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the world and these reveal that there were many more climatic cycles during Pleistocene time, which is now known to have extended back to i .8 million years ago.

In the deep oceans sediment accumulates slowly on the seabed virtually without interruption. Luckily for historians of Earth Time, the continuous oceanic 'rain' of fine-grained sediment includes the debris of ocean life, especially the tiny shells of minute single-celled protists called foraminiferans (or more conveniently forams). Most importantly, in the 1950s, it was discovered that the composition of foram shells is influenced by the chemical composition of ocean water when they were growing and that in turn reflects global ice volume and ocean water temperature. So the shells provide a proxy measure of past climates. By analysing the shell composition of forams from successive layers in sediment cores retrieved from the ocean floor, an excellent detailed record of climate change over the last 2 million years and more has been recovered. This has been supplemented by evidence of iceberg abundance and distribution in the North Atlantic. The drifting bergs carry rock debris, which is released when they melt with the debris falling onto the oceanbed. In addition, ice-core records from Greenland and Antarctica provide an independent measure of climate change in both the northern and southern hemispheres over the last 300,000 years.

Altogether, these records tell us that there have been frequent and often rapid changes in climate over the last 2 million years. At least 30 cycles of cold and warm oscillations have been measured from the ocean sediment record over the last million years. By comparison, the land-based record of glaciation is much more problematic. Much of the difficulty arises from the tendency for one glacial ice advance to disturb or eradicate evidence of the previous cycle of glacial erosion and subsequent deposition. Researchers have had great difficulty in finding good records of the succession of events. Nowhere on land is there a continuous record. Short sequences have been discovered, but they are scattered all over the place and the problem has been to match them together to form a more continuous history of events. Over 150 years after it was first realised that there has been a recent succession of ice ages, we are beginning to get somewhere. Much has been due to the discovery of the oceanic and ice-core records plus improvements in dating methods, so that there is now an increasingly reliable chronological framework within which the sediment and fossil record can be placed.

For many years now there has been a considerable problem of matching the classic land-derived record of the Ice Ages to that of the ocean floor sediments with the oxygen isotope stages (OIS). Only recently has it become clear that some parts of the early Pleistocene are preserved in East Anglia by deposits representing the Baventian (perhaps over 1.5 million years old), the Pastonian, Cromerian (around 500,000-600,000 or perhaps as much as 800,000 years ago) and Anglian stages (around 450,000 years ago).

Reconstruction of early Pleistocene river floodplains shows that an enormous amount of rock material was worn away from British landscapes since that time. Furthermore, during cold phases river flow was much more powerful and effective in both erosion and deposition. The removal of such a weight of rock material, in addition to the loss of the weight of the glaciers and ice sheets, resulted in the landscapes rising through a process known as isostatic readjustment. Just as icebergs continue to float as they melt, so do continents tend to rise as rock (or ice) is stripped off their surfaces. The added complication is that as the ice melts sea levels rise, but the two processes work at different rates and so they are rarely synchronous.

The Anglian Stage is generally matched to OIS 12 (around 478,000 years ago), which was one of the coldest phases of the mid-Pleistocene record. The following warm Hoxnian interglacial (around 423,000 years ago) represented at Swanscombe seems to equate with OIS 11 or perhaps 9. Stanton Harcourt in the upper Thames Valley is Wolstonian and OIS 7 at between 245,000 and 186,000 years ago. The latter is similar in age to the famous Pontnewydd Cave in North Wales with its Neanderthal remains, which is one of the few sites that can be dated using radiometric and thermoluminescence dating methods. The Trafalgar Square deposits are considered Ipswichian in age (OIS 5e) and around 125,000 years old.

Among the fossils found in the Thames Valley are stone tools and very rare human remains, which show that our ancestors walked across from mainland Europe when sea levels allowed them to do so. As hunters they were probably following the migrations of the animals such as horse, deer, wild oxen, bison and even mammoth, which they relied on for food, and a variety of materials for clothing, shelter and tools. When it was first accepted that our ancestors lived alongside these extinct animals of the Ice Ages, the possibility arose that their development of stone tool technology would follow a recognisable chronology. The assumption was that the evolution of the technology would be linear from crude basic forms to more advanced and sophisticated forms. All that had to be done was to work out the evolution of form and type of tool and wherever tools were found the deposit could be dated relatively within the overall scheme.

However, it was soon realised that such technological developments are culturally determined and develop at different rates within separate peoples, especially those isolated by geography, language and so on. Correlation can work, especially within contiguous regions such as northwest Europe, but it requires very detailed and meticulous studies backed up by modern dating techniques.

Naming names

The names used to label the various phases of prehistory can be really quite difficult to cope with. Even students of geology have a hard time trying to remember even the main subdivisions of Earth Time and usually have to fall back on various handy mnemonics, which can be unprintably rude. The problem is that the divisional names have grown in an 'organic' and historical way, from terms first used by quarrymen and miners as far back as mediaeval times. When the more academic geologists began to try to systematise matters they adopted the standard scientific procedure of using words derived from classical Latin and Greek. Most of these scholars knew these languages, which were the 'lingua franca' of science for centuries. Today we can no longer assume that students can automatically access the etymology of the words and they just seem incredibly arcane and meaningless. The Ice Ages form one phase of Earth Time that has a readily recognisable common name, although its scientific equivalent (Pleistocene) is much less familiar. Since even 7-year-olds today have at least heard of the Jurassic, perhaps there is some hope that in the future the Ordovician and Triassic might be just as familiar once their characteristics become better known, although I will not bet on it.

M. Jules Desnoyers, 1801-77, French stratigrapher, vertebrate palaeontologist and librarian at the National Museum of Natural History in Paris who first formally defined the Quaternary (1829) in the modem scientific sense. He also co-founded the Geological Society of France in 1830 with Ami Boue, Constant Prevost and Paul Deshayes and became its secretary in 1831.

Giovanni Arduino, 1714-95, Italian mining engineer and agriculturalist, inspector of mines in Tuscany who became professor of mineralogy in Venice. From studies in northern Italy, he first proposed a division of rock strata into Primary, Secondary etc., recognised the igneous origins of granite and basalt and that mountain building was a long process.

In modern scientific terminology the epoch of the Ice Ages is referred to as the Pleistocene epoch of Earth Time and its deposits as the Pleistocene Series (the old Diluvium). The name Pleistocene, meaning 'most recent', was coined by Charles Lyell in 1839 to include those deposits that lie on and and are therefore younger than the older Pliocene Series deposits. Lyell distinguished the deposits of the Pleistocene Series on the high percentage of modern molluscs they contain.

Lyell had previously (in 1833) developed an earlier series of divisions using the proportion of living as opposed to extinct molluscs found within them. He named the divisions, in descending order of age, as Pliocene (meaning 'more recent'), Miocene ('less recent') and Eocene ('early recent'). It might have seemed a good idea at the time, but it created a lot of problems. Furthermore, the latter three divisions when grouped together formed the Tertiary System of strata, as we shall see. The Pleistocene and younger Holocene series of

Arduino's 1760 divisions of the Earth's rock formations

Pianure - alluvium Tertiary monti terziari — fossiliferous sands, clays and gravels, volcanic rocks Secondary monti secondari — fossiliferous limestones and marbles Primary monti primari - sandstones and conglomerates (unfossiliferous) vetrescibili - mineral-rich crystalline rocks roccia primigenia — schists sediments form a post-Tertiary division, and have been grouped together as the Quaternary System. The name Quaternary was used as long ago as 1829 by the French geologist Desnoyers for certain geologically young deposits in the Paris region that were originally laid down on the seabed, although in strict historical terms the name is even older and was used by an eighteenth-century Italian naturalist Giovanni Arduino. In 1759 Arduino wrote to a colleague, Professor Vallisneri, proposing a fourfold division of the succession of rock formation on Earth into Primary, Secondary, Tertiary and Quaternary.

Million Years Ago

1 g J ice ages

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