Complex sentence: The Patagonian Shelf, which in ancient geological time was part of Gondwana and which broke from what is now Africa, drifted westwards relative to Africa is now the widest continental shelf in the world covering.
Split into simpler sentences: The Patagonian Shelf, which in ancient geological time was part of Gondwana and which broke from what is now Africa, drifted westwards relative to Africa. It is now the widest continental shelf in the world covering.