Complex sentence: The rocks of the Macarenia date back to 1.2 billion years ago; they are a western extension of the Guiana Shield of Venezuela, Guyana, and Brazil; whose rocks are considered the oldest in the world.
Split into simpler sentences: The rocks of the Macarenia date back to 1.2 billion years ago. They are a western extension of the Guiana Shield of Venezuela, Guyana, and Brazil, whose rocks are considered to be among the oldest in the world.