Most fjords have a submerged ridge at their entrance that acts as a gate between deep inner waters and the open ocean.

Fjords are formed through the process of glaciation, where massive ice sheets carve deep U-shaped valleys into the bedrock. As these glaciers move toward the sea, they carry vast amounts of rocks and sediment known as moraine. When the glacier reaches the ocean and begins to melt or retreat, it deposits this debris at its furthest point of advancement. Over thousands of years, this accumulation creates a terminal moraine that forms a shallow underwater ridge called a sill.The presence of a sill significantly impacts the oceanography of the fjord. Because the sill is much shallower than the basin behind it, it restricts the inflow of cold, dense, and nutrient-rich saltwater from the open ocean. This creates a stratified water column where fresher, less dense water from mountain runoff sits on top of the heavier saltwater. In some cases, this lack of circulation can lead to anoxia, where the deepest waters become depleted of oxygen.A prime example is the Sognefjord in Norway, which is the largest and deepest fjord in the country. While the main basin reaches a staggering depth of 1,308 meters, the threshold at the mouth is only about 100 meters deep. This dramatic difference in depth was documented by researchers from institutions like the University of Bergen. These sills are not just geological curiosities but are vital for supporting unique ecosystems that thrive in the calm, sheltered waters protected from the turbulence of the North Sea.