Complex sentence: Its northern boundary is the Brecon Beacons National Park with the rugged backdrop of the Black mountain, the river Amman, with its fringe of ancient oak woodland, forms its southern boundary.
Split into simpler sentences: Its northern boundary is the Brecon Beacons National Park with the rugged backdrop of the Black mountain. By contrast, the river Amman, with its fringe of ancient oak woodland, forms its southern boundary.