Complex sentence: At this time the sea level was about 127 m (417 ft.) lower than it is today, so Britain was joined to Ireland and by the area known as Doggerland to the continent of Europe.
Split into simpler sentences: At this time, the sea level was about 127 m (417 ft.), lower than it is today. Therefore, Britain was joined to Ireland and by the area known as Doggerland to the continent of Europe.