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[Keldor]

The actual depth of the cavern layer varies greatly, and seems to be influenced by the local topology.  Hence, on a flat plain, the caverns will be near the surface, while in an area with steep slopes, they will tend to be many, many Z levels down.  Try generating a world that's completely flat.  Then the caverns will be only about 5 Z levels down.  However, on a mountain, it's more like 50 Z levels down, or more.  The distance between the layers also grows and shrinks proportionally.

[G-Flex]

Generally, the upper levels are where you'll find the richest veins of ores; down in the caverns, it's a lot of really nice stuff like native gold and other precious metals/gems, but the upper levels are where you'll find your iron ore.

I sincerely doubt the game weights things like this. I'm pretty sure that the density/identity of minerals is based solely on the layer stone they appear in; there's nothing in the raws (or anywhere else) to suggest otherwise, to my knowledge. You're probably finding a greater proportion of iron ore in the upper layers simply because magnetite occurs in large clusters, and those get broken up more by caverns.

[Urist Imiknorris]

Generally, the upper levels are where you'll find the richest veins of ores; down in the caverns, it's a lot of really nice stuff like native gold and other precious metals/gems, but the upper levels are where you'll find your iron ore.

I sincerely doubt the game weights things like this. I'm pretty sure that the density/identity of minerals is based solely on the layer stone they appear in; there's nothing in the raws (or anywhere else) to suggest otherwise, to my knowledge. You're probably finding a greater proportion of iron ore in the upper layers simply because magnetite occurs in large clusters, and those get broken up more by caverns.

Iron is found mainly in the "shallows" because it only occurs in sedimentary (and for hematite igneous extrusive) rock, which is generally only found near the surface. Gold is found in all igneous rock, and the deeper levels of the map have mainly metamorphic and igneous intrusive rock.

[G-Flex]

Generally, the upper levels are where you'll find the richest veins of ores; down in the caverns, it's a lot of really nice stuff like native gold and other precious metals/gems, but the upper levels are where you'll find your iron ore.

I sincerely doubt the game weights things like this. I'm pretty sure that the density/identity of minerals is based solely on the layer stone they appear in; there's nothing in the raws (or anywhere else) to suggest otherwise, to my knowledge. You're probably finding a greater proportion of iron ore in the upper layers simply because magnetite occurs in large clusters, and those get broken up more by caverns.

Iron is found mainly in the "shallows" because it only occurs in sedimentary (and for hematite igneous extrusive) rock, which is generally only found near the surface. Gold is found in all igneous rock, and the deeper levels of the map have mainly metamorphic and igneous intrusive rock.

Right, although for what it's worth, igneous extrusive stone can appear deep as well.

So yeah, I guess the original statement was true to some degree, since sedimentary stone only appears up top it seems.

[Djohaal]

Ok here's a complete (I hope) list of underfeatures you can have in DF 31.03

1: Magma pipe, which starts from the magma sea and piereces all the way up to one of the three cavern layers, most commonly to layers 2 and 3 . If it pierces all the way to the surface it digivolves into a volcano.

2: Underground lakes, which can be limited or connected to the corner of the map. Connected lakes refill as time passes, while limited ones can obviously be drained.

3: Chasms, which are round like hollow magma pipes that connect two cavern layers or more. They don't have bottomless tiles in their bottom so they make poor dump zones. A magma pipe can also continue above the magma level as a chasm. Chasms don't seem to ever breach into the surface.

Spoiler (click to show/hide)

4: Demonic fortress: A slade tower that goes all the way from hell to the middle cavern layer, it has an adamantine weapon in the floor somewhere in the middle, which if removed opens the glowing floor beside it unleashing hell upon you. Its upper (demon free) layers are usually filled with undead friends.

5: Paths to lower levels, that are discrete one tile ramps that connect layers 1 and 2 or 2 and 3.

6: Caves, which are similar to the 40d caves, have a megbeast inside usually and often filled with loot around. They also are passages to cavern layer 1

For Z-layer thickness, it seems toady used an average volume system for making caverns. If you have the passage worldgen parameter set at 0-100% and openess at 0-0 you get very tall caverns made of spaghetti-like bendy passages, which can span anything from 5 to 20 layers or more. If you set openess to 0-100% and passages to 0-0% you get very wide spreading flat caverns that span at most some 6z levels, but don't have the annoying spaghetti-passages. It seems the total volume of a cavern section made of passages or openess is the same, but open caverns are flatter since they have less "noise" from the passages. The total thickness of your underground relies on the cavern height span. Worlds made of 0-100% openess caverns and no passages (0-0%) are quite flat and average 30 to 50 z levels between surface and the magma ocean (and beyond). Default DF worlds have an average of 120 to 150 (way more in some unusual sites) Z levels between surface and the magma ocean.

[G-Flex]

Yeah, the "chasms" aren't really chasms, more like vertical connections between layers so that they're actually contiguous.

The non-surface-piercing magma pipes you speak of are the magma pools I mentioned earlier. Since they reach the bottom, you can effectively call them "pipes", but the game still calls them "pools". They don't seem to appear as often as they should, though.

[Urist Imiknorris]

Ok here's a complete (I hope) list of underfeatures you can have in DF 31.03

1: Magma pipe, which starts from the magma sea and piereces all the way up to one of the three cavern layers, most commonly to layers 2 and 3 . If it pierces all the way to the surface it digivolves into a volcano.

2: Underground lakes, which can be limited or connected to the corner of the map. Connected lakes refill as time passes, while limited ones can obviously be drained.

3: Chasms, which are round like hollow magma pipes that connect two cavern layers or more. They don't have bottomless tiles in their bottom so they make poor dump zones. A magma pipe can also continue above the magma level as a chasm. Chasms don't seem to ever breach into the surface.

Spoiler (click to show/hide)

4: Demonic fortress: A slade tower that goes all the way from hell to the middle cavern layer, it has an adamantine weapon in the floor somewhere in the middle, which if removed opens the glowing floor beside it unleashing hell upon you. Its upper (demon free) layers are usually filled with undead friends.

5: Paths to lower levels, that are discrete one tile ramps that connect layers 1 and 2 or 2 and 3.

6: Caves, which are similar to the 40d caves, have a megbeast inside usually and often filled with loot around. They also are passages to cavern layer 1

For Z-layer thickness, it seems toady used an average volume system for making caverns. If you have the passage worldgen parameter set at 0-100% and openess at 0-0 you get very tall caverns made of spaghetti-like bendy passages, which can span anything from 5 to 20 layers or more. If you set openess to 0-100% and passages to 0-0% you get very wide spreading flat caverns that span at most some 6z levels, but don't have the annoying spaghetti-passages. It seems the total volume of a cavern section made of passages or openess is the same, but open caverns are flatter since they have less "noise" from the passages. The total thickness of your underground relies on the cavern height span. Worlds made of 0-100% openess caverns and no passages (0-0%) are quite flat and average 30 to 50 z levels between surface and the magma ocean (and beyond). Default DF worlds have an average of 120 to 150 (way more in some unusual sites) Z levels between surface and the magma ocean.

@5: All the ones that I've found are two tiles.

7: Adamantine pipes.

[Vercingetorix]

Right, although for what it's worth, igneous extrusive stone can appear deep as well.

So yeah, I guess the original statement was true to some degree, since sedimentary stone only appears up top it seems.

I did hit a small amount sedimentary rock in a cavern that was fairly open; it appears that there might be some collapsing involved that pushed the sedimentary layer further down.  I have a feeling the openness of caverns affects the way things wind up being distributed.

Either that or it was just a quirk caused by overlapping biomes.