In almost everybody’s natural lifetime, the sea is one of the great unchanging certainties of life. There is land; there is sea; and in between is that magical place, the seaside, which is sometimes knocked about a bit by the waves, but always manages to recover for that next idyllic summer. There are, one remembers, those faintly disquieting legends, about a remarkably well-organized and ecologically aware person called Noah, and about a Deluge. But these, of course, should not be taken seriously. They were a jumpy and superstitious lot, our ancestors, always prone to making up scary stories. It was a good way to keep the children in order. With a longer perspective, things seem a little different. Take any one location on the globe, for instance. Track it over millions of years. At that one location, there may be a change from deep ocean, to shallow sea, to a shoreline, and thence to terrestrial swamps and flood plains. And then, perhaps, to the absence of evidence, a horizon of absolutely no thickness at all within a succession of rock strata, in which a million years or a hundred million years—or more—may be missing, entirely unrecorded. It is that phenomenon called an unconformity, all that is left of the history of a terrestrial landscape pushed up into the erosional realm. On that eroding landscape, there may have been episodes of battle, murder, and sudden death among armoured saurians, of fire, flood, and storm, and of the humdrum day-to-day life of the vast vegetarian dinosaurs, chewing through their daily hundredweights of plants. Of this, no trace can persist. Only when that landscape is plunged again towards sea level, and begins to be silted up, can a tangible geological record resume. The Earth’s crust, as we have seen, is malleable, can be pushed downwards or thrust upwards by the forces that drive the continents across the face of the globe. Many of the sea level changes that can be read in the strata of the archives are of this sort, and mark purely local ups and downs of individual sections of crust, with no evidence that global sea level was anything other than constant.
Indicators of Holocene sea level changes along the coast of the states of Pernambuco and Paraíba, Brazil