Early in the Ordovician , the continents Laurentia ( present-day North America ) , Siberia , and Baltica ( present-day northern Europe ) were still independent continents . Early in the Ordovician , the continents Laurentia ( present-day North America ) , Siberia , and Baltica ( present-day northern Europe ) were still independent continents ( since the break-up of the supercontinent Pannotia earlier ) , but Baltica began to move towards Laurentia later in the period , causing the Iapetus Ocean to shrink between them .