Complex sentence: When ploughing was done with large teams of small oxen (as it was during the Early Middle Ages), the team and plough together were many metres long, and this led to a particular effect in ridge and furrow fields.
Split into simpler sentences: When ploughing was done with large teams of small oxen (as it was during the Early Middle Ages), the team and plough together were many metres long. This led to a particular effect in ridge and furrow fields.