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[Featherblade]

Lately I've been going across a ton of small embark sites, simply digging a stairwell straight to the magma sea. From the stairwell I can map out the rock layer sizes and depths, to see if there is space for a potential fortress beneath the last cavern layer.

What I have noticed is that in particularly deep areas (over 100 deep), the game seems to run out of rock layers to add. Essentially, after ten or twelve rock layers, I noticed filler bedrock layer will fill the rest of the world's bottom until the magma sea.

The typical rock layer tends to be within 3-7 z-levels high. However, the bedrock layer seems to expand indefinite amounts relative to the height of the surface world. I have found 50 deep granite layers, and equally deep gabbro layers, giving masses of the minerals they wield (e.g, once I kept knocking into large amounts of bismuthinite clusters, while another time in gabbro nearly every second layer breached a native gold vein).



I'm just wondering how much is known about this 'bedrock' phenomenon, or if this is the first mention of it ever. Another question that comes to mind is that is it possible for bedrocks of metamorphic rocks, and if so, could a 50 deep marble layer form?




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Related side note - With the new method of 'frequent' and 'sparse' generation of minerals, has anyone noticed that this effects the diversity of the minerals, and not so much the quantity? Sometimes the same ock won't contain certain minerals, but other times is teeming with it (e.g, horn silver, galena, native gold). If this is so, how many types of minerals/gems/metals are there allowed at different frequencies of ore generation?



This just seems like an awfully important topic to discuss, and I have seen nothing from the wiki. Any links to others mentioning of these happenings would be appreciated.

[Patchy]

I've noticed these bedrock regions too from time to time. They always seem to be of the igneous intrusive type, gabbro, granite or diorite. I've never seen them of any other layer type. My current fort has a gabbro bedrock layer of about 90 z-levels, and has 10 rock layer types above it crammed into 55 z-levels.

[NecroRebel]

Dwarf Fortress geology is loosely based on real-world geology. In the real world, the vast majority of the Earth's crust is made out of a heavy igneous rocks, especially gabbro and basalt IIRC. The other rocks, comparatively less dense, tend not to sink when the crust subducts into the mantle, so they get stuck on top of the continents. So, you get a wide variety of rocks closer to the surface, but deep down you get pretty much just gabbro. The game probably forces the lower crust to be an igneous intrusive rock, and might well choose from the very densest of those, so a marble bedrock is probably unlikely.

[Patchy]

I don't think the game chooses the densest rock, as I've seen embarks with both diorite and granite filling in that "bottom" layer. I'd bet if he changed marble to igneous intrusive in the raws, it'd be possible to see a marble bottom.

Edit: And just checked the raws. All 3 have the standard rock density set. So maybe the game does pick densest, but since they all have the same it just picks randomly.

[Featherblade]

I don't think the game chooses the densest rock, as I've seen embarks with both diorite and granite filling in that "bottom" layer. I'd bet if he changed marble to igneous intrusive in the raws, it'd be possible to see a marble bottom.

Edit: And just checked the raws. All 3 have the standard rock density set. So maybe the game does pick densest, but since they all have the same it just picks randomly.

That's interesting, although the wiki claims most stones are of the density 2670, so it probably isn't set by density. It does seem to be set from an igneous intrusive rock, which would make valuable bedrocks impossible to get.



Mineral occurrence in rocks is also important - in the game I have now, I found cassiterite (ore of tin), which is particularly rare. At the same time, there are some three or four veins of the stuff over several layers of granite in this fortress, which would be almost impossible to miss in other areas also made of granite. Yet, none of my other maps with granite have any tin at all. This suggests that only a certain number of minerals can exist in the layers of a particular biome area.

Can anyone else confirm this sort of distribution of ores?

[JmzLost]

The number of minerals in a particular biome depends on the scarcity setting.  Very scarce means few or none, frequent may have several ores, plus several gem types. (Note: I have no idea what the exact numbers are.)  When scarcity was first implemented, someone posted a screenshot of a layer that was pure marble, with no ores or gems. 

I've also noticed the "bedrock" issue.  If you run dfprospector with "-ab", you can guess what the bedrock is because it's the last material on the list (they're ordered from lowest quantity to greatest).  This will also tell you what ore(s) you're likely to find down near the magma sea ("near" being a relative term).

JMZ

[Patchy]

Well did some testing yesterday. I adjusted the density of gabbro up, diorite down and granite I left the same. Then genned a new world and went looking for the deep maps with a visable "bedrock layer" I still found areas of bedrock of all 3 types, so I'm fairly sure density doesn't factor in.

Next I changed marble from metamorphic to igneous intrusive. Regenned and went looking. Sure enough I found a bedrock area with some 60 z-levels of solid marble. So it seems that the game picks one of your igneous intrusive layer stones for the bedrock.

[Featherblade]

Sure enough I found a bedrock area with some 60 z-levels of solid marble. So it seems that the game picks one of your igneous intrusive layer stones for the bedrock.

If only that were canon... Well, I'm glad that the bedrock question is answered



My other side note was on mineral generation.

I'll do a bit of counting, and see how many crystals/minerals/ores appear at different scarcity levels in each of the layers. I believe each layer seems to pick what it wants to contain (or perhaps it's each rock type), but what I've seen is that two different stone layers that could contain the same minerals sometimes don't, even if one layer or the other has this common mineral.

I hope that makes sense, the data might help visualize what I mean.

[Featherblade]

Just looked at a few rock strata in game, and I found interesting results.


For example, I found three layers of granite within the same embark. You can compare these results (Frequent mineral generation). Two of these layers (layer two and three) are actually both bedrock layers over two biomes.



Granite layer one:
Gems: Zircon (red, black & brown), Milk quartz, Prase, Amethyst, Wood Opal, Turquoise, Honey Yellow Beryl
Metals: Tetrhedrite, Bismuthinite
Minerals: Cobaltite, Pyrolucite
Total: 10 Gem, 2 Metal, 2 Mineral, 14 total.

Granite layer two:
Gems: Goshenite, Amethyst, Golden Beryl, Red Tourmaline, Black Zircon, Prase
Metals: Native gold, Native silver, Bismuthinite
Minerals: Pyrolucite, Orthoclase
Total: 6 Gem, 3 Metal, 2 Mineral, 11 Total.

Granite layer three:
Gems: Cat's Eye, Heliodor, Blue Garnet, Turquoise, Smokey Quartz, Clear Tourmaline, Morganite, Aquamarine
Metals: Bismuthinite
Minerals: Cryolite, Pitchblende
Total: 8 Gem, 1 Metal, 2 Mineral, 11 Total.



Matching minerals:
Layer 1 & 2: Bismuthinite, Black Zircon, Amethyst, Pyrolucite
Layer 2 & 3: Bismuthinite
Layer 1 & 3: Bismuthinite, Turquoise


I think that the number of matching minerals is low enough to say that there is no relation between rock layers, same type or not. This creates scarcity and differences is resources even within each layer, regardless of the layer itself. As for global data, each rock layer had an average of 4-5 gems, 1 mineral (like cobaltite or Orthoclase) and two thirds a metal. This makes these granite layers richer than the rest of the layers: This suggests there could be some weighting agasnt or for certain rocks, OR I need more samples.

[Reelyanoob]

Can anyone else confirm this sort of distribution of ores?

DF Reveal shows how minerals are distributed: In each 1x1 embark area (48x48 Urists), you get 1 major CLUSTER per z-level, randomly picked, and 2-3 VEINS, and number (8-10?) of CLUSTER_SMALL (gems etc). I capitalized the tags from the raws here to highlight the in-game types. I had no large clusters at all when upping the Minerals value to a high number, but still plenty of VEINS and CLUSTER_SMALL it appears. There didn't ever seem to be more than two three types of veins or small clusters in any z-level, and it was per-rock-type too. Most layers had no veins or anything, but this will be different at the other end of the Mineral spectrum.

So, as in an unmodded game you're just not going to get a CLUSTER of a metal (except magnetic near the surface I think, in sedimentary layers), it's a matter of chance what exact veins you will get. e.g. 5 layers of granite should give you two vein types. The next, say 6 layers of Gabbro will have it's own two vein types. One part of my embark had only gold veins, but just as many individual veins as the other layers, so it might pick Vein Type#1, Vein Type#2 and Vein Type#3 but all can turn out the same ore type by chance.

I don't know if the RNG rolls up new vein types individually per per site or per biome or per z-level whatever. Looking over my site now, it looks quite uniform between two metals, so it may be per-site (or per stone layer). EDIT: Maxing out minerals made it a lot more diverse, but no higher max veins in a layer