This essay was written for the Greenwell Cup on the processes which form Dartmoor’s landscapes.
Assess the relative importance of both endogenic and exogenic processes in the formation of Dartmoor’s characteristic landscape.
Geomorphological processes that happen underground are described as endogenic, whereas exogenic geomorphological processes are those that occur above ground: both are necessary in forming any landscape, but endogenic processes are more important in the formation of Dartmoor’s characteristic landscape.
The key to understanding the nature of Dartmoor’s unique landscape is the predominant rock, granite, which forms 65% of the area’s geology (Dartmoor National Park Authority, 2017). Dartmoor’s granite, an igneous rock, was formed 280 million years ago at a destructive plate boundary during the Hercynian Orogeny, a mountain range-forming event (Dartmoor National Park Authority, 2002): the lower temperature of the magma (600oC at a destructive boundary against 1000oC at a constructive boundary) meant that only minerals with low melting points were liquefied, and so the magma had high silicon content and viscosity (Eton College Geography Department, 2021), and the resulting granite contained mica, a high silicon mineral. Larger feldspar crystals within the granite evidence the slower cooling of the magma to form the rock, showing that the intrusion at the subduction zone must have been deep (Veevers, 2011). The close interlocking of high silicon mica, large feldspar crystals and quartz crystals is what makes the granite such a hard rock (Dartmoor National Park Authority, 2002), a characteristic responsible for many of its and Dartmoor’s geomorphological features. Since granite and its key properties are caused by the deep intrusion of the crust at a destructive plate boundary, an endogenic process, all features of the landscape that rely on the granite geology can be said to be the result of endogenic processes.
The high altitude of Dartmoor is one key characteristic of its landscape, both in itself and in the further features it contributes to, and it is formed largely by exogenic processes, but the granite is the reason these processes have their particular effect. Dartmoor reaches 621m above sea level at its highest, and more than half of the moor is 300m above sea level or more (Punnet, 2003), whereas, as seen in figure 1, few areas surrounding Dartmoor National Park are over 200m above sea level. The weathering and erosion of rocks, an exogenic process, at different rates leads to the higher relative altitude of Dartmoor, since granite is an igneous rock, harder, less permeable and less porous than the surrounding sedimentary country rocks, and so other rock is weathered faster, leaving Dartmoor granite at a higher altitude (Eton College Geography Department, 2021). The high altitude of Dartmoor also contributes to its distinctive landscape by its hydrology: it creates radial drainage patterns, where stream channels flow down from the high altitude areas (there are two main high-altitude plateaus the channels flow from), per figure 3; it increases Dartmoor’s precipitation due to relief rainfall seen in figure 2 and therefore also increases river’s runoff, discharge and erosive power. The contribution of exogenic processes to Dartmoor’s landscape is significant in relative altitude, but it is the granite geology of Dartmoor causing the differential weathering and erosion rates of the rocks, and thus its higher relative altitude and all that begets too. Furthermore, the mostly impermeable granite geology also affects the hydrology as less water is lost to percolation from the soil into the rock, thus increasing runoff and discharge even more.
Exogenic processes also were significant in the formation of Dartmoor river valleys and gorges. With the end of the Pleistocene and the advent of the Holocene around 12,000 years ago, the thawing of the permafrost (in the soil and rock—glacial ice never reached Dartmoor) led to torrential flow of the rivers, eroding the granite, surrounding metamorphic rock (produced by the excess heat from the magma intrusion that created the granite batholith) and country rock more fiercely to produce steep-sided gorges and valleys (LUC, 2017), such as Lydford Gorge. The high altitude of the area as previously discussed contributed further to the erosion of the gorges as the water has greater erosive power due to its increased velocity. The geology of Dartmoor is important in forming the gorges too, however, as gorges must be formed in hard rock such as granite (Rutledge, et al., 2011) since softer rock would mean wider channels would be eroded rather than these steep and narrow gorges, and so endogenic processes are a prerequisite for these characteristic exogenic formations. The more common form of river valley that can also be seen in Dartmoor is the v-shaped river valley shaped by the erosion of rivers, an exogenic process. However, interlocking spurs within these valleys, formed due to the river meandering away from areas of hard rock (here granite) encountered in its downhill flow, provide evidence that even these valleys are shaped by the endogenically-formed geology of Dartmoor.
The contribution of exogenic processes to Dartmoor’s unique landscape is evident in the formation and shape of tors, but here endogenic processes are ultimately more significant. There are two processes of weathering which form a tor: hydrolysis and freeze-thaw weathering. Hydrolysis, the dominant process in the tertiary period (65 to 2.8 million years ago) due to the tropical environment of Dartmoor at that time (Dartmoor National Park Authority, 2002), is evidenced by the rounded shape of the rocks in tors as in figure 4, showing that the weathering must have been chemical, as opposed to the jagged edges produced by physical weathering. The appearance of growan, inert quartz crystals unaffected by hydrolysis in contrast to mica or feldspar, the two other components of granite, at the base of tors also confirms hydrolysis, and thereby exogenic processes, must have been important (Linton, et al., 1955). Freeze-thaw weathering also can be observed to have contributed to the formation of tors and the Dartmoor landscape by the clitter seen in figure 5—angular granite rocks that have broken off the main tor—arranged in stripes down the slopes from tors due to solifluction. Freeze-thaw weathering would have mainly affected Dartmoor’s granite tors in the Quaternary Period (from 2.6 million years ago) in the periglacial environment. Thus, these exogenic processes can be seen to have had a significant effect on Dartmoor’s landscape.
However, due to how these processes act to form tors, their impacts are in fact consequences of prior endogenic processes. Both hydrolysis and freeze-thaw weathering act through liquid water entering cracks and joints within the rock. This is why tors have core stones (large stones at the base with no joints—see figure 4): hydrolysis and freeze-thaw cannot act upon these core stones, and this is why these tors are still standing. Hydrolysis acts chemically, by solution, due to the slight acidity of rainwater caused by atmospheric CO2 dissolved within; freeze-thaw weathering acts by the water in the joints in the granite freezing and expanding (water expands by 9% when frozen), breaking the rock apart. These initial joints in granite are there due to endogenic processes alone. The horizontal joints in the tors, known as pseudo-bedding planes (see figure 4), are created by pressure release. Granite is intruded at great depths, and so it is under great pressure from the overlying country rocks. As these rocks are weathered away over millions of years and the denuded granite no longer has this pressure applied to it, it expands: cracks form horizontally due to the expansion. The vertical joints within granite are formed by contraction as the magma cools to form the rock. Both of these processes which form the cracks in granite are endogenic, as is the formation of granite itself. Furthermore, there is a theory that much of the apparent chemical weathering of granite that leads to tor formation is in fact pneumatolytic, meaning gases trapped under the granite and heated by the magma rotted the rock, which would mean that even the apparent exogenic weathering of hydrolysis was in fact mostly done by an endogenic process named pneumatolysis (Palmer & Nielsen, 1962). Therefore, every way in which these exogenic processes acted to form tors, one of the most characteristic aspects of Dartmoor’s landscape, is either potentially enacted by an endogenic process in pneumatolysis or the ultimate result of longer-term endogenic processes.
In conclusion, the exogenic processes which occur on Dartmoor are necessary for the formation of many of the features of the landscape, such the river gorges and tors and even the altitude. However, endogenic processes are ultimately more important: exogenic processes of erosion, chemical and physical weathering occur in many other places, and yet it is Dartmoor which has its characteristic landscape, as every characteristic feature of Dartmoor can be traced to its granite geology, and thus to the endogenic processes which created this granite.
Broadbent, M., 2014. Open University Geological Society, Burrator Reservoir. [Online]
Available at: https://ougs.org/southwest/event-reports/322/burrator-reservoir/
[Accessed 13 10 2021].
Dartmoor National Park Authority, 2002. Dartmoor Factsheets: Tor Fomation. [Online]
Available at: https://www.dartmoor.gov.uk/__data/assets/pdf_file/0025/72097/lab-tors.pdf
[Accessed 14 10 21].
Dartmoor National Park Authority, 2017. Dartmoor National Park: Basic Factsheets. [Online]
Available at: https://www.dartmoor.gov.uk/learning/basic-factsheets
[Accessed 12 10 2021].
Eton College Geography Department, 2021. Dartmoor Field Course 2021 Pamphlet. Eton: s.n.
Linton, D. et al., 1955. The Problem of Tors: Discussion. The Geogrpahical Journal, 121(4), pp. 482-483.
LUC, 2017. A Landscape Character Assessment for Dartmoor National Park, Bristol: Dartmoor National Parks Authority.
Palmer, J. & Nielsen, R., 1962. The origin of granite tors on Dartmoor, Devonshire. Proceedings of the York Geological Society, Issue 33, pp. 315-340.
Punnet, N., 2003. Granite Landscapes: Dartmoor. GeoFile Online, p. 1.
Rutledge, K. et al., 2011. National Geographic Encyclopedia Entry–Gorge. [Online]
Available at: https://www.nationalgeographic.org/encyclopedia/gorge/
[Accessed 14 10 21].
Veevers, J., 2011. Intrusive Igneous Activity and the Landscape. GeoFactsheets, Issue 270, p. 5.
(Featured Image: © Alison Day)