Henry Treece 
The Golden Strangers [EPUB ebook] 

Then as darkness fled and sun smiled warmly on the land, the rains began to fall. Trees planted themselves wherever they could find a footing—birch and pine, oak and elm, hazel and indomitable alder.
So the hunting grounds became fewer, for the trees’ great army had occupied the land, and at last men looked towards the sea for food, forgetting the sweet taste of deer and giant ox, grateful gleaners of salt shellfish now.
At length, majestic, sea roared down the tree-hung valley, and one golden morning an island stood separate, new-born, afraid with loneliness. Those who came now must cross in boats, must dare death in the valley, among the blackened oaks.