While I'm not disputing the importance of terroir, your example is not the best:
- Far from being "literally across the river", the two regions are ~200km apart by road (Pouilly-Fumé is from the Loire valley, Pouilly-Fuissé from the Mâconnais).
- The two wines are made from different grapes: Pouilly-Fuissé is a Chardonnay, while Pouilly-Fumé is a Sauvignon Blanc.
Maybe you meant Pouilly-Fumé and Sancerre? Same grape, opposite sides of the Loire?
- Far from being "literally across the river", the two regions are ~200km apart by road (Pouilly-Fumé is from the Loire valley, Pouilly-Fuissé from the Mâconnais).
- The two wines are made from different grapes: Pouilly-Fuissé is a Chardonnay, while Pouilly-Fumé is a Sauvignon Blanc.
Maybe you meant Pouilly-Fumé and Sancerre? Same grape, opposite sides of the Loire?