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“Investigate the impact of Human activity on the long-term evolution of Dartmoor”
Introduction
This report looks into the impact of human activity on the long-term evolution of
Dartmoor (Fig 1), by incorporating GIS analysis, field observations and relevant
literature. A pre-Holocene geological and climatic context of Dartmoor is given,
illustrating the landscape conditions before human activity. Humans began activities
on Dartmoor including deforestation, agriculture and mining, during the Holocene.
Evidence of anthropogenic perturbations can be provided, by comparing against the
more natural Smoky Mountains (Fig 2).
Fig 1. Map to show the location of Dartmoor in Devon. Showing the location of the
granite batholith (shaded blue), with its expected underground locations (light blue)
(Dartmoor National Park Authority 2004).
2
Fig. 2 a) map to show location of the Smoky Mountains in the USA, showing that it is
spread across Tennessee and North Carolina (Alley 2011). b) Map of Smoky
Mountains (National Park Service 2012)
Pre-Holocene geological and climatic context
290-270 million years ago granite intruded the landscape forming the Cornubian
batholith (Hagg 2008; Dartmoor National Park Authority 2004). The granite intrusion
met surrounding sedimentary Carboniferous rocks, resulting in contact
metamorphism (Hagg 2008). This created a baked zone between sedimentary rocks
and granite (Hagg 2008). This baking led to cracks in which minerals could flow (Park
1979; Hagg 2008). Minerals including cassiterites (tin ore) formed as the granite
cooled. These processes led to the formation of the Cornubian batholith’s largest
pluton; Dartmoor (Fig 1).
Dartmoor’s landscape has been shaped by climate variations (Molnar 2003). As the
Permian and Triassic developed the mountains were exposed to semi-arid
conditions, in a period of rapid erosion and weathering. It is believed glaciation cycles
have been the main driving force of pre-Holocene landscape change (Hagg 2008). It
is widely accepted that Dartmoor remained beyond the limits of glaciation during the
Pleistocene (Gerrard 1988; Campbell et al. 1998; Hagg 2008). Being south of the
glacial limit during the four ice ages in the Pleistocene period, resulted in
periglaciation, which had a large impact on the landscape (Hagg 2008). During the
Devensian, climate on Dartmoor was significantly cooler (by 10-25°C) (Watson 1977;
Hagg 2008).This led to permafrost development and formation of periglacial
landforms (Hagg 2008).
b) a)
3 As the Holocene began climate warmed, rainfall increased and vegetation developed
(Caseldine & Hatton 1994; Hagg 2008). Erosion of overlaying soils led to exposure
of granite, particularly at tors. Tors are tabular clumps of highly weathered and
jointed granite which have resisted wasting away (Hagg 2008). Denudation of
sedimentary rock surrounding the granite batholith resulted in the unroofing of the
Dartmoor pluton, which now forms an extruded upland located in Devon, England,
with elevations up to 621m (Fig 1) (Hagg 2008).
Human settlement patterns and activities
Dartmoor has evolved under considerable human interference during the Holocene
(Caseldine & Hatton 1994). It is a semi-natural landscape produced directly and
indirectly by the activities of prehistoric and historic human communities (Caseldine &
Hatton 1994). Human activities including deforestation, agriculture, and mining have
taken place on Dartmoor for at least 8000 years (Caseldine & Hatton 1994). Intensity
of activity varies according to the technological advancement, population changes
and climatic conditions.
The landscape between 9000 BP and 6500 BP is likely to have consisted of
complete woodland cover except isolated pockets of heath communities on the
highest exposed tors (Caseldine & Hatton 1994). It is probable that Mesolithic
communities played a key role in changing Dartmoor vegetation (Caseldine & Hatton
1994). Radiocarbon-dated studies give evidence of a continuous record of
microscopic charcoal between 7700 and 6300 BP (Caseldine & Hatton 1994). This
coupled with radiocarbon-dated studies of pollen showing a gradual reduction in tree
pollen and an expansion of peat-forming plants reflects the Mesolithic use of fire
(Caseldine & Hatton 1994). This reduced woodland cover and increased peat cover,
mainly in high latitude areas (Caseldine & Hatton 1994). Human use of fire has been
interpreted as a strategy for hunting game (Caseldine & Hatton 1994). Regeneration
of woodland was prevented by browse and exposure of acidic soils to heavier
precipitation, leading to the spread of blanket peat (Caseldine & Hatton 1994).
As Dartmoor entered the Neolithic period (circa 4,500 – 2,300 BC) much of the
woodland remained (Caseldine & Hatton 1994; Dartmoor National Park Authority
2004). The Neolithic period saw a shift to a more sedentary style of agriculture where
pastoral farming became more prevalent, instead of Mesolithic nomadic hunter
4 gatherer style (Dartmoor National Park Authority 2004). This led to further tree
clearance and lowland farms being created (Fleming 1988). Animals were
domesticated, as the population became more dietary reliant on cattle and sheep
(Dartmoor National Park Authority 2004). By around 3400 BP much of the tree cover
had been cleared from the high slopes of Dartmoor to maximise agricultural
opportunities, whilst the steep valleys remained heavily wooded (Caseldine & Hatton
1994).
Towards the end of the Neolithic, as Dartmoor entered the Bronze Age (2,300 – 700
BC) the landscape saw the radical reshape through the intensification of agricultural
practises (Caseledine & Hatton 1994; Dartmoor National Park Authority 2004).
Evidence of human existence at this time is shown by burial mounds known as cists
(Fig 3a), and reaves (Fig 3b) (Caseldine & Hatton 1994; Fleming 1988). Reaves are
the remnants of low walls used by Bronze Age communities to divide their land, to
enclose animals (Fig 3b) (Dartmoor National Park Authority 2004).
a)
Fig. 3 a) Photo to show burial cist on Dartmoor, near Bellever Tor.
5 b)
Fig. 3 b) Photo to show reave layout on Dartmoor, near Bellever Tor.
By the middle of the Bronze Age the majority of trees had been cleared from upland
Dartmoor for its conversion to farmland (Dartmoor National Park Authority 2004). The
combination of grazing, widespread clearance of trees, and changing climate
accelerated the formation of peat on the moor, and lead to the poor soil conditions of
today. It is believed that towards the end of Bronze Age the weather became colder
and wetter, with soils turning more acidic (Dartmoor National Park Authority 2004).
This made it increasingly difficult to farm and as the catastrophically wet conditions
persisted, communities abandoned Dartmoor, moving to lowland areas (Dartmoor
National Park Authority 2004). Dartmoor remained sparsely populated until the
Saxon and Medieval period (700 – 1540) when climate warmed, leading to an
increase in human population and hamlets appearing on Dartmoor (Dartmoor
National Park Authority 2004).
There is a long history of tin extraction on Dartmoor (Dartmoor National Park
Authority 2004). Hydrothermal activity, when Dartmoor first formed, lead to local
concentrations of mineral veins containing tin ore, in both granite and the surrounding
rocks (Park 1979; Hagg 2008). Tin was found in alluvial deposits and it is believed
the earliest form of tin extraction dates back to the 12th century; known as tin
streaming (Park 1979; Dartmoor National Park Authority 2004). Tin streaming
involved the collection of pure tin gravel from alluvial deposits in river beds, through
separating unwanted gravel by passing a stream of water over the gravels (Park
1979; Dartmoor National Park Authority 2004). This left extensive arrangements of
spoil heaps and channels (Park 1979; Dartmoor National Park Authority 2004). Leats
and reservoirs were constructed to divert water, and trenches dug along the valley
6 enabled loosened unwanted materials to be removed, leaving the tin, and ridges and
scars in the valleys (Park 1979; Dartmoor National Park Authority 2004). This
process is an example of equifinality as it created deep narrow river channels giving
them an appearance as though a knickpoint had migrated upstream. Dartmoor’s
industrial past of tin streaming has had a significant influence on the landscape seen
in figure 4.
Fig. 4 photo showing surficial tin mining on Dartmoor, showing how new drainage
had been constructed by cutting into the landscape. This has reshaped the hillside
leaving it unnaturally rugged.
Tin streaming had an influence on the landscape, because it altered discharge. A
small change in discharge can lead to a large change in the sediment carrying
capacity of a river, influencing the erosion rates (figure 5) (Cornwell et al. 2003). This
leads to sediment eroding or protecting the river bed, depending on the amount of
sediment (Sklar and Dietrich 1998; Dietrich et al. 2003). Generally rivers are erosive
systems in Britain, rather than depositional. This increase in sediment, causing
erosion or protection, alters the local base-level, leading to the hillslopes changing
angles in response; changing the landscape (Fernandes & Dietrich 1997; Reinhardt
et al. 2007).
7
Fig. 5 graph to show that a small change in discharge can lead to a large change in
the sediment carrying capacity of a river (source: Cornwell et al. 2003).
Infrastructure development from the late 18th century onwards increased
accessibility, providing further opportunity to quarry, mine and farm, seeing the
development of Dartmoor’s industrialisation (Dartmoor National Park Authority 2004).
In 1951 Dartmoor was designated a National Park, leading to its protection and
recreational use (Dartmoor National Park Authority 2004).
8 Presentation of Data
Fig. 6 Graph to show hillslope distributions of Dartmoor and the Smoky Mountains.
Fig. 7 Graph to show Hypsometry of Dartmoor and the Smoky Mountains.
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0 20 40 60 80 100
No
rmalised
are
a (
Km
2)
Hillslope gradient (degrees)
Dartmoor
Smoky
0
10
20
30
40
50
60
70
0
60
120
180
240
300
360
420
480
540
600
660
720
780
Are
a (
km
2)
Zeroed elevation(m)
Dartmoor
Smoky Mts
9 Dartmoor
Value Count Stream Order Ratio
1 78109
2 43127 0.552
3 25051 0.581
4 13573 0.542
5 4895 0.361
Mean: 0.509
Smoky Mountains
1 17679
2 7977 0.451
3 4030 0.505
4 2543 0.631
5 936 0.368
Mean: 0.489
Table 1 Stream order ratios for Dartmoor and the Smoky Mountains
10
Fig. 8 Map of Dartmoor created in GIS, showing the Dartmoor structures, the area of
granite, and the 5 main rivers studied on Dartmoor. These have been labelled from 1
to 5, for reference when looking at long profiles.
1. River Tavy
2. River Teign
3. Wray Brook
4. East Dart
5. River Walkham
11
a)
b)
c)
12 d)
e)
Fig. 9 Graphs to show the channel long-profiles of the 5 main rivers on Dartmoor.
Showing long-profile distance and elevation, where rivers cross granite boundaries,
and the knickpoint locations. Knickpoints are numbered for reference purposes. a)
long-profile of the River Tavy (river 1) which crosses a granite boundary at 6938m
upstream. b) long-profile of the River Teign (river 2) which crosses a granite
boundary at 9209m and 13169m upstream. c) long-profile of Wray Brook (river 3)
which crosses a granite boundary at 377m upstream. d) long-profile of the East Dart
River (river 4) which crosses a granite boundary at 18117m and 18577m upstream.
e) long-profile of the River Walkham (river 5) which crosses a granite boundary at
4435m and 13410m upstream.
13 a)
b)
c)
14 d)
e)
f)
Fig. 10 Graphs to show the channel long-profile of the 6 main rivers on the Smoky
Mountains, for comparison against the main rivers of Dartmoor. Geological
information is unavailable for the Smoky Mountains, therefore information on where
rivers cross geological structures or rock type boundaries cannot be given. Long-
profile distance and elevation with knickpoint locations are given on the profiles for a)
river 1, b) river 2, c) river 3, d) river 4, e) river 5, f) river 6.
15 a)
Dartmoor Equilibrium channel incision (mm.kyr-1)
River Sigma 10-6 10-7
1 -0.2 1.21 0.121
2 -0.2 0.47 0.047
3 -0.2 0.57 0.057
4 -0.5 33.53 3.353
5 -0.1 0.055 0.0055
Mean -0.2 7.167 0.7167
b)
Smoky Mountains Equilibrium channel incision (mm.kyr-1)
River Sigma 10^-6 10^-7
1 0.2 3.51 0.351
2 -0.4 22.02 2.202
3 -0.3 5.89 0.589
4 -0.4 56.71 5.671
5 -0.2 2.68 0.268
6 -0.6 270.66 27.066
Mean -0.3 60.245 6.0245
Table 3 showing channel-slope drainage-area relations shown in the sigma values
and estimated rates of channel incision (mm.kyr-1) for a) Dartmoor, and b) Smoky
Mountains. In both a) and b) two theoretical estimated rates of equilibrium channel
incision have been calculated. This is because in the equation used to estimate the
rate of river incision (E=kAθS, where E = the rate of river incision, k = an erosion
coefficient, A = drainage area (meters), S = mean channel gradient) the true value of
‘k’ is not known. Therefore two calculations have been conducted to show the range
that the equilibrium channel incision lies between, because the erosion coefficient for
granite is known to lie between 10-7 and 10-6 m.yr-1. By using the end-members of this
range the possible range of theoretical channel incision rates can be determined.
16
Catchment Erosion rate (mm.kyr-1) Error (mm.kyr-1)
1 20.6 1.3
2 30.2 1.7
3 21.2 0.9
4 36.6 1.7
5 31.0 1.7
6 26.5 1.3
7 40.0 2.6
Mean 29.443 4.429
mean erosion rate + error
33.872
mean erosion rate -error
25.013
Table 4 Cosmogenic nuclide based erosion rates for 7 catchment areas on Dartmoor
measured by Hagg (2008). The mean is given, with mean erosion rate + mean error,
and mean erosion rate – mean error.
Tor Erosion rate (mm.kyr-1) Error (mm.kyr-1)
1 32.00 3.20
2 29.34 3.00
3 18.63 1.86
4 27.65 2.77
5 45.61 4.49
6 16.00 1.70
7 20.33 2.04
8 14.00 1.40
Mean 25.445 7.716
mean erosion rate + error
33.161
Mean erosion rate - error
17.729
Table 5 Cosmogenic nuclide based erosion rates for 8 Tors on Dartmoor measured
by Hagg (2008). Mean Tor erosion rate is given, with mean Tor erosion rate + mean
error, and mean Tor erosion rate – error.
17
Fig. 11 Map created on GIS to show the location of the catchment areas used by
Hagg (2008) to calculate mean erosion rates of Dartmoor (table 4). Tor locations are
also given that Hagg (2008) used to calculate mean Tor erosion rates (table 5)
18 a) b)
Table 6 a) cosmogenic nuclide based mean erosion rates (mm/kyr-1) for the Smoky
Mountains. b) Cosmogenic nuclide based erosion rates for exposed bedrock on the
Smoky Mountains (Matmon et al. 2003). Means are given at the bottom of each
table, with mean + mean error, and mean – mean error. Erosion rates for Smoky
Mountains are given to compare against Dartmoor.
Mean 25.43±21.95 Mean + error 47.38 Mean – error 3.48
Mean 29.14±13.15 Mean + error 42.29 Mean – error 15.99
19 Analysis and discussion of results
A distinct difference can be seen between the hillslope distributions of Dartmoor and
the Smoky Mountains (figure 6). Dartmoor shows a more plateau shaped landscape
compared to the steep mountains of the Smoky Mountains. As previously mentioned,
Dartmoor has experienced large scale deforestation due to human interference
(Caseldine & Hatton 1994). Deforestation reduces the slope stability and increases
soil water, enabling the soil to move downslope through creep processes (Meyles et
al. 2006). This in turn increases sediment present in the convergent valley bottom
and decreases sediment on the divergent ridge top (Dietrich et al. 2003).
Deforestation also resulted in the exposure of the inherently acidic soil allowing it to
become water logged, leading to the formation of peat-land, covering the majority of
Dartmoor (Caseldine & Hatton 1994). The resulting process when combined with the
spread of peat acts to smooth the landscape, further denuding the slopes of
Dartmoor. This denudation has resulted in the plateau shape of Dartmoor as seen in
figure 6. The Smoky Mountains have not experienced the same scale of
deforestation, retaining the majority of its forest cover, with slopes and mountain
crests soil-covered and heavily vegetated (Matmon et al. 2003). As a result the
hillslopes of the Smoky Mountains have not experienced the same denudation. Due
to deforestation Dartmoor has experienced more denudation than the natural
Smokey Mountains, providing evidence that the entire Dartmoor landscape has been
influenced by human activity.
Landscape hypsometry describes how an area changes with elevation, making it a
useful descriptor of topography. The hypsometry (figure 7) illustrates further that
Dartmoor has a significant plateau of elevations, indicating that Dartmoor is an
upland massif surrounded by a rim of higher slopes (Dartmoor National Park
Authority 2004). Relief is a fundamental property of a landscape, as seen in figure 7
relief is higher in the Smoky Mountains than in Dartmoor, providing further evidence
that Dartmoor has experienced denudation. It may be expected that the Smoky
Mountains would have higher erosion rates, due to higher elevation. As can be seen
in figure 7 relief of both Dartmoor and the Smoky Mountains is less than 1000
meters, illustrating the landscapes are low-relief and transport limited. This indicates
that both areas lie within the linear tectonically inactive end of the spectrum of
mountain ranges. It can be also seen in figure 7 that the Smoky Mountains have a
20 similar structure to Dartmoor, giving justification for comparison between Dartmoor
and the Smoky Mountains.
The quasi-length stream order ratios give Strahler ordering for Dartmoor and the
Smoky Mountain river networks (table 1). These ratios relate to Hortons (1945) law of
stream lengths which states a geometric relationship exists between the average
length of streams of a given order and the corresponding order. The mean quasi-
length ratio of Dartmoor (0.509) and the Smoky Mountains (0.489) are similar,
suggesting both locations have similar river network structures. It would therefore
appear that the fluvial processes at both sites have similar natures. This similarity
demonstrates the scale invariance of stream order ratios, the fractal nature, and
ultimate aim of rivers to reach base-level regardless of worldwide location (Horton
1945; Fernandes & Dietrich 1997). This similarity provides further evidence that
these locations are comparable. As form follows process we can infer that the river
networks on Dartmoor remain in a natural unaltered form, as the comparable Smoky
Mountains are relatively unaffected by human influence.
The 5 main rivers on Dartmoor are shown in figure 8 and the long-profiles of these
main rivers are given in figure 9. The straight river profiles and presence of
knickpoints show disequilibrium. Whipple (2001) observes that river longitudinal
profiles move towards an equilibrium form which best facilitates transport of the
sediment loads and erosion of the bed; a simple concaved shape. Therefore the
straighter a channel the more it is in disequilibrium. Knickpoints have been pointed
out and numbered on the river profiles (figure 9). In attempt to see whether human
activity has caused knickpoints, they have been identified on an Ordnance Survey
(OS) map using GIS software (figures 12-20).
Fig. 12 Map to show section of the River Tavy where knickpoint 1 is located (figure
9a), taken from OS.
21
Fig. 13 Map to show section of the River Tavy where knickpoint 2 is located (figure
9a), taken from OS.
Fig. 14 Map to show section of the River Teign where knickpoint 1 is located (figure
9b), taken from OS.
Fig. 15 Map to show section of the River Teign where knickpoint 2 is located (figure
9b), taken from OS.
Fig. 16 Map to show section of Wray Brook (River 3) where knickpoint 1 is located
(figure 9c), taken from OS.
22
Fig. 17 Map to show section of Wray Brook (River 3) where knickpoint 2 is located
(figure 9c), taken from OS.
Fig. 18 Map to show section of Wray Brook (River 3) where knickpoint 3 is located
(figure 9c), taken from OS.
Fig. 19 Map to show section of the East Dart (River 4) where knickpoint 1 is located
(figure 9d), taken from OS.
Fig. 20 Map to show section of the River Walkham (River 5) where knickpoint 1 is
located (figure 9e), taken from OS.
As can be seen from the OS maps, some knickpoints are in areas where no evidence
of human activity has taken place (figure 13, 14); their presence is therefore likely to
be due to base-level lowering. Other natural knickpoints (figure 17 and 18) are
located on structural boundaries, which suggest static knickpoints. Figure 12 also
23 shows a natural knickpoint on the River Tavy, likely to be caused by the river
excavating the horizontal and vertical joints in the bedrock, because of the steep
slopes. By presenting the long-profiles of the main rivers of the Smoky Mountains
(figure 10) we can see that these rivers also have knickpoints with straight profiles.
This demonstrates that rivers in natural environments can be in disequilibrium.
The other knickpoints on Dartmoor rivers are located near human settlements or
have evidence suggesting humans have caused them (figure 12, 15, 16, 19, and 20).
A study by Park (1979) confirms that extensive tin streaming took place surrounding
knickpoint 2 (figure 15) which significantly narrowed the channel and altered the
valley shape. Knickpoint 1 on the East Dart River (figure 19) is located near
Postbridge, in an area of significant past human activity including an ancient
settlement and mine shaft, suggesting its presence is due to human activity. The
transient knickpoint on the River Walkham is located by Merrivale mine (figure 20).
Photographic evidence further suggests that Merrivale mine has had an impact on
the surrounding area (figures 21-23). These transient human induced knickpoints are
present on all of the 5 main rivers in Dartmoor, illustrating the extent of anthropogenic
perturbation.
Fig. 21 Photo to show Merrivale mine on Dartmoor.
24
Fig. 22 Photo showing the flattened v-shaped valley upstream of Merrivale mine. The
landscape shows response to deforestation during the Mesolithic-Bronze Age period,
through its blanket peat and flattened, declining hillslopes. No effects of Merrivale
mine can be seen, as this is upstream of the mine.
Fig. 23 Photo to show the steeper v-shaped valley downstream from Merrivale mine.
This potentially illustrates that Merrivale mine has had an influence on the discharge
and/or sediment levels in the river, resulting in increased erosion. This illustrates
possible human impact of the mine on the landscape. Alternatively this change in the
landscape may be due to base-level lowering.
Hack (1960) explains the strong relationship between drainage area and channel
slope. Table 3 gives the sigma values which equate to the concavity index of the
river. Research suggests that sigma values greater than -0.2 are indicative of debris
flows being the dominant process (Stock & Dietrich 2006). Debris flows occur in
steep mountain channels where they tend to create straight uncurved channels, but
such flows do not arise from natural circumstances in Dartmoor – therefore a value of
-0.1 or -0.2 indicates human disturbance. The concavity indices of the Dartmoor
rivers (table 3) are all between -0.1 and -0.2 apart from The East Dart River (river 4),
25 which is -0.5. Therefore all Dartmoor rivers, apart from river 4, indicate human
disturbance. The mean concavity index for Dartmoor (-0.2) also varies significantly
(by 0.1) from the mean concavity index of the Smoky Mountains (-0.3) suggesting
that the Dartmoor rivers are operating in a different way to Smoky Mountain rivers.
This could be due to the difference in human activities on each landscape, as
Dartmoor has experienced large scale human interference, whereas the Smoky
Mountains have remained relatively unaffected by humans (Matmon et al. 2003).
The majority of estimated rates of river incision for Dartmoor (table 3) are therefore
invalid and ‘nonsense’ results, as the rivers appear to be in disequilbrium, illustrated
by river profiles and concavity indices. This is except for river 4, which has a
concavity index of -0.5 (table 3), typical of low relief shallow angle rivers observed on
Dartmoor. Therefore the channel incision value for river 4 is valid; equilibrium
channel incision for river 4 lies between 3.35 – 33.53 mm.kyr-1. The concavity value,
long-profile (figure 9d), and erosion estimate suggests that the East Dart River (river
4) is in equilibrium. Although human activity may have caused the transient
knickpoint (figure 19) it appears that the river has had enough time to reach a new
equilibrium. This demonstrates the importance of time scale in landscape process
(Schumm & Lichty 1965).
The cosmogenic nuclide based erosion rates (table 4) measured by Hagg (2008)
show the mean erosion rates of 7 catchment areas on Dartmoor (figure 11). These
erosion rates can be compared against cosmogenic nuclide based erosion rates of
the Smoky Mountains measured by Matmon et al. (2003). The mean erosion rate for
Dartmoor is 29.44±4.43 mm.kyr-1, and the mean erosion rate for the Smoky
Mountains is 25.43±21.95 mm.kyr-1. A rate difference of <~5 mm.kyr-1 is not
significant; it therefore appears Dartmoor and the Smoky Mountains are eroding at a
similar rate. According to Matmon et al. (2003) the Smoky Mountains are in dynamic
equilibrium, the fact that measured erosion rates in Dartmoor and the Smoky
Mountains do not differ significantly suggests Dartmoor is also in dynamic
equilibrium.
An Additional method to determine whether Dartmoor is in equilibrium is to see
whether the Tor and catchment means overlap within their error bounds. The mean
erosion rate range when the mean error is taken away and added to the mean
26 erosion rate is 25.01 – 33.87 mm.kyr-1, and the Tor mean erosion rate range is 17.73
– 33.16 mm.kyr-1. These measurements overlap, showing that the Tor and catchment
means are indistinguishable, giving further evidence that Dartmoor is in equilibrium.
It is important to bear in mind limitations. The limitation of cosmogenic nuclide
analysis is that it assumes a constant rate of erosion over an integration time. This is
not the case for Dartmoor, as human activity has altered and skewed erosion rates
over time. The human impacts on the landscape may not show in cosmogenic
nuclide results as the perturbation is within the integration time of the analysis.
Furthermore as landscapes take a long time to respond to change, the effects of
human activity on the landscape will unlikely manifest themselves in an observable
form for some time. River channels are more sensitive to perturbation and although
this makes them a weaker method of analysis they will demonstrate anthropogenic
perturbations more clearly than bedrock and catchment analysis.
When looking at river channels the majority of the 5 main rivers on Dartmoor appear
to be in disequilibrium (apart from the East Dart River). This is because rivers are
more responsive to human influence, so we will see them responding quicker than
the rest of Dartmoor’s landscape. There are however limitations associated with the
methods used to retain these results. Digital elevation models (DEM) were used to
compare hillslope distributions, plot channel long-profiles, calculate channel slope
angles, and estimate erosion rates. DEMs are not true representations of a river
network (figure 24) (Montgomery 1993). This is because DEMs are made up of cells,
usually of 30m2 or 100m2, each cell being an average elevation of that 30/100m2.
DEMs are therefore made up of averages and could be described as pixelated
(Montgomery 1993).
27 a) b)
Fig. 24 a) and b) show the misrepresentation of rivers in digital elevation models
(DEM) (created in GIS). White shows the ‘actual’ river and blue shows the DEM river.
Pixilation of ‘actual’ rivers can be seen, which sometimes shows rivers to flow in a
straight line instead of their natural course, giving evidence that DEMs are not true
representatives. The blue line is also shown to not follow the exact course of the
‘actual’ river, illustrating that DEMs are not true representatives of the landscape,
which may cause bias in results.
Conclusion
To conclude, it is evident that Dartmoor has evolved through a series of climate
driven processes. It appears that Dartmoor’s evolution is natural, until the Holocene.
Human perturbations within the Holocene are evident in Dartmoor’s river profiles.
Human impacts can also be seen by comparing Dartmoor against the similar, but
more natural landscape of the Smoky Mountains. These human perturbations are not
evident in other parts of the Dartmoor landscape, because rivers are more sensitive
to human influence. It is shown in the East Dart River that over time rivers can reach
a new equilibrium, even after human impacts. It may be expected that given time
other rivers will reach a new state of equilibrium. Depending on spatial scale different
responses are taking place. At the river channel scale disequilibrium, due to human
activity can be seen, but at the river network and catchment scale it appears that
Dartmoor is in equilibrium. This may be because humans have only had an effect at
the localised scale, however hillslopes must respond to changes in river profiles and
base levels. It is therefore likely that the extent to which humans have altered the
landscape will not be seen in the entire Dartmoor landscape until the distant future.
Overall this demonstrates the importance of temporal and spatial scale when
analysing landscapes (Schumm & Lichty 1965).
28 References
Alley, R. (2011) Textbook 4.1: Plate Tectonics III, The Great Smoky Mountains,
24/04/12, <https://www.e-education.psu.edu/geosc10/l4_p2.html>
Campbell, S., Gerrard, A.J., Green, C.P. (1998) Granite landforms and
weathering products. In Quaternary of South-West England, Vol. 14 (eds. S.
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