The Caves of Bluestone by Philip C. McCormac
 
The year is 2094. The population of Britain have abandoned the countryside and live in Megacities. Isolated communities not wanting to be constrained by city life eke out a living in the resulting Wilderness. When Savages attack the small farm where Becki and Karal live, Becki is taken and Karal sets out to find her. With the help of stranded townies hopelessly at sea in the Wilderness, the group sets out on their quest to save Becki, avoid the Savages and their vile children the Brats, and find refuge in the last land reputed to live in peace and harmony—the Caves of Bluestone.

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