If you look at maps that show the extent of the ice sheets during the last glacial maximum you will expect to see the extent of the ice stop just north of the Severn estuary. Up until recently this was believed to be correct by most climate scientists. I have always enjoyed visiting Dartmoor and Exmoor, the open wilderness of the place and the contrast to most other landscapes I am used to in the south west, is breathtaking. I remember being told in school and by my parents that the tors on the top of hills on the moor were formed due to weathering processes, however a month ago one of my lecturers mentioned that he had found evidence to suggest that Dartmoor was shaped not by the periglacial processes of freeze thaw, but by the same processes that have shaped more well known glacial landscapes such as the Lake District.
The extent of the northern ice sheets over the UK changed the landscape that we see today forever. During the last glaciation, which ended around 11,500 years ago, ice sheets resulted in a reduction of sea levels meaning that the UK was once connected to mainland Europe. Many islands around the British coastline were also once part of mainland UK such as the Isles of Scilly of the tip of Cornwall. If we were able to look at that the country then, we probably wouldn’t recognize it. This illustrates how glacial processes and landscape can change the landscape for the future. This idea could be applied to Dartmoor’s evolution. The previous theories suggested that the ice sheets never reached as far as the south west of England and so this supposedly ruled out the possibility of glaciers influencing the landscape here as they do in Scotland and Northern England. The alternate explanation for the features we see on Dartmoor today was that they were formed by periglacial processes. These took place over land that fluctuated in temperatures above and below freezing during day and night and were present were the glaciers were not. Periglacial processes include freeze thaw, a term many would know from GCSE and A level geography lessons. Freeze thaw occurs when water during daytime temperatures flows into a small crack in a rock or the ground, then when temperatures drop below freezing during night temperatures, this water freezes and expands putting pressure on the surrounding rock, forcing it to be pushed further apart creating a larger crack. This process repeats until the crack becomes large enough to lead to the breaking up of the rock. This process has been observed on Dartmoor and this is not what is incorrect. The incorrect assumption is that it was the only cause of the way we see Dartmoor today.
The ice cap that covered the northern parts of Dartmoor is thought to have been 31 square miles wide and around 100m thick. Evidence for this ice cap consists of many familiar glacial features such as moraines (deposits of rock debris carried by glaciers), small U shaped valleys and drumlins. One of the reasons why this evidence has been missed is because it has been so subtle, despite there being a large understanding of what land forms and features can result from a glacial environment. However, the research that made this discovery, which involved both ground and aerial observations, can be useful in the future for making sense of other potential glacial landscapes that have been overlooked somewhat in the past. This discovery will change my view of Dartmoor; when I visit it again i will see the landscape in a different light. I think it is amazing to think of a landscape I know well to once have been covered in a thick blanket of ice and it just shows how the variations in the climate system can have such a dramatic impact on our natural landscapes. It also shows how vulnerable the earth is to climate change. Abrupt changes in the climate in the past, such as the Younger Dryas period, resulted in conditions in northern Europe changing over a matter of years from a relatively warm period to a climate of near glacial conditions. This raises questions about how vulnerable our current human civilisation is to these abrupt changes that have been suggested to occur again in the future.
- last ice age date: http://www.bbc.co.uk/science/earth/water_and_ice/ice_age (credit: BBC)
- evidence for Dartmoor glaciation: http://www.bbc.co.uk/news/uk-england-18422207 (credit: BBC)
- All images in this post are owned by me
- information on abrupt climate change sourced from my personal lecture notes