Dark Sun Silt Sea... How deep?

[Elkad]

Running a Darksun campaign, and it has gotten to a point where the players basically want to create a stronghold in the silt sea, away from everyone. One of the casters has access to a sand to stone spell which creates 2 10ft cubes of stone per caster level.

The only thing that I can find is that there is a deep silt layer about 50ft down that rarely ever gets disturbed. But nothing about how deep that goes. Ideally they want to get down to bedrock. How far should that be?

It's described as round, with a large island in the center.

That sounds a LOT like a meteor crater to me. At 400 miles across, depth of a crater is probably somewhere 10 and 100 miles. But it's in a volcanic region, and described as having islands, so having rock formations that don't reach the surface, even very large ones (underseailt mountain ranges), is quite feasible.

Of course who knows what lives on the top of those rock formations, far under the silt, but not willing/able to venture into the true depths. And how they will feel about someone sticking giant stone stilts into the middle of their homes.
If the deep silt is air to them, but rock is still rock, you've just given them a tower to the heavens. With tasty adventurers at the top!