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Why do some fjords have shallow rock thresholds near their mouths?
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valley
The rugged Atlas Mountains surround a valley in Morocco.
Fjords are deep, glaciated valleys that often feature a shallow rock threshold, or sill, near their seaward mouth. This feature is primarily a result of the glacial processes that carved the valley.12
The formation of these thresholds is attributed to the following factors:
- Glacial Erosion Patterns: Glaciers are most erosive in their middle and upper reaches, where the ice is moving most actively and vigorously. As a glacier flows toward the sea, its erosive power diminishes, resulting in a shallower valley floor at the terminus compared to the deep basins carved further inland.12
- Glacial Deposits: Many fjords possess a submerged ridge or sill near the mouth that is composed of glacial deposits, such as a moraine shoal. These deposits accumulate as the glacier terminates and retreats, creating a physical barrier that restricts water depth.34
Because of these shallow thresholds, the deep basins of many fjords are often isolated from the open ocean, which can lead to the presence of stagnant water and the accumulation of sediment.13