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I have this construction site right outside my house, which I would like to give you some geographical background about before asking my question. Have a picture first though

Construction Site

There used to be industrial buildings of red clay bricks like the piece of wall still remaining. These walls, however, had a white stone facade around them so I'm not too sure the wall still standing was indeed part of the structure before demolition.

The city I live in is Dresden, Germany and the district I live in was a major bomb dropping zone during WWII. For some reason in my district they decided to just let some walls of otherwise vanished old structures be and use them to mark estate borders, or to just be there really, which looks quite beautiful in some cases.

Seeing this construction site I cannot help but notice this black layer of earth under what seems to be a layer of rubble. Even though its not that noticeable on the left side, its continuous throughout the hole and starts appearing at the same height.

As I said before, this area used to be exposed to heavy bombardment. So now I'm wondering if that might be the "fire layer" where the excessive heat hast left its mark in the ground.

Or is it, perhaps, a natural occurrence in slightly deeper depths, for the soil to turn that dark?

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    $\begingroup$ A really good way to answer this question, which probably needs a site inspection to be sure, would be to do a soil decomposition test. Get a large jar, and crush up some soil as fine as you can (might help to dry it out first), then fill the jar about 1/3 full of soil, and then fill almost to the top with clean water. Now shake the bejeesus out of it, until it's mixed really well. Then sit it down somewhere, and wait for it to settle. Once it's settled, you'll be able to get a basic estimate of the composition - gravel at the bottom, clay at the top, and hummus below the clay or floating. $\endgroup$
    – naught101
    Sep 9, 2014 at 0:47
  • $\begingroup$ A close up image of that bank would be useful, too. $\endgroup$
    – naught101
    Sep 9, 2014 at 0:49
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    $\begingroup$ Being a professional landscaper, my instinct would be to do like I always do, and send a sample to a lab for analysis. I do it to fnd the pH and nutrient levels, and organic matter content, but you could get a complete analysis of all the composites of that particular sample. $\endgroup$
    – J. Musser
    Sep 9, 2014 at 18:15
  • $\begingroup$ Hi thanks for the suggestions. Unfortunately I have no way to access the construction site, so getting samples is not really an option. $\endgroup$
    – Spraygun
    Sep 11, 2014 at 23:52
  • $\begingroup$ @Spraygun Why can't you walk in? I'm assuming that equipment didn't spawn in there. Or maybe, you'd have to ask first. $\endgroup$
    – J. Musser
    Sep 16, 2014 at 0:39

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Regarding your question:

So now I'm wondering if that might be the "fire layer" where the excessive heat hast left its mark in the ground.

The short answer is no. The layer seems to be about 1 meter thick. No bombardment related fire can cause blackening 1 meter deep.

Now, why is it "black"? First of all, it's not black. It's dark brown. I've seen black soils, and trust me - they are pitch black. There are several reasons for it. My best guess is that it's simply moist. You know how things appear darker when wet? Same here. My hint is the floor of the site. I'm guessing it's the same material, but it's drier so it looks lighter.

There are more reasons, some concerning the organic content of the soil (for example, peat - a highly organic black soil) or the mineral content of the soil (soil composed of high iron contents will also appear black).

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  • $\begingroup$ I agree that on the picture it looks more like a darker brownish layer. My camera does not seem to be working that well. In real life it is even darker, to the point of being black. However, what you're saying about the thickness of the layer makes sense and I thought about it too. The city was burning for days with bombardment being stretched over a period of days also, so I thought the already disturbed earth might make it easier for explosive material to enter deeper grounds, hence the thickness. Anyway, if your expert opinion is that it is not so, I'm fine with it also :D $\endgroup$
    – Spraygun
    Sep 12, 2014 at 0:00
  • $\begingroup$ Michael, In my area, the soil contains lots of iron. Rather than being darker, it is red, from oxidization. Humus shouldn't be found in a layer like that, unless some excavator in the past had been clumsy enough to bury the topsoil. In my area, some areas of subsoil varies from the regular red or orange, and are darker, like what is pictured above. I think it's mostly shale, in those areas. $\endgroup$
    – J. Musser
    Sep 16, 2014 at 0:38
  • $\begingroup$ I meant iron as in iron bearing minerals, mostly shales. Not proper "iron minerals" such as hematite and various iron oxy-hydroxides. $\endgroup$
    – Gimelist
    Sep 16, 2014 at 3:49
  • $\begingroup$ After it was raining all day yesterday the "blackness" of the layer was a lot clearer to see. Directly underneath the rubble and quite distinct from the darker brown underneath it. Unfortunately I didn't have my camera operational, but I'm hoping tomorrow it's still as visible as today and that I get a better shot of it. $\endgroup$
    – Spraygun
    Sep 17, 2014 at 0:45
  • $\begingroup$ How thick is the black layer and is this thickness constant throughout the site? $\endgroup$
    – Gimelist
    Sep 17, 2014 at 5:06

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