In the 17th century, a theologian called James Ussher worked out the age of the universe based on the chronology of the Bible. He concluded that it was created in a week in October 4004 BC. Even today, millions of people still believe that this 17th-century calculation is about right and that the overwhelming evidence of geology, cosmology, paleontology, chemistry, astrophysics, geophysics, stratigraphy, and biology is wrong. So let's take a look at what that evidence is, starting with a principle that even creationists don't dispute. The golden rule is that newest sedimentary deposits are always laid on top of older ones. So except where rocks have been folded, this orange bed would be older than the green one and the green one is older than the blue one. Geologists can then follow a single bed to make similar deductions in a different locality. The dark gray bed may be at the surface or even at a higher point than the others, but it's older than all of them, and the light gray bed is older still. What we end up with is what's called the geological column. To get the absolute dates of these rocks, scientists use a variety of different methods. I'll start with one of the easiest and most recent. We all know trees have rings because in summer they grow faster than in winter. But seasonal growing conditions vary according to the weather, so the bands will vary in size. Each time period in each geographical location has its own distinctive pattern of tree rings, like a fingerprint. So if we take a piece of wood of an unknown age, we can match the oldest part of it with the youngest part of another piece of wood, and so on back through time, back ten thousand...