Complex sentence: The Chaillu Mountains is a mountain range straddling southern Gabon and the Republic of Congo which named after French explorer Paul du Chaillu, who explored and documented this region in the 19th century.
Split into simpler sentences: The Chaillu Mountains are a mountain range around the south border between Gabon and the Republic of Congo. The mountains are named for French explorer Paul du Chaillu, who documented them in the 19th century.