If you have a spherical planet, and it's possible to locate the stars or the sun, then people will be able to run along a parallel even if they have no compass.

Also I'm not a geologist or a physicist but I don't think it would be possible for that to work short of magic anyway, so you might as well just go with "magic fuddles the mind and all devices of navigation" rather than "magic created an enormous amount of cobalt and gave it a massive magnetic charge so that it can affect compasses thousands of metres away."