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4345 Views 6 Replies Last post: Dec 14, 2013 10:01 PM by Dan RSS
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Dec 13, 2013 4:31 PM

Millstone grit formation?




I have been doing a lot of research lately, trying to figure out what the formation is where I find my fossils, in Ossett (North England, West Yorkshire.). I know the fossils are carboniferous in age, but I aren't surtain what the rock formation is.  It is in the right place for Millstone grit, but I don't know what it looks like.  The area where I find the, is a combination of hard sandstones and semi sandstones.





Any idea what the rock formation could be?  (I have attached some images of some rocks from Ossett.)

  • Currently Being Moderated
    Dec 13, 2013 9:42 PM (in response to Dan)
    Re: Millstone grit formation?




    It depends exactly on location. Ossett itself is on the Pennine Coal Measures Group. The Millstone Grit Group is to the west, appearing roughly on a line through Halifax and Huddersfield.



    But step back a moment. It is important point to understand the difference between lithology and stratigraphy.


    There are various types of stratigraphy, eg. lithostratigraphy, biostratigraphy, chronostratigraphy.

    They are all concerned with the stratification of rock, but they look at it in different ways.

    Lithostratigraphy is concerned with rock types (lithology). Biostratigraphy is concerned with the fossils (palaeontology). Chronostratigraphy is concerned with time (chronology).


    The Pennine Coal Measures Group and Millstone Grit Groups are lithostratigraphical units.


    You use the term 'formation'...

    'Formation' is a lithostratigraphical term, used to denote part of a Group.

    The Pennine Coal Measures Group, for instance, comprises:

    - Pennine Upper Coal Measures Formation

    - Pennine Middle Coal Measures Formation

    - Pennine Lower Coal Measures Formation


    The Pennine Coal Measures Group is a lithostratigraphical division, not a lithology. As such, it can comprise a mixture of rock types. In its particular case, it comprises sandstone, grey siltstone, grey mudstone, coal and seatearth. The PCMG occurs in the Westphalian Stage of the Carboniferous Period (chronostratigraphy).


    The Millstone Grit Group (which includes a Millstone Grit formation) occurs in the Namurian Stage of the Carboniferous. It comprises mainly a coarse sandstone (lithology).



    So, back to your question...


    Your specimens could tell us about the lithology (better if they included a scale), but not really about the lithostratigraphy. That's because some similar sandstones occur in both the Pennine Coal Measures Group and the Millstone Grit Group.


    To decide which lithostratigraphic unit the specimens come from, you need to:

    1. know if they were in situ (eg. in a cliff) or ex situ (eg. loose in a river bed)

    2. find the location on a detailed geological map

    If ex situ it would be hard to be 100% sure. They could have been transported a long way by glaciers and/or rivers, possibly with man's influence as well.

    For an online geological map, I refer you to

    There, you can enter Ossett into the 'Go to location' location search, then adjust the transparency slider so you can see the towns and roads, then find your location(s), then slide the transparency to show the geology again (preferably bedrock and superficial), then click on the locality and wait for the popup to show the details.






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