Announcement

Collapse
No announcement yet.

Really dumb geology question

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

    Really dumb geology question

    If you see the map of the UK



    then the exposed rocks reveal a history of sedimentary stuff being laid down over hundreds of millions of years, broadly getting older the more north-west you go from Dover. So chalk, on top of limestone, on top of older stuff, etc. Devon's "Jurassic coast", for example, that would be what, 70 million years old. The Carboniferous stuff in the north west, where the coal and oil is, 300 million years old.

    Okay, here's my really dumb question - these are all sedimentary rocks, so must have been laid down on top of each other, layer on layer on layer. It's 300 miles from here (Lancashire) to Dover. At some point, did Britain tip up on end to the point where 300 miles' worth of bedrock flipped up in the air? How or what caused that? And what's happened to all the more recent strata that are no longer above me, or the Peaks, or the midlands, etc?

    #2
    Really dumb geology question

    something to do with erosion by ice / weather?

    Comment


      #3
      Really dumb geology question

      I don't know the answers to your questions, Rogin, but I can be an irritating pedant and point out that if the "Jurassic coast" were only 70m years old, it would be very badly misnamed:

      http://en.wikipedia.org/wiki/Jurassic

      Comment


        #4
        Really dumb geology question

        That jarred with me, too. I assumed I'd missed something.

        Comment


          #5
          Really dumb geology question

          Yeah, the difference is mainly erosion caused by sea levels, flood plains, continental drift (the UK was squished together about 400 - 500 million years ago - talk about your North/South divide) and mainly ice. Glaciers are big heavy abrasive things.

          Comment


            #6
            Really dumb geology question

            This is good.

            Comment


              #7
              Really dumb geology question

              Yeah, I get continental drift, but to reiterate then, at some point in the last - 50m - years? something has happened to tip all of these previously buried sedimentary strata up to the surface, and more importantly expose and erode the layers that were on top.

              What exactly was that event? What could tip our entire island up on edge by 30% or whatever it is? Did it happen suddenly, as a cataclysmic thing, or gradually?

              Comment


                #8
                Really dumb geology question

                They aren't necessarily previously buried. There isn't always a 100m year old stratum on top of a 200m year old one, because many areas haven't been submerged in that time. If some rock was laid down 300m years ago, and never had ocean over the top and never had huge volcanic outflows onto it, and was never glaciated away, then there's still 300m year old rock on the surface.

                (Also, younger rocks tend to be softer, having not been compressed over time, so younger rock tends to erode quicker from under fluvial and glacial flows)

                Comment


                  #9
                  Really dumb geology question

                  A combination of a lot of the above, you'll likely find Rogin. Partly you'll note from hobbes' video (right at the start) that the north-west British Isles were partners to Canada, as opposed to the rest of the islands, so they have a naturally different geological makeup. Then during the Pleistocene ice age (from 2million to around 10,000 years ago) the northern half of the British Isles (roughly speaking) was heavily glaciated compared to the south. Having a whacking great ice sheet on top of half the island also resulted in isostatic depression (basically compression and depression of the land relative to sea level, the same thing Antarctica experiences), which I guess you could characterise as 'tipping' the island.

                  As LLG notes, as glaciers retreat (as they did at the end of the Pleistocene, a relatively recent event in geological terms) and glacial outflows take place, younger sedimentary rock is then easily eroded (the glacial till that forms the coastline of east riding of Yorks, for example, has been eroded by the sea relatively quickly since, say, Roman times, than other parts of the UK's coastline).

                  Comment


                    #10
                    Really dumb geology question

                    So the sands and mud of Morecambe Bay are the same as the hills of the Chilterns and Downs further south, and have been scraped away and washed there by glacial retreat?

                    I think I'm getting it, now. No, seriously, I think I am. Apart from the bit about us being "tipped" on edge to start with. But getting there. Thanks.

                    Comment


                      #11
                      Really dumb geology question

                      The geology of Britain is very unique in that it spans almost the entire history of the Earth. There are sedimentary rocks in Scotland that are approx 3 billion years old. These are some of the oldest known sedimentary rocks.

                      The large variation in visible surface geology in Britain is (as far as I know) due to the Caledonian Orogeny. Orogeny means mountain bulding and Caledonia is Scotland, of course. This happened waaay before the Antlantic Ocean had started to form, and Europe and North America were basically pressed up against each other causing mountains to form. The mountains of Scandinavia and the Appalachian mountains were a result of the same process.

                      Before this mountain building process, all the layers of original low lying and ocean sediment were once laid down in horizontal layer upon horizontal layer, like a very long lasagna. Then, due to two continents colliding during plate tectonics, these layers became squashed from the sides and forced upwards to form very large mountains. Imagine if you took a very long lasagna and squashed it from both ends; all the layers would no longer be horizontal, they would be in evey which direction, sometimes completely vertical. These Caledonian Orogeny mountains, when newly formed, would have been really huge, like the Himalayas today (which are quite new by geological standards). But, over hundreds of millions of years, they have been weathered away by rain and wind. So now imagine if the same weathering process happened to your squashed up lasagna and weathering lopped the top off it. All the original layers would be visible to an ant walking over over the new top surface of the lasagna...

                      Younger geology (from dinosaur times onwards, White cliffs of Dover etc) in Britain is the result of various higher sea levels than today which deposited marine sediment in the shallower areas of Britain. That's why you see it in the flat parts of England but not in Scotland, where the elevation was (and still is is) higher. Also, the ice sheets (which are like huge erasers) weren't as intense in the south of England, so the softer and more recent dinosaur times stuff still remains. Then you have, in some areas, the top stuff on top of that, which is basically the sand, muck and mud that was deposited more recently (past few millions and thousands of years).

                      That's all of the top of my head so liable to be wrong.

                      Comment

                      Working...
                      X