One of the many fascinating things about Riddley Walker is the little map at the front of the book which makes sense of some of the corrupted place names in the book and puts the whole thing in some sort of geographical context - it's basically set in Kent after it's been radically re-landscaped by the ill-defined nuclear disaster and what we can assume to be some accompanying rise in sea level.
Here's the map (right-click and open in a new tab to enlarge):
Note how both the Isle of Thanet (aka The Ram) and Dungeness (aka Dunk Your Arse) have been severed from the mainland, while the low-lying land separating Sandwich from the sea and most of the Isle of Sheppey have been submerged. The funny thing about this is how similar it looks to this map of the same area at around the time of the Saxon invasion of Britain, so probably 5th century AD or so.
I'm not sure when Dungeness was joined up to the mainland, but the Isle of Thanet remained a proper island until the late 17th/early 18th century, separated from mainland Kent by a waterway known as the Wantsum Channel. This eventually silted up, but its course - south from Reculver, then east to the mouth of the modern-day River Stour between Sandwich and Ramsgate - can be clearly seen on modern-day Ordnance Survey maps:
In the light of what's been happening to the Somerset Levels recently, it's sobering to reflect, while making use of this virtual sea-level readjustment tool, that it would take a sea-level rise of no more than about 3-4 metres to return Thanet (and Dungeness) to being an island.