The north-south divide has been the butt of jokes in Britain for years, but research has shown the Watford Gap, which separates the country, was in fact established centuries ago when the Vikings invaded Britain....And then the explanation:
Adams was struck by the absence of Scandinavian placenames south-west of Watling Street, the Roman road that became the A5. “There might be one or two names, but I don’t think there are any, and there are certainly hundreds and hundreds north-east. Clearly the Scandinavian settlers stopped at Watling Street,” Adams said.
“I began to notice that all the rivers’ sources stop pretty much on the line of Watling Street. North-east of that line, all the rivers flow into the Irish Sea or the North Sea. South and west of it, they all flow into the Severn or the Thames.”...I have always thought that people who really love a place show it when they understand the way the rivers flow. But that is dependent on the modern sensibility, in which it really doesn't matter to you which way the rivers flow for the most part. You'd only need to know if you cared about the place. For much of history, and never more than for the Vikings, where those rivers go was a big deal.
“These days, we’re unaware of which way rivers face and where they flow out to. It doesn’t make any odds to us. We just put bridges over them. But, for most of history, such things have mattered. Your natural trading routes are along rivers and all the medieval monastic estates used the rivers as their arteries of power. So clearly the geography of power has always mattered … Geographically, it slaps you in the face as soon as you figure it out.”
I don't often recommend video games, but if you want to get a sense of this, try this historical semi-simulation. It's not super cheap -- you have to buy the base game plus this add on, or else one of the extended editions that includes it. Still, it's got a pretty good map of the western part of the Viking world.