Knowledge Of Ice Ages and Mammoths – The Quaternary.
In his Chapter 13, Peter Toghill notes that Northern Britain has a wealth of dramatic mountain scenery produced by ice action, in particular valley glaciers and ice sheets during the last two million years; and that there is also a great variety of lowland features formed by sands, clays and gravels left behind by melting ice sheets. As he says: it is hard to imagine that during the early nineteenth century scientists would not accept that ice had ever covered Britain in the past, and sought to explain the origin of what we now call glacial features by reference to drifting ice bergs in a sea which covered the highest mountains during the biblical flood. By as late as 1830, such “scientists” were going only so far as to accept that certain features and deposits were due to drifting icebergs from the cold north which melted and deposited sand, gravel and large boulders, but they could not yet accept the evidence for continuous ice cover of the land. Indeed, they explained the presence of marine shells in these sands and gravels by inundations of the sea to depths of around 1200m in Britain and to even greater depths to explain the presence of such shells at greater altitudes in the Alps. However, in 1837, Louis Agassiz put forward his glacial theory and eventually persuaded, such as the above, to accept that glaciation over Europe and Britain was much older and more widespread than had previously been thought (believed). He based his conclusions on the Alps where he proved the previous extent of existing glaciers by evidencing the moraine deposits exposed by their more recent retreats. It was easier to accept this for the Alps where glaciers still existed, but in Britain, it took longer to be accepted. However, now we can look at the deep U-shaped valleys in Snowdonia and the Lake District and conclude that they once had glaciers in them which scraped and polished the valley sides and over-deepened them while eroding boulders from near the mountain summits and transporting them down to lower altitudes as glacial erratics, and sand and gravel to be exposed as the glacier retreats as it melts and thus exposes them. In the Alps today, we can see such moraines many kilometres down the valleys beneath current glacier snouts, thus showing, the older more widespread extent of the ice.
In his Chapter 13, Peter Toghill presents what is now known of Quaternary climate change, Global warming, Pleistocene glaciation, Devonian glaciations, erosional features in highland regions, features of tundra areas, and Lowland glacial features, not all which sub-sections need concern us here. However, as to climate change, he notes that towards the end of the Tertiary Period repeated cycles of warm and cooler climates became established, each with periods of about 100,000 years. these became more pronounced in the last two million years, and during the cooler periods expansion of glacial ice occurred in the form of continental ice sheets, of which seventeen cycles of cold and temperate climates are recognised to have occurred in Britain during the last two million years, and ten in the last million years. However, we now have evidence of three glaciations over the whole of Britain as recently as in the last 300,000 years. Again there is evidence of possibly two earlier episodes in the North Sea basin within the last 1.5 million years. These three glacial episodes were separated by interglacials when the climate was somewhat warmer than at present.
We now live in an interglacial which started around 10,000 years ago. but now temperatures are lower than they were 2000 years ago and we may be heading towards another glaciation in about 5000 years’ time. Deep-sea cores from the North Sea, where a complete Quaternary marine sequence is available and from other oceanic areas provide for the earlier seventeen hot/cold cycles. A number of plant and animal groups occur as fossils in Quaternary sequences, and as they are often closely related to living species, they are excellent climate indicators. These include pollen, beetles and marine micro-organisms. Oxygen isotope ratios (Oxygen-18/Oxygen-16) in the calcite shells of marine organisms can reveal when oceanic water is being stored in glacial ice during a cold period and vice versa, because water molecules containing the light isotope are preferentially evaporated from sea water to be stored in rain and snow and hence is depleted in marine shells during cold episodes. These animals also have known modern temperature ranges so that sea temperatures can be estimated by this means also. By analogy with present ranges of beetles and plants we can estimate the ancient climates of glacial and interglacial sediments laid down on land. Pollen analysis is a very accurate way of recognising old climates, particularly in peat deposits.
In Britain we have good evidence for three major glacial episodes (Anglian, Wolsonian and Devensian) during the last 300,000 years, separated by interglacials. As Peter Toghill has said, we are now living in an interglacial with temperatures cooling, which started 10,000 years ago; that cyclical evidence from the past suggests it will end in about 5000 years; and that there is no evidence that this cyclical sequence is at an end. The Quaternary is divided into the Pleistocene and the Holocene which are further subdivided into stages based on climate change. The Holocene started only 10,000 years ago and is considered by many as merely the latest interglacial within the Pleistocene. The Development of continental ice sheets is relatable to atmospheric and ocean circulation, and it appears that that the first ones were formed in Antarctica during the Oligocene around 30 Ma with a well-established sheet in existence by the late Miocene, 14 Ma and those in Europe and Iceland around 2.5 Ma. In Britain, the oldest true glacial deposits are around 300,000 years old, although as we have seen, older glaciations occurred in the North Sea Basin, and it is likely that the Scottish Highlands began to be glaciated over 2 Ma when cold phases began in earnest, though no sediments or fossils occur of this age. In north-west Europe the Quaternary includes an early pre-glacial phase, 2.5 to 0.6 Ma and a later phase, 0.6 Ma to the present day. Glacial episodes in Europe and North America led to ice sheets up to 4 km thick covering many areas, though in Britain they probably did not exceed 2 km. Continental ice sheets such as in Antarctica and Greenland lock up oceanic water and sea levels fell globally during the Pleistocene glacial phases by up to 150 m compared with today. Ice around the north pole is simply frozen sea, not an ice sheet. The weight of ice on land depresses continental masses into the molten magma beneath them and in parts of northern Britain, this depression was greater than the fall in sea level, so that old beaches were taken below earlier sea levels, while with the removal such ice sheets by melting, the land has risen again (isostatic readjustment) by as much as 30m above present sea levels in north-west Scotland, total isostatic effect has been as much as 150m.