Field Guide to the Deep History Coast

The Pleistocene

By analysing cores from deep sea sediments and ice-caps we can see a record of world climate for the recent geological past. The ratio of the isotopes of oxygen 16 to oxygen 18 trapped in the microscopic remains of calcareous plankton was once thought to be indicative of sea surface temperatures. But Shackleton et al have shown that the variations in this ratio is caused in large part by expansion and contraction of glaciers (global ice volumes). A graph of these values plotted against depth (which is equivalent to age) produces a spiky line. This shows that as well as gradually cooling from 2.3 million years ago, the world's climate began to oscillate between colder and warmer phases. There are over 100 of these oscillations in the Pleistocene which permits this period to be divided into marine isotope stages (abbreviated to MIS) and they are numbered backwards from the present (warm) one. Thus, warm periods have odd numbers and cold ones are even. While many factors and feedback mechanisms are involved in generating an ice age, the primary drivers are subtle wobbles and variations in the Earth's orbit, named Milankovich cycles after their discoverer and each of these parameters has a different periodicity. At times they reinforce each other and drive climate change - warmer, colder, warmer, colder. The most dramatic oscillations have come in the past half a million years and they have generated glaciations, with expansion of the polar ice-caps into mid latitudes. Between the cold periods are interglacials when climate returned to 'normal' like the present, or sometimes even warmer.

While the steady accumulation of sediment in the deep oceans provides a continuous record, that on land is far from perfect, with deposits that can be both dramatic and complex. Unravelling this complexity and relating it to the marine record has been the subject of much research and fieldwork, not least in Norfolk, where due to its geography near the limits of successive ice sheets, it bears the marks of glaciations and preserves fragments of the interglacial sedimentary record. Norfolk is especially useful because the coastal section provides exposure of sediments both glacial and interglacial and in north Norfolk these can also be related to landforms associated with the margins of a Middle Pleistocene ice sheet.

The 'pre-glacial' Pleistocene

It is convenient in Norfolk to consider the sediments below the tills as 'pre-glacial', even though glaciation may have occurred elsewhere, but without ice reaching Norfolk. So, the deposits sandwiched between the Chalk below and the tills above form a mappable unit that can be found more or less continuously around the coast from Weybourne in north Norfolk to Pakefield in Suffolk and appearing occasionally in inland pits. Closer examination shows that these comprise marine gravels, sands and clays known nowadays as the Wroxham Crag (previously called the Weybourne Crag) and freshwater and estuarine deposits, sometimes organic, known as the Cromer Forest-bed. That this represented a long period of time was recognised by palaeontologists even before radiometric dating, for the older deposits yield a distinctly antique suite of fossils, especially vertebrates, that contain many extinct and primitive ancestral forms, while the younger deposits have many species that are are familiar from our modern day fauna. Precise dating is still the subject of research, but it is believed that these beds cover a period approximately 1.8 million years ago (Ma) or more, to 0.5 Ma.

Middle and Upper Pleistocene glaciation

There have been numerous cold oscillations of climate but only two glacial episodes are known to have directly affected Norfolk. The greatest of them is named the Anglian Glaciation and is equivalent to to the Elsterian of the northern European continent and correlates with Marine Isotope Stage 12 (MIS 12). At it greatest extent the whole county was ice-covered and at its limit the icesheet reached as far south as the London suburbs of Finchley and Hornchurch. Several distinct tills are recognised in north Norfolk but recent research using OSL dating (optically stimulated luminescence) has confirmed that they all belong to the same Anglian stage, which lasted some 50,000 years and finished around 424,000 years ago. The last glacial period, called the Devensian, began 120,000 years ago but only produced extensive icesheets in Britain around the glacial maximum about 26,500 to 20,000 years ago. Ice only got into the very northwest corner of Norfolk leaving a thin pale coloured till but it was responsible for diverting the River Stiffkey.

References

D. Brand1, S.J. Booth and J. Rose, 2002. Late Devensian glaciation, ice-dammed lake and river diversion, Stiffkey, north Norfolk, England. Proceedings of the Yorkshire Geological Society Vol. 54, pp.35-46.