Complex sentence: It was much smaller than its later forms and its connection to the main sea was much narrower; it may have been a complex of lakes and marshes and channels, rather than one lake; over time these lakes gradually eroded their soft peat shores and spread.
Split into simpler sentences: It was much smaller than its later forms and its connection to the main sea was much narrower; it may have been a complex of lakes and marshes and channels, rather than one lake. Over time these lakes gradually eroded their soft peat shores and spread.