SearchShield Wall
The Shield Wall is a rock formation which trisects the Known World into the Southlands, the Northlands, and the Tablelands. The elevation of the Southlands is considerably higher (as are all lands south of the Shield Wall and north of the Sea of Eternal Dust [q.v.]) than that of the Tablelands or Northlands.
Many scholars have speculated on why this is so, one such hypothetical proposition being that the realms north of the Shield Wall were once in fact the bed of a large sea of water. One would expect, however, that if one travelled north far enough, he/she should find the 'opposite shore' of this dry sea, another Shield Wall to the far north. This has never been proven to exist, and so any such beliefs remain untested.
The Shield Wall tends to separate weather patterns somewhat, especially blocking the storms of the Red Desert (q.v.) from entering the Scrub Plains (q.v.). Several large cracks do exist, however, and so a certain amount of spillover does occur.