Montreuil was a thriving port before the river silted up in the 1300s (the sea is now about 25 km away).
The sea receded up to 25km away from Montreuil.
mnli hypothesis: The sea receded up to 25km away from Montreuil. premise: Montreuil was a thriving port before the river silted up in the 1300s (the sea is now about 25 km away).