 This is a portion of the Geological Society of America's version of the geologic timescale, starting with the Sanizoic era. So this is today, and then time goes backwards down this axis and picks up again here in the Mesozoic about 65 million years ago, and then time keeps on going backwards down to about 250 million years ago. But what I want you to see is that next to these ages, so here's 5 million years ago, 10 million years ago, 15 million years ago, and so on, the direction of the Earth's magnetic field is recorded. And so each of these black bands, like this one, is showing times of normal polarity, in which the Earth's magnetic field is aligned the same way it is today. And then each of these white bands, for example, right here, down here, these are showing times where the field was reversed compared to today's field. So you can see that there are several reversals, many, many, many, and they're not necessarily in any particular periodic way, and they happen for different lengths of time.