Earth Sciences / 73Land that is now free from permanent ice was glaciated for rather more than half of the Pleistocene.
It is reasonable to suppose that the global climate now fluctuates between glacial and interglacial
conditions and that the present interglacial may be near its end, unless the subsequent cooling is
overridden by warming induced by the greenhouse effect. The alternation between glacial and
interglacial is the most extreme climate change imaginable, but the historical evidence suggests that
living organisms adapt to it fairly robustly. As conditions deteriorate enough of them migrate to
more favourable environments for their species to survive, from which they return when opportunity
allows. In certain local areas, called refugia, communities survive even without migrating, because
the older conditions prevail. There are several Pleistocene refugia in Britain, Upper Teesdale being
probably the best-known example. Although undoubtedly inconvenient for humans, rapid climate
change does not necessarily imply the extinction of species, merely their absence until the next
change allows them to return.
19. Dating methods
Consider the world as it might appear to an intelligent mayfly. As it emerges from the stream in
which it has spent most of its life, the insect sees a world where the Sun shines, trees are in full leaf,
a summer world. To the mayfly, this is how the world is, the only appearance it can ever present.
Long before leaves start to fall and even longer before water starts to freeze and the ground is
covered with snow, the mayfly will have died.
Unlike the mayfly, we know the world changes, that there is winter as well as summer. Yet our lives,
too, are brief and deny us any opportunity to observe at first hand the fact that those aspects of the
world we regard as permanent are no more so than the summer sunshine and leaves. Our Earth is
changing constantly. Continents move, mountains are thrust upward and then eroded into plains, ice
ages come and go, species evolve only to vanish again, but these changes occur on a time-scale that
seems long to us. Compared with the 4.6 billion years during which our planet has existed, a human
lifespan is ephemeral indeed.
If we are to understand the environment in which we find ourselves, to observe ways in which it may
be changing, and to predict future changes and our own influence upon them, we must learn to
appreciate the time-scale on which such events occur. We must try to discover how the environment
arrived at its present condition if we are to discern trends and compile forecasts. We must study the
history of our planet and the first requirement for any historical reconstruction is a reliable means for
dating events. We must know when and in what order past events happened.
The first step in reconstructing the past is fairly straightforward. Sedimentary rocks begin as
sediments, most deposited beneath water, and they can be seen to form layers. Obviously, the
older layers must have been precipitated before those lying above them and changes in the
composition of layers must reflect changes in the depositional environment. Unfortunately,
sediments seldom remain undisturbed, so although it is easy enough to recognize their layers it is
more difficult to determine their relative ages; to do that it is necessary to determine which way up
they were when they were precipitated.
It was Georges Cuvier (1769–1832) who first realized that fossils might be used to identify sedimentary
strata. He and Alexandre Brogniart (1770–1847), an engineer, applied this idea to a study of the
Paris Basin, describing their discoveries in Descriptions géologiques des environs de Paris, published
in 1811 (BOWLER, 1992, pp. 213–220). Their scheme was based on the observed fact that
fossil invertebrates found in some strata are absent from others and, therefore, that certain