Complex sentence: Eventually the County roughly corresponded to the two present-day Regions of Upper and Lower Normandy of the modern French Republic, plus the Channel Islands, which remain under the British crown as successors of the dukes of Normandy.
Split into simpler sentences: Eventually the County roughly corresponded to the present-day regions of Upper and Lower Normandy of modern France. In addition, the Channel Islands remain under the British crown as successors to the dukes of Normandy.