The ground that changes while you watch: what the slope loses, the valley keeps
For most of its life the land in The Long Watch aged behind your back. Water ran downhill and wore the ground thinner, but you only found out about it the next time the world opened. Now a slope reshapes while you are standing in front of it, and the rock the water lifts off it is set back down somewhere you can walk to.
Sit above a runnel and give it sixteen beats of the world’s clock. At the setting we ended up shipping, the worst-affected patch of ground under you will have moved about a fifth of a voxel: a fifth of one of the blocks the world is built from, less than the width of a hand. You will not see it move. What changed this week is that you no longer have to leave the world and come back to learn that it did.
The land used to be regrown, not remembered
A world here is never stored as finished scenery. Every time you open it the land is regrown from its seed, with your shaping replayed on top, and everything else that has happened since is layered back on with it. That is why the carving could work and stay invisible for so long: the erosion record was faithfully kept and faithfully re-applied, but the ground already drawn in front of you had been drawn before any of it, and nothing ever went back to tell it otherwise. How slow that carving is on purpose, and why we wanted it slower than you can catch, is where the carving started.
So we built the missing seam. Ground already on screen is now rewritten in place, a patch at a time, as its erosion drifts. A patch is left alone until it has moved at least half a voxel since it was last redrawn, and the redrawing is dribbled across many frames rather than swept over the whole world at once — the pattern this borrows — so the land reshapes without the game ever hitching. The awkward case was the lip of a cliff whose neighbour had moved but which had not: it would sit a few voxels out of date, a strip of stale edge you could actually see, until we gave every patch a marker saying it still owed a redraw, and made it hold that marker until it got one — the engineering account of that stale edge goes further into it.
What the water takes, it sets down
The older half of this always promised silt settling downstream. Making the carving visible raised the question we had been able to ignore while it was too fine to see: where does the rock go? For a long time the answer was that it went nowhere. The ground was lowered and the transaction ended there — fine arithmetic, poor geology. A hillside that wears down has to put its material somewhere.
So carved rock became a thing in its own right: suspended material, lifted into the water and carried by the same flow that lifted it, handed downhill to the lowest neighbouring ground and dropped there. Carve, carry, settle. The three sum to zero by construction rather than by luck, so the world holds exactly as much rock as it started with, for as long as it lasts. The hand-off is built so that a flowing cell always has somewhere in bounds to give its load to, which means nothing ever washes off the edge of the map.
The ground is never destroyed, only moved. What one slope loses, another gains, and the total never changes.
That turns erosion from a wearing-away into a redistribution, and it is the same shape as something the world already does with fertility, where a dead plant’s nutrients go back into the exact ground it grew on rather than leaving the books. Valleys fill. Deltas build at the mouths of the channels the water has cut. A coast slumps in one place and thickens in another. The land had been weathering since before anything lived on it; now the weathering keeps its accounts.
One dial between a dent and a gorge
That left a single number to settle: how readily carried rock drops back out of the water. It sounds like a detail of bookkeeping, and it turns out to govern the whole character of the land.
Set it low and material travels a long way before it settles, so the water carves deep and reaches far. The process also feeds itself at that end: a steeper slope erodes harder, which makes it steeper, which makes it erode harder still, and a landscape given long enough runs away into gorges. Set it high and the rock drops close to where it was picked up, so the land dents itself a little and then calms down. Between the softest and harshest settings we measured about twelvefold difference in how much ground moved over the same stretch of time, and every setting in that range conserves rock exactly. The choice was never about correctness. It was about what kind of country you want to be standing in.
Three separate readings were taken, each asking a different question: whether a setting stayed faithful to what we had written down about erosion, whether a player would actually notice it, and whether the land it produced looked stable and natural rather than melted. They came back at three tenths, a half, and seven tenths of the way along the dial. We kept the middle, the crossover where the ground visibly changes and still refuses to run away with itself. At that setting a stretch of low ground carries about five voxels of built-up material by the hundred-and-twenty-eighth beat: a mound you would notice, arrived at without a single lurch.
Rock that is still in the water when you close the game
Conservation only holds if it survives being put down. Material in transit is real ground temporarily off the land; forget it on the way into a save and the world comes back lighter than it went in, its ledger quietly reset with nobody the wiser. So a saved world now carries its suspended material too, a couple of hundred kilobytes on top of the carved ground already written there — and why the carved field had to be saved at all is a story of its own. Close the game mid-carve, and the rock that was in the water is still in the water when you return.
What a visible change broke
Erosion large enough to see reached places it had never reached before, and one of them was a shortcut. When the world builds a block of ground it has a fast way of deciding that the whole block is solid rock, or the whole block is empty air, without looking inside: it works out where the surface must be and rules the block out. That reasoning still described the land as the seed grew it, before any water touched it. Once deposition got five voxels deep, the shortcut started being wrong, and a new mound could be sliced flat or a carved valley filled back in the moment that ground was next built. Folding erosion into the judgement fixed it — the same shortcut was handing the land back wrong on reload as well.
The last check was the horizon. A carving made near the camera has to reach the ground drawn coarser with distance too, or a bank would be worn away close by and pristine a few hundred metres out. It gets there.
The line in our goals for this stage of the project is a short one: a mature world should have a forest that has aged, a meadow grazed flat, and a coast that has eroded. The coast was the hard third of that sentence, because a coast that has eroded only means anything if the ground it lost went somewhere, and if someone could have been standing there to watch it go. Both are true now. The rest is time — and time is the one thing this game has in abundance.
