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...
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).