This paving slab is part of the floor of the new atrium in Kings Cross Station in London:

Kings cross paving

The paving slabs are mostly grey but some contain areas of this reddish colouration. They are all fine grained but seem to contains some bands of slightly different colours.

What is the rock and what is the source of the red colouration?

  • $\begingroup$ Most likely iron oxide staining, aka rust. $\endgroup$ – Gimelist Aug 17 '15 at 3:47
  • $\begingroup$ Why didn't you edit out that ugly cigarette butt? ;-) $\endgroup$ – Jan Doggen Aug 17 '15 at 7:00
  • 2
    $\begingroup$ @JanDoggen I thought of it as a useful scale bar ;-) $\endgroup$ – matt_black Aug 17 '15 at 9:16

My guess (without being able to crawl around through the cigarette butts and lick the floor) is that this is a mudstone or siltstone. The difference between the red and grey does not look like a primary deposition structure of different sediment layers but seems to be an oxidation-reduction (also called redox) 'front' between areas of orange oxidised iron and grey reduced iron. The front would have formed in the sediments after being formed - probably related to groundwater of different chemical composition moving through the material. Sometimes the redox change can be induced by different components in the rock - even fossils but that doesn't seem to be the case here.

Note the darker red line right at the interface between the two colours. The banding in the orange, oxidised area mostly parallels the interface in a way that couldn't happen in sedimentary deposition. The banding appears similar to Liesgang rings.


This looks like a red sandstone, with the red colour coming from iron oxides in the matrix (similar process to rust). The layers represent periods of sedimentation and have different colours due to slightly differing composition and environment of deposition.

  • $\begingroup$ The vast majority of the labs are grey not red with only a few having partial red colouration. I'd have expected a sandstone to be more uniformly red. $\endgroup$ – matt_black Aug 17 '15 at 9:17
  • $\begingroup$ I don't think the majority of the floor tiles are red sandstone, but I think some sandstone may have been included in this particular slab. It is hard to tell what the grey rock is from just the photograph, however if it is igneous (cooled magma) then it may have been intruded into the sandstone creating what you can see in your picture above. This contact may then have been made in to this particular paving slab. It is hard to say for sure without seeing the rock. $\endgroup$ – Justin Aug 17 '15 at 9:43
  • $\begingroup$ Also I am going through King's Cross tomorrow, can you remember where the slab was so I can take a look? $\endgroup$ – Justin Aug 17 '15 at 9:46
  • $\begingroup$ The entire floor of the new atrium is made up of this rock. All the paving seems to come from the same source. $\endgroup$ – matt_black Aug 17 '15 at 9:53

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