The Forces that Created Dartmoor

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‘Assess the significance of both endogenic and exogenic forces in shaping the landscape and landforms of Dartmoor’

Dartmoor is a geographical goldmine, and as Alan Lee quotes: “Dartmoor contains such a rich variety of landscape, as many boulders, foaming rivers and twisted trees as my heart could ever desire”. Dartmoor was declared a national park in 1951, and it covers 954km of rolling moorland.The forces which carved this amazing landscape of steep valleys and searing granite “Tors” are millions of years old, and happened through a mixture of exogenic and endogenic forces; exogenic forces are the ones which happen on the surface of the earth, while endogenic forces are ones caused from inside the earth.

Firstly, we must discuss the two different theories which have been proposed for the formation of the famous Dartmoor “Tors”. The Linton theory is the idea that the dominant process in creating the tors was hydrolysis. He believes that in the tertiary period, deep chemical weathering caused the granite to break up, and then the peri glacial mass movement from the ice age caused the removal of the rotten waste products; The evidence for this is that the tors are rounded and surrounded by growan, both show that hydrolysis is likely to have occurred. The second theory is by Palmer and Nielsen, who believed that the dominant process was Freeze-thaw. They believe it was a one stage happening during the ice age within the quaternary period, and the harsh cold and wet from peri glacial conditions  led to the soft original surface breaking away and the tors being protruded. Their evidence for this theory is that the tors are surrounded by clitter fields (jagged debris which surrounds the tors) and the fact that this clitter is arranged in stripes, which signifies solifluction. Both these theories are trying to show how the exogenic forces have shaped the amazing protruding nature of Dartmoor Tors. After these tors were formed, weathering such as freeze thaw and chemical weathering, pin pointed the vertical splits and pseudo beds, and thus further exaggerated these features.

The exogenic effects on the landscape (talked about in the previous paragraph) have had an effect on shaping the appearance of the Tors into their iconic shapes. However, the way these piles of granite actually arrived in Dartmoor in the first place is a different story. Over 300 million years ago, endogenic forces pushed the granite up in plumes to the surface due to the rock becoming super heated; some of these plumes reached the surface, while others lay just below, waiting to be uncovered by future exogenic processes. These granite plumes are visible as a chain stretching along the centre of south-west England. The granite is much more resistant than the surrounding rock, which was subsequently stripped away to show the rises of granite. When the Granite was uncovered, and the heavy weight of the land above was taken away, a huge amount of pressure was released from the plumes, and so like a recently compressed spring they began to expand. This create the pseudo beds which line across the tors from where cracks have occurred due to  the expansion.

The endogenic forces have also shaped the complexion of the granite of Dartmoor. The granite is an igneous rock, and consists of Quartz, Feldspar and Mica. There are three types of granite in Dartmoor: Contact granite (granite fused with minerals from “country rock”),Tor granite (Large grains), and Blue granite (fine Grains). As the granite cooled after being pushed up to the surface by thermal energy, hydrothermal forces led to veins of minerals being formed through the granite, such as: Tin, Copper, Zinc and small amounts of gold. When the granite of the tors are inspected closely, it is noticeable that the quartz grains are aligned to each other, showing the direction of flow.

However, the landscape of Dartmoor does not solely encompass the tors. Another extremely noticeable feature is the barren nature of the area, and how it is predominantly moorland with very little ground cover. This is due to the acidity of the soil, and how over a third of Dartmoor is covered in 50cm of Peat. The acidity mens that only Heather and other bush grasses can survive, while the bog land causes there to be high moisture content in the soil because rainwater is absorbed and slowly dispersed throughout the soil, meaning that that the soil is rarely dry. the reason why these bogs are always full is also due to the frequent precipitation in Dartmoor. Dartmoor has a very high relief due to the endogenic forces causing the hills to be created from the movements of the tectonic plates below. This means that Princetown (town in Dartmoor) has an annual rainfall of 1974.2mm, while Teignmouth (a town on the south Devon coast) has an annual rainfall of 850mm. This proves how the endogenic forces have had an indirect affect on the appearance of Dartmoor.

In conclusion, Dartmoor is a unique landscape with unique features and appearance. Exogenic forces have affected this landscape through the weathering and climates causing the removal of waste rock from the hill tops and leaving the iconic tors. Endogenic forces have been significant by superheating the magma to near 1000 degrees, and then letting it cool near the surface, meaning that as the heat and pressure was released, the rock began to deform and create the pseudo bedding and vertical joints on the tors. These conflicts within the Earth caused the land to buckle as well, and so gave the high relief of Dartmoor, and thus the National Park’s famous weather patterns (rain). In history, the tors were believed to be old sea stacks or ancient horse droppings, however now we know that it was due to a combination of exogenic and endogenic forces, pushing and moulding the granite into the formations which we see today.

 

Bibliography:

Wikipedia –https://en.wikipedia.org/wiki/Dartmoor

Tor Theories –http://www.landforms.eu/cairngorms/tortheory.htm

Discovering Landscape in England and Wales, Andrew Goudie and Rita Gardner, Chapman & Hall, Pg. 156-157

Geofile 436, Granite Landscapes: Dartmoor, Neil Punnet, January 2003

Dartmoor National Park, geology and Landforms- http://www.dartmoor.gov.uk/learningabout//lab-geologylandforms

Devon County Council – http://www.devon.gov.uk/geo-dartmoorgranite.pdf

Dartmoor National park, Climate and Weather – http://www.dartmoor.gov.uk/learningabout//lab-climateweather