Complex sentence: The Caledonian Forests were formed at the end of the last ice age, when trees began to recolonise what is now the British Isles over a land bridge which is now beneath the English Channel.
Split into simpler sentences: The Caledonian Forests were formed at the end of the last ice age. Trees began to recolonise what is now the British Isles over a land bridge which is now beneath the English Channel.