the length into the wall

No mortar. That’s the first thing to get, and it’s the whole thing. The wall does not hold together because something was poured between the stones. It holds because each stone leans its weight into the ones below and the ones below lean back, and the argument of all that weight, resolved, is a wall. Friction and gravity. Nothing wet, nothing that dries.

The beginner’s mistake is to lay a stone the pretty way — the long flat face out, so the wall looks like a wall from three feet back. Tracing, they call it. The waller stops you. Turn it. The length goes into the wall, not along it, so what you see from the field is the stone’s short end, its knuckle. You’re stacking firewood you’ll never burn. A wall built the pretty way is a skin of stone pretending, and the first hard winter finds the pretense and opens it.

Behind the two faces, the hearting. This is the part no one photographs — the gut of the wall, packed with the stones too shapeless to face with. And here the instinct is wrong again: you want to pour in gravel, chinking, handfuls of the small stuff, fill it like sand in a jar. Don’t. Fewer larger stones, each one set by hand into the exact void it fits, tight enough that it carries load and doesn’t just sit there. A wall hearted with gravel is a wall waiting to slump. The inside you can’t see is doing more of the work than the faces you can.

Then, at about the height of your knee, a throughstone. One stone long enough to reach clear across, touching both faces at once, tying the two skins into one body. Without them the wall is two walls standing back to back, agreeing to lean on each other until they don’t. The throughs are the wall admitting it’s a single thing. You set one about every meter along, and you can feel, when you set them, the wall stop being a stack and start being a structure.

The faces tilt in as they rise — the batter, an inch back for every six up — so the whole thing is a long shallow A, wider at the ground, gathering toward a top course that a passing sheep can’t knock. Cross the joints as you go: every stone bridges two below it, so no seam ever runs straight up through more than a course. A seam that runs is a crack that hasn’t happened yet.

The maxim the old wallers keep is: handle a stone once. You look at the gap, you look at the pile, you pick the stone that is that gap’s shape, and you set it, and it stays. Pick it up, turn it, set it down somewhere else, and you’ve spent the stone’s willingness — you’re fighting it now, wedging, shimming, breaking off a corner that the next frost will find. The good waller is not strong. The good waller is unhurried enough to see, before lifting, which stone the wall is already asking for.

They last centuries like this. No mortar to fail, nothing to crack and let water in and freeze and split. When a dry wall does finally go, it doesn’t shatter — it slumps, quietly, stone settling back toward stone, and a waller comes and takes it down to the sound course and builds it back up out of the same stones. The wall was never the stones. It was the leaning. The stones are just what happened to be lying in the field.