
A valley glacier may erode parts of its floor below sea level. When ice retreats and seawater enters, a deep corridor forms. A bedrock or depositional sill at the mouth can be shallower than the inner basin. Depth reflects former ice thickness, rock structure and erosion patterns. Sea-level changes and land uplift or subsidence jointly shape the final coastline.
Why water circulation varies
A sill can restrict exchange between deep water and the open sea. River inflow may create a fresher surface layer while different conditions develop below. Not all fjords stratify identically: wind, tides, sill depth and seasons matter. Steep slopes also constrain transport and settlement. Understanding a fjord requires underwater topography and glacial history in addition to its visible scenery.