Complex sentence: The Sauk sequence was the earliest of the six cratonic sequences that have occurred during the Phanerozoic; it dates from the late Proterozoic through the early Ordovician, though the marine transgression did not begin in earnest until the middle Cambrian (Monroe and Wicander, 533).
Split into simpler sentences: The Sauk sequence was the earliest of the six cratonic sequences that have occurred during the Phanerozoic. It dates from the late Proterozoic through the early Ordovician, though the marine transgression did not begin in earnest until the middle Cambrian (Monroe and Wicander, 533).