"The Atom Curtain" by Nick Boddie Williams

Sometimes I reach for the Ace Doubles instead of the Gold Medals when I'm looking for a comfort read. And more often than not, I regret it. The Atom Curtain is a weird little lost world novel with a post-apocalyptic twist. The USA, in the hopes of averting a world war, raises an impregnable nuclear shield around the entire North and South American continents. The rest of the world sort of... gets on with things. Our hero is an English jet fighter (but of fine American stock!) who gets sucked past the curtain in a fluke storm. There he discovers that radiation has scientifimatically unevolved the American people, so they're now... cave people. Also, there are creepy mole people. With a supermind. 

It is transparently, lazily based on The Time Machine, but with the added bonus of meandering social theory. Special perk: because everyone is, like, radioactively unevolved and stuff, our jet fighter spends most of the book having sex with his 12 year old cavewoman bride. BUT, YOUR HONOUR, SHE LOOKED AN EVOLVED 18. (Jared, Pornokitsch)