Our attempts to reconstruct the climate of the distant Archaean in Chapter 1 might seem a little like reading a volume of Tolstoy’s War and Peace recovered from a burnt-out house. Most of the pages have turned to ash, and only some scattered sentences remain on a few charred pages. The Proterozoic Eon that followed began 2.5 billion years ago, thus is not quite so distant from us in time. We know it a little better than the Archaean—at least a handful of pages from its own book have survived. And this book is long—the Proterozoic lasted nearly two billion years. This is as long as the Hadean and Archaean together, and not far short of half of Earth’s history. Like many a soldier’s account of war, it combined long periods of boredom and brief intervals of terror—or their climatic equivalents, at least. The latter included the most intense glaciations that ever spread across the Earth. Some of these may have converted the planet into one giant snowball. The earliest traces of glaciation on Earth are seen even before the Proterozoic, in rock strata of Archaean age, 2.9 billion years old, near the small South African town of Pongola. These rocks include sedimentary deposits called tillites, which are essentially a jumble of rock fragments embedded in finer sediment. The vivid, old-fashioned term for such deposits is ‘boulder clays’, while the newer and more formal name is ‘till’ for a recent deposit and ‘tillite’ for the hardened, ancient version. Many of the ancient blocks and boulders in the tillites of Pongola are grooved and scratched—a tell-tale sign that they have been dragged along the ground by debris-rich ice. This kind of evidence is among the first ever employed by scientists of the mid-nineteenth century, such as Louis Agassiz and William Buckland, to tell apart ice-transported sediments from superficially similar ones that had formed as boulder-rich slurries when rivers flooded or volcanoes erupted. Ice, then, appeared on Earth in Archaean times.
A numerical study for convection in a cylindrical model with continuously varying viscosity
