How does Move Earth work?

Question

Move Earth is a 6th-level 2-hour concentration spell useful in landscaping. Before investing in learning it, I would like to understand the uses and limitations a little better. The spell says

Choose an area of terrain no larger than 40 feet on a side within range. You can reshape dirt, sand, or clay in the area in any manner you choose for the duration. You can raise or lower the area's elevation, create or fill in a trench, erect or flatten a wall, or form a pillar. The extent of any such changes can't exceed half the area's largest dimension.

and

At the end of every 10 minutes you spend concentrating on the spell, you can choose a new area of terrain to affect.

What happens to the originally transformed area, does it remain reshaped while you concentrate on another area? Or will it revert? If it stays in form and you choose another area, can the new area be one you already affected before, so you could continue piling on effects?

Answer 1

Reshaping is permanent

My read of the spell is that "for the duration" here applies to the active reshaping of the earth, not the new-shape-of-things, which is essentially mundane. It's a magical backhoe and bulldozer.

That is to say, for the duration you can shape the earth, and the changes you've made will remain forever afterward (or at least until altered by the natural forces of wind, water, and gravity -- or a guy with a shovel and sufficient time). That's the point of the spell, to be able to manipulate the landscape at your whim, to build defensive earthworks, undermine a structure, or alter the landscape around your home. If the changes only lasted for the 2 hour duration, this wouldn't be worth a sixth level spell slot, and the spell would need to be a lot more clear about how the earth reverts to normal after the duration expires. The fact that the spell makes no mention of the affected earth moving back to its original shape is strong evidence that the developers never intended that to be the effect.

You can stack effects

To answer your other question, you aren't required to pick a "new area" every ten minutes -- it says you can choose a new area, not that you must. So yes, you can continue to reshape the same 40x40 area or pick a new area to reshape. And as far as I can see, there's no reason to think you can't overlap the areas, so when you pick a new area it could be half of the old area plus another 20 feet, or whatever. If that means you can make a wall taller and taller or dig a deeper and deeper trench, well, that seems to be the intent, so long as you spend 10 minutes per 20 feet of movement. However...

Impossible landscapes

There's an interesting question of what happens if you shape the earth into an unsustainable form. If you bring sand up into a 20 foot pillar, that obviously can't just stand on its own. A sixty-foot-tall earthen wall or 200-foot-deep trench can't possibly retain its shape under the force of gravity; it will collapse into a less extreme angle unless supported by a solid object. (This is called the "angle of repose" and it's a major factor in a number of civil engineering problems.)

I'd almost certainly rule that the changes remain as long as the spell is running on the area in question, but when the spell ends or you move the target area so that it doesn't include that spot, ordinary physics reasserts itself. At that point the pillar collapses into a heap, the wall slumps, the trench falls in, and so on (but to fit with the spell, I'd have to rule that the magic releases slowly enough that it still can't trap creatures or damage them by dropping stuff on their heads).

Answer 2

You can affect different areas every 10 minutes, and the previous ones remain reshaped until you end concentration.

The spell description says (emphasis mine):

Choose an area of terrain no larger than 40 feet on a side within range. You can reshape dirt, sand, or clay in the area in any manner you choose for the duration.

The changes in one area occurs during 10 minutes, in which the caster has to concentrate. After these 10 minutes, they can shape another area with a side of 40 ft:

At the end of every 10 minutes you spend concentrating on the spell, you can choose a new area of terrain to affect.

Since the concentration keeps on, the previous area still remains reshaped: indeed, the description says every 10 minutes you spend concentrating on the spell, not on a particular area.

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Note anyway that a strict reading of the description implies that after two hours the reshaping stops and it is not permanent: the duration is 2 hours, hence the reshaping last just two hours. There are no indication about how the reversion to the original shape occurs, so it is up to the DM how to rule it. If I were the DM, I would rule that the reversion is slow, in a similar way as the modification occurred.