Where the ghost of the Western Interior Seaway still runs, where it goes to ground, and the one stretch of old floor where a door can be opened. Drawn to Mabel's logic: seats sit where lines cross; the door sits where the line ends.
Six kinds of ground, ranked by what they can do for a crossing. The logic underneath all of them: a ley line is the ghost of a seaway current, so it needs old, flat, undisturbed floor to run on, and it needs somewhere to end.
The door. The northern shore of the old sea, and the one stretch of it neither mountain-building nor ice ever touched: the Cypress Hills stand as an unglaciated island with pines that belong five hundred miles west, which in-world makes them the oldest unbroken run of line on the continent. The current lands here. It terminates against something, and a line that ends is a line you can open.
It also sits under the aurora, closest to Malikin's regard, so the same ground that makes a door possible makes it watched. The seventh seating must happen under a dark sky.
The one place something punched up through the seaway floor from below. This is where the builders anchored the lines: Devils Tower is a terminal with machined grooves, and more lines converge and cross here than anywhere else. Convergence means leaks. You can fall through at Bear Butte, which is what Mabel did, but it is a crack, not a door, and it opens on no one's timetable. The Bighorns are the western wall where the line surfaces before the mountains scatter it, hence the wheel.
The western shoreline of the old sea. The line is clean and easy to find, which makes this the place to learn to seat a stone, and the place Alistair can first appear. But it is wading depth. Nothing crosses here.
The abyss of the seaway, the deepest water, the chalk full of mosasaurs and sixty-foot fish. By the chart's own logic the ghost current is strongest here, maybe too strong, and there is nothing to seat it to: no barrows, no high ground. The line runs at full flood and never surfaces. Drowned ground. Held in reserve for later books, as the place Alistair will not go, or the place the keepers come from.
Where the seaway drained to the Gulf: the line's outfall, not its source. Strong enough to feel, useless for a crossing. With the boreal edge at the other end, it is why the work must run along the spine, south to north or north to south, and never sideways.
Everything north and east of the Missouri was planed by the ice and then buried under glacial Lake Agassiz. The lines there are broken and still: a lot of stagnant little leaks and no throughway. The river itself is the seam between intact line and broken line, which is what the Fred Robinson Bridge chasm is.
Flat, old ground carries the line cleanly; mountains scatter it; ice breaks it. The Front is the western wall, the Missouri is the northern-and-eastern seam, and everything a crossing needs lies between them.
Seats sit where lines cross, which is what the concentric circles on Mabel's map mean. The door sits where the line ends. The lodestone tunes one stretch at a time; a seating fades once the hand that made it leaves the line.
The 100th meridian is drawn as the eastern limit of where the line can be felt on the ground (dry ground carries; wet ground grounds it out). The boreal edge is where the line goes under for good; what it runs into up there is a Book 2 question.
Medicine wheels are drawn but never used. They are living sites belonging to people who know more than Elaine does.