Complex sentence: The oldest rock in southern Sarawak is schist, which was formed during the Carboniferous and Lower Permian times, while the youngest igneous rock in this region is andesite, found at Sematan.
Split into simpler sentences: The oldest rock in southern Sarawak is schist, which was formed during the Carboniferous and Lower Permian times. While the youngest igneous rock in this region is andesite, found at Sematan.