By Joe Shimmin
The Quaternary comprises the Pleistocene and the Holocene and is the youngest of the geological periods. It dates from approximately 1.8 million years ago right up to the present, with the large majority of this time being filled by the Pleistocene. The Holocene spans a geological ‘blink of an eye’, beginning only 10,000 years ago at the start of the present interglacial and continues today.
What sets the Quaternary apart from other geological periods is a suite of high frequency climate fluctuations, with very cold stages being interspersed by warmer stages. This type of climate fluctuation is believed to have occurred at various other times in the Earth’s history, but most of the evidence for these has been wiped out over millions of years. However, the glacial/interglacial or warm/cold stages of the Quaternary have, in many cases, left us enough evidence of their existence for the Quaternary scientist to be able to attempt to reconstruct these past environments with some degree of success.
Serbian mathematician, Milutin Milanković, formulated the accepted theory for why climate oscillations have occurred in this period, in the first half the twentieth century. According to ‘Milanković, Quaternary climate was, and is, influenced by three factors:
- Factor 1: the shape or ‘eccentricity’ of the Earth’s orbit around the sun, which varies over a cycle of approximately 100,000 years.
- Factor 2: The tilt or ‘obliquity, of the Earth’s axis, which varies over a cycle of approximately 41,000 years.
- Factor 3: The wobble of the Earth’s axis of rotation, otherwise known as ‘the procession of the equinoxes’, which varies over a cycle of approximately 21,000 years.
It is beyond the scope of this small article to go into the ins and outs of the above, but suffice to say that these astronomical factors affect the amount of solar radiation that the Earth receives and hence its climate.
No one is sure exactly how many cold and warm periods there have been in the Quaternary. Evidence is scattered and fragmented on land and, in areas affected by glaciation, has often been obliterated by subsequent glaciations. A more complete record is held in the sediments of the deep oceans. Here can be found sequences of sediments that have accumulated, uninterrupted, over long periods – in some cases the whole of the Quaternary. Within these sediments are found the shells of micro-organisms and, by analysing these shells, inferred climatic data can be collected.
This deep ocean record is an excellent resource with which Quaternary scientists, researching deposits on land, do their best to correlate the sediments that they are investigating. The fragmentary nature of the deposits left on land mean that a resource such as this is an important tool, as it provides a framework within which to fit the various pieces of data obtained from studying terrestrial sites.
The Quaternary Period in Kent
I live in an area of England that was not covered by glaciers at any point during the cold stages of the Quaternary. Here, in Kent, instead of ‘U’-shaped valleys, corries, striations, moraines and all the rest of the glacial features that can be found (for instance, in the Lake District), we have landforms brought about by successive cycles of periglacial and temperate climates.
These landforms may be less dramatic than those that make up the scenery one admires in the Lake District, but they can contain a great deal more information. Of most interest are river terrace deposits. These are found as patches of sediments or gravels on valley sides and were formed in response to the Quaternary climate fluctuations, coupled with regional crustal uplift.
During the cold stages, there was less vegetation, bedrock was exposed and the cold weather caused a great deal of erosion. Water seeped into cracks in the rock and, when it froze, it expanded and enlarged the cracks. Over time, rocks were broken down and large volumes of gravel accumulated on a river’s flood plain. Most water was locked up, as ice and the relatively small quantity of liquid water that did flow in rivers did not have enough energy to erode through this accumulating sediment in response to the regional uplift. However, when the cold stage began to subside and the ice melted, the rivers became swollen and had enough energy to cut down into the accumulated gravels and sediments.
Successive cycles of aggradation during the cold stages and down-cutting during the warm stages leave us with a staircase pattern of deposits, with the oldest found at the top and the youngest at the bottom.
Investigating Terrace Deposits
Within terrace deposits can be found a wealth of relics such as animal fossils (including beetles, molluscs, fish bones and mammal bones and teeth), plant fossils (such as wood, seeds, leaves and pollen) and artefacts produced by our human ancestors. In addition, there are numerous diagnostic sedimentological features as well as an array of other things that give us clues as to the age of the deposit, the climate at the time it was formed and the habitat type which prevailed in the area at that time. Information relating to how our ancient human ancestors lived can also be found.
It is important, when investigating any Quaternary deposits, to study the overall assemblage of evidence that is present. The various sediments in a section should be investigated, animal and plant material should be collected for identification, counting and statistical analysis, and human artefacts should be drawn, catalogued and studied. Using all available lines of evidence together, a reliable and comprehensive environmental reconstruction can be often be achieved.
The digging involved when studying Quaternary sediments is often extremely hard work, but the results are well worth it. Within the gravels and sediments that I have already extracted, I have found foraminifera, ostracods, molluscs, small mammal bones and teeth, an elephant tooth and a large number of flint flakes produced during the production of stone tools. I find this work extremely rewarding and feel lucky to have such a topic to sink my teeth into.
Over the next few years I hope to investigate many more sites in Kent with the aim of revising and broadening the information provided by previous work in this area. I hope to be able to build up a picture of north-east Kent’s previous environments over a timescale of hundreds of thousands of years and I know that, during this work, I will hold in my hands some fascinating pieces of natural history. The task will be a big one, but I can’t wait.
J. J. Lowe and M. J. C. Walker (1997). Reconstructing Quaternary Environments. Longman
Bridgland, D., Maddy, D. and Bates, M (2004). River terrace sequences: templates for Quaternary geochronology and marine-terrestrial correlation. Journal of Quaternary Science, Vol. 19, 203-218.