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Downstream from this point the river has shaped a large bed in the sand dunes creating a significant bottleneck at the confluence.The occupation of the hill that dominates this narrowing of the valley developed through a gradual spread across the lowlands by building embankments and the aggradation from flood soil.
The course of the Adour was changed in 1578 under the direction of Louis de Foix and the river returned to its former mouth, returning business lost to Bayonne for over a hundred years.
Subsequent to these deposits there was a rise in sea level in the Holocene period (from 15,000 to 5000 years ago) which explains the invasion of the lower valleys with fine sand, peat, and mud with a thickness of more than 40 metres (130 feet) below the current bed of the Adour and the Nive in Bayonne. In the late Quaternary the topographic physiognomy we know today was formed—i.e. The promontory of Bassussarry–Marracq ultimately extended to the labourdin foothills, dying out at the Grand Bayonne hill is an example.
Similarly, on the right bank of the Nive, the heights of Château-Neuf (Mocoron Hill) met the latest advance of the plateau of Saint-Pierre-d'Irube (height 30 to 35 metres (98–115 feet)).
The meeting point of the two rivers coincides with a narrowing of the Adour valley.
Above this the alluvial plain extends for nearly thirty kilometres (19 miles) towards both Tercis-les-Bains and Peyrehorade, and is characterized by swampy meadows called barthes which are influenced by floods and high tides.