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How do river deltas form at the heads of fjords?
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valley
The rugged Atlas Mountains surround a valley in Morocco.
River deltas form at the heads of fjords after the retreat of the glaciers that originally carved these deep, U-shaped valleys. Once the ice disappears, the river that formed the original valley reestablishes itself on the upper valley floor.1
As this river flows into the standing water of the fjord, it is forced to deposit its bedloadâthe coarser gravel and sandâat its mouth. This process follows a specific pattern of sediment accumulation:2
- Foreset beds: Coarser materials are laid down directly at the stream mouth as steeply inclined layers.2
- Bottomset beds: Finer, suspended silt and clay drift farther into the fjord, settling as flat-lying layers.2
- Topset beds: As the sediment builds outward, the river deposits a thin, subhorizontal veneer of gravelly material over the foreset units.2
Over time, this accumulation of sediment creates a delta at the head of the fjord. These deltaic deposits are often significant, as they frequently provide the only level land suitable for human settlement, such as villages and farms, within the steep-walled fjord environment.1