What was the escape route to the Fringes in The Chrysalids?
The telepaths escaped from Waknuk and headed to the Fringes.
Waknuk is part of Labrador, a peninsula-like area of northwest Canada. To get to the Fringes, the telepaths have to go South.
[If] you were to go three hundred miles or so east, north, or north-west you would come to [the sea] sooner or later. But south-west or south, you wouldn't; you'd get to the Fringes and then the Badlands, which would kill you. (ch 4)
Waknuk is pretty remote and isolated, and near to the Fringes. It does not take long for the land to get desolate. David notes that the Fringes are “little more than seven miles away” from his farm (ch 4).
We went on by the erratic route that Rosalind had picked to hide the trail. There were more stony places and more streams until finally we urged the horses up a steep bank and into the woods. (ch 12)
The “Wild Country becomes tractable, and Badlands country slowly gives way to habitable Fringes country” (ch 6). Much of the land in the Fringes is habitable, but the people have a difficult time there because they have almost nothing. They are all outcasts with only their ingenuity to keep them alive, and law and order are not well-maintained.
It takes bravery and knowledge for the young telepaths to successfully flee Waknuk to the Fringes. Not only are the Fringes dangerous, but they have been told all of their lives that the people there are heathens. When they are captured almost the minute they arrive, the warnings seem accurate.