The midline ridge is a continuous raised landform running around the circumference of Coriopolis near the midpoint of its length, dividing the habitat into roughly equal northern and southern halves.

The division it marks is demographic rather than administrative. The northern half, which holds the city, is far more heavily populated than the southern half, which grades into the nature preserve and the Southern Park. The ridge is the conventional boundary between the two, and the point at which a traveler moving south leaves developed country.

Because it lies close to the origin of the habitat’s coordinate system, the ridge is also a convenient physical reference for axial position: a feature is commonly located as so many kilometers north or south of the midline rather than by coordinate.