The river also dissects the Ardennes forests during the Hercynian orogeny. It is more than 900 miles in length. River French Broad, USA: This river is over 300 million years old.

Green bars stand for the Hercynian contribution and blue bars for the whole continental area excluding the Hercynian orogeny. All fluxes are normalized to the global weathering flux at 350 Ma (in %).