Here's why:

* Glacial Erosion: Fjords are formed by glaciers carving deep valleys into the landscape. The immense weight and movement of the ice erode the bedrock, creating steep, U-shaped valleys.

* Submerged Valleys: When the glaciers melt and sea levels rise, these valleys often become flooded, creating the distinctive narrow, deep, and often winding inlets that characterize fjords.

* Water and Sediment: While the bedrock is the primary component, fjords also contain water and sediment. The sediment can be carried in by rivers, deposited by glaciers, or even carried in by ocean currents.

So, while a fjord is filled with water, the defining feature is the rock that forms its walls and defines its shape.