Science of Discworld III

Bishop Ussher, which is why the idea remains controversial among certain Christian fundamentalists, whohave bizarrely chosen to fight their corner on the weakest of grounds, completely needlessly. Deep Time is supported by so much evidence that a truly committed fundamentalist has to believe that his God is deliberately trying to fool him. Worse, if we can’t trust the evidence of our own eyes, then we can’t trust the apparent element of ‘design’ in living creatures either. We can’t trust anything.
    Lyell realised that the age of the Earth must be many millions of years, when he looked at sedimentary rocks. These are rocks like limestone or sandstone which form in layers, and have been deposited either underwater, as muddy sediments, or in deserts, as accumulating sand. (Independent evidence for these processes comes from the fossils found in such rocks.) By studying the rate at which modern sediments accumulate, and comparing that with the thickness of known beds of sedimentary rock, Lyell could estimate the time it had taken for the layers of rock to be deposited. Something in the range 1000–10,000 years would produce a layer about a metre thick. But the chalk cliffs of the south coast, around Dover, are hundreds of metres thick. So that’s several hundred thousand years of deposition, and we’ve only dealt with one of the numerous layers of rock that make up the geological column – the historical sequence of different rocks.
    We now have many other kinds of evidence for the great age of our planet. The rate of decay of radioactive elements, which we can measure today and extrapolate backwards, is in general agreement with the evidence of the rock layers. The rate of movement of the continents, when combined with the distances they have moved, is again consistent with other estimates. We’ve seen that India was once attached to Africa, but about 200 million years ago it broke off, and by 40 million years ago it had moved all the way to its current position, butting up against Asia and pushing up the Himalayas.
    When continents move apart – as Africa and South America, or Europe and North America, are doing now – new material forms on the ocean floor, flowing out from the mantle beneath to form hugemid-ocean ridges. The rocks in the ridges contain a record of the changes in the Earth’s magnetic field, ‘frozen in’ as the rock cooled. They show a long series of repeated reversals of the field polarity. Sometimes the ‘north’ magnetic pole is at the northern end of the Earth, as now, but every so often the polarity flips, so that the magnetic pole near the northern end is the ‘south’ one. Mathematical models of the Earth’s magnetic field predict that such reversals occur roughly once every five million years. Count the number of reversals in the ocean-ridge rocks, multiply by five million … again, the numbers fit reasonably well, and careful comparisons and a lot of disputation by experts lead to revised numbers that fit even better.
    The Grand Canyon is a deep gash through layers of rock one mile (1.6km) thick. You have a choice. You can understand what the record of the rocks is telling you here: it took a very long time to lay down those rocks, and quite a long time – though less – for flash-flooding in the Colorado river to erode them again. Or you can follow one book that until recently was displayed in the ‘science’ section of the Grand Canyon bookstore, until a lot of scientists complained, and assert that the Grand Canyon is evidence for Noah’s flood. The first choice fits huge amounts of evidence and geological understanding. The second is an excellent test of faith, because it fits absolutely nothing. A flood that lasted only 40 days could never have produced that kind of geological formation. A miracle? In that case, the Sahara desert could equally well be hailed as evidence for Noah’s flood, miraculously not forming a deep canyon. Once you admit miracles, you can’t pursue a logical thread.
    Anyway, that’s the second ingredient – Deep Time. It takes huge amounts of time to change organisms into entirely new species, if all you can do – as Darwin believed – is make very gradual changes. But even Deep Time, when combined with heritable variation, is not enough to lead to the kind of organised, coherent changes that are needed to create new species. There has to be a reason for such changes to occur, as well as opportunity and time. Darwin, as