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1017 Views 19 Replies Last post: Jan 14, 2014 9:32 PM by Tabfish RSS
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Dec 25, 2013 11:07 PM

Can bad snow/rain damage fossils?

Hi,

 

 

I was just wondering, can bad weather damage fossils in a short space of time?  I was wondering because when there has been bad snow, there is loads of fossils, but they are always very fragile and it's hard to get them out of the ground without damaging them, but in dry conditions, they are really strong fossils, which are much harder than they are then when they have been in bad weather conditions.

 

 

Is it just a coincidence or can bad weather damage fossils in a few hours, or do things like erosion take years?

  • Currently Being Moderated
    Dec 25, 2013 11:23 PM (in response to Dan)
    Re: Can bad snow/rain damage fossils?

    Hi Dan

    My own opinion is that nature always takes back the fossils that it has made, especially were we collect on the Holderness coast.

    Nature has one big advantage, Time.

    It's hard to comprehend time over millions of years.

     

    Tabfish

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    • Currently Being Moderated
      Dec 26, 2013 8:28 AM (in response to Tabfish)
      Re: Can bad snow/rain damage fossils?

      Dan,

       

      "Can bad weather damage fossils in a short space of time?"

      Yes it can.

       

      With the fragile fossils you mention, they are often in friable clays or siltstones, which may have micro-cracks. When we try to remove a fossil, it will often break (or already be broken) along one of those micro-cracks. ...Or the fossil may be sufficiently weak that it will break almost anywhere.

       

      During rains, erosion will gradually remove particles and in doing so, fossils will become exposed or more-exposed. If such a fossil is already broken (by micro-cracks) or very weak, it may break simply through removal of the particles of rock that were supporting it.

      During summer drought, the clays/siltstones may shrink significantly. That can exaccerbate the cracking - breaking fossils in-place and allowing rainwater to penetrate deeper next time it rains. That is, there can be damaging shrink/swell cycles in the rock.

       

      During freezing conditions, if the rock is wet, the ice will expand in the micro-cracks and prise little chunks of the rock apart - perhaps breaking fossils in the process, the pieces later falling away when the ice melts.

      Repeated freezing-thawing cycles can have a significant erosional effect, even on much harder rocks. In such cyclic cases, the ice wedges force cracks to open wider and wider, which can result in even large blocks of rock  becoming loose. In mountains above the tree line, this sort of erosion is responsible for the jagged appearance of the rocks as a whole (in combination with other factors).

       

      Fossils can be harder when they are dry either because the rock supporting them is harder, or because they are strengthened by a temporary cement. Let me explain that last bit... In some rocks, the cement holding the particles together is rather soluble in water. When the rocks are subjected to cycles of wet and dry, water soaks in and dissolves the cement as it goes, then as it dries, the fluid migrates to the surface, where the dissolved cement may partly re-precipitate. That results in two things: a) the rock perhaps including fossils becoming harder (than when wet) at the surface; b) the slightly deeper rock becoming weaker because the cement is gradually removed. This whole process can result in hollows forming behind the surface, giving a strange honeycombed effect. If the rock contained veins of different, less-soluble, minerals, they are likely to remain, forming at least some of the walls of the honeycomb.You can see that on some coastal cliffs.

      Some examples - http://www.geopoem.com/2013/03/stone-lace.html

      (I should say that is a simplification: there are several processes involved, and there is much about them that we still don't properly understand.)

       

      Time scale:

      Erosion operates on a wide range of time scales.

      There are types of erosion that are extremely slow - maybe taking thousands of years to remove 1mm thickness of rock.

      Other types can change things extremely quickly - eg. landslides may remove many metres thickness of rock in a few seconds.

       

      Mike

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