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Description
Pentz introduces the idea that the present is the key to the past. To support this view he poses some questions about fossils. By studying the modern environment we have clues as to how fossil sedi
...ments were formed.
Wilson emphasises the view that the present is the key to the past by showing that the rocks are only a static record of a period in geological time.
| 26 | 00:02:01,000 | 00:02:05,000 | And then when that’s over the sediment has separated out… |
| 27 | 00:02:06,000 | 00:02:09,000 | ..into its different grain size components on each sieve. |
| 28 | 00:02:10,000 | 00:02:14,000 | The next step then is to take these off the sieve and weigh each one… |
| 26 | 00:02:11,000 | 00:02:18,000 | Well, let’s see if one’s in here. Yes here we are. |
| 29 | 00:02:15,000 | 00:02:20,000 | ..in turn and this gives us a grain size distribution by weight of each grain size. |
| 27 | 00:02:20,000 | 00:02:22,000 | You can see that it’s about two or three inches long but when it’s in… |
| 30 | 00:02:22,000 | 00:02:24,000 | Now, having obtained that data the next thing to do is… |
| 28 | 00:02:23,000 | 00:02:27,000 | ..the burrow it can extend up to two or three times this length… |
| 31 | 00:02:25,000 | 00:02:28,000 | ..to plot it up so that you can compare different sediments. |
| 29 | 00:02:28,000 | 00:02:31,000 | ..and by expanding and contracting, it’s able to move about. |
| 32 | 00:02:29,000 | 00:02:32,000 | The simplest way of doing this is to plot a histogram as we’ve done here… |
| 30 | 00:02:32,000 | 00:02:36,000 | It takes in sediment and digests out the organic material. |
| 33 | 00:02:33,000 | 00:02:37,000 | ..for the dunes sediments. This is a rather different kind of histogram, |
| 34 | 00:02:38,000 | 00:02:42,000 | say to a rainfall bar chart, because what we’ve done is filled each tube with… |
| 35 | 00:02:43,000 | 00:02:49,000 | ..sediment to a height proportional to the weight that was retained on each sieve size. |
| 36 | 00:02:50,000 | 00:02:54,000 | On this side we have sediment less than 0.5 of a millimetre in diameter, |
| 37 | 00:02:55,000 | 00:02:59,000 | it increases in grain size over here, and if this bottle had anything in it, |
| 38 | 00:03:00,000 | 00:03:03,000 | the grain size would be greater than 1.5 millimetres. |
| 39 | 00:03:04,000 | 00:03:07,000 | You can see that this gives a rather jumpy looking appearance. |
| 40 | 00:03:08,000 | 00:03:10,000 | It’s only an approximation to the real grain size. |
| 41 | 00:03:11,000 | 00:03:13,000 | If we had an infinite number of sieves we would get a… |
| 42 | 00:03:14,000 | 00:03:18,000 | ..much smoother distribution which would give us a frequency curve like this. |
| 43 | 00:03:21,000 | 00:03:26,000 | Here what we’ve done is to smooth out that jumpy bar chart. |
| 44 | 00:03:27,000 | 00:03:30,000 | I think it’s a pretty fair thing to do and you can see it does express… |
| 45 | 00:03:31,000 | 00:03:34,000 | ..much more naturally the grain size distribution and we can do… |
| 46 | 00:03:35,000 | 00:03:37,000 | ..the same thing for the other two sand environments, |
| 47 | 00:03:38,000 | 00:03:41,000 | the storm beach and the intertidal sands. |
| 48 | 00:03:42,000 | 00:03:46,000 | Can you see any differences between the grain size distributions of each one? |
| 49 | 00:03:47,000 | 00:03:50,000 | Well, I think they’re standing out very well. For a start, both… |
| 50 | 00:03:51,000 | 00:03:55,000 | ..the storm beach and the intertidal sand have a much larger… |
| 51 | 00:03:56,000 | 00:04:00,000 | ..range of grain size, these last three bottles have sediment in them. |
| 52 | 00:04:01,000 | 00:04:06,000 | But the storm beach has very little sediment in this fine grain bottle. |
| 53 | 00:04:07,000 | 00:04:10,000 | Because it’s so turbulent when the storms are washing the material up… |
| 54 | 00:04:11,000 | 00:04:15,000 | ..on to highest part of the beach, all this fine material gets winnowed away. |
| 55 | 00:04:16,000 | 00:04:19,000 | But because the turbulence only lasts a short period of time… |
| 56 | 00:04:20,000 | 00:04:23,000 | ..because the storms probably soon die away, there hasn’t been enough time… |
| 57 | 00:04:24,000 | 00:04:29,000 | ..to even out the grain size distribution at the courser end of the frequency curve. |
| 58 | 00:04:30,000 | 00:04:34,000 | But this has been possible down in the intertidal sand area. |
| 59 | 00:04:35,000 | 00:04:38,000 | Here the waves are continually breaking and washing the sand… |
| 60 | 00:04:39,000 | 00:04:43,000 | ..to and fro so there is just a very small course tail to this curve. |
| 61 | 00:04:45,000 | 00:04:49,000 | But then contrast that, the intertidal sand, with the dune. |
| 62 | 00:04:50,000 | 00:04:54,000 | Here you see the wind has transported the sand from the beach onto the dune… |
| 63 | 00:04:55,000 | 00:04:59,000 | ..and so there’s been an extra process involved, first the waves and then the wind. |
| 64 | 00:05:00,000 | 00:05:04,000 | So we have a much sharper peak to this curve so it’s better sorted. |
| 65 | 00:05:05,000 | 00:05:09,000 | So, we can see then that there are differences in the grain size… |
| 66 | 00:05:10,000 | 00:05:13,000 | ..distribution according to the environment. And the question arises; |
| 67 | 00:05:14,000 | 00:05:18,000 | can we use these geologically to detect different kinds of environments? |
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