The Laurentian Divide carves the continent into two watersheds. The southerly side drains to the Gulf of Mexico; to the north, drainage goes to the Hudson Bay.
In the Dakotas, as with the rest of North America, and much of the world, treaties and borders drawn on maps have been erased and moved over time. The Dakotas are unceded land.
This drawing and re-drawing is disrespect for not only nation-states and their residents, but for geography. Geography is treated as mutable.
Geology, with its heavy embeddedness, is immutable.
The Air Fence deploys the Laurentian Divide as an impenetrable border. The Divide is deepened until hot magma at the earth’s core is freed, to superheat air and create a fence of geo and atmo. The lines of geo and atmo are blurred, but the line of the Air Fence is impassable.