A new approach to the relief of Great Britain

Geomorphology 27 Ž1999. 173–189

A new approach to the relief of Great Britain III. Derivation of the contribution of neotectonic movements and exceptional regional denudation to the present relief Keith Clayton ) , Nadhim Shamoon School of EnÕironmental Sciences, UniÕersity of East Anglia, Norwich, NR4 7TJ, UK Received 12 March 1998; revised 13 May 1998; accepted 4 July 1998

Abstract This paper builds on the contribution of relative rock resistance to contrasts in the relief of Britain, to establish the contribution of other factors. These include the slope of the major rivers towards the sea, itself in part a function of the size of material eroded from the headwater uplands and thus of their lithology and the steepness of their valleyside slopes. The pattern of denudational unloading and thus of isostatic uplift is mapped and compared with actual mean altitude. It is shown that the sum of predicted isostatic uplift and local base-level is closely related to the current mean altitude of our uplands. The predicted mean height based on the six equations for each rock resistance class relating mean altitude to river distance can also be compared with actual mean elevation. This shows positive departures which are most readily explained by neotectonic uplift, notable in the Scottish Highlands, the Alston Block of the northern Pennines and Fforest Fawr in South Wales. Negative departures may be due to relative neotectonic subsidence, as in Buchan, where there is coincidence with positive gravity anomalies. However, most negative departures can be linked with severe glacial erosion, especially in such areas as westernmost Scotland, the Lake District, the Vale of Belvoir and the WashrFens Basin where evidence for deep glacial erosion is already strong. Regional height differences related to the major structural regions of Britain are calculated; they make a smaller contribution to differences in average elevation, and thus to the overall relief of Britain. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Great Britain; relief; megageomorphology; denudational unloading; isostatic uplift; glacial erosion; neotectonics

1. Isostatic uplift as a consequence of denudational unloading In Paper I of this series ŽClayton and Shamoon, 1998a., the ‘freeboard’ at the margin of Britain was

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noted from a comparison of the general level of the land and of the continental shelf close to the present coastline and a difference of almost 180 m established. While a small part of this may be linked with water loading during the Flandrian transgression, the bulk was matched by a measure of denudational unloading which was linked to isostatic uplift of the land. Although it has played little role in most accounts

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of the evolution of the land surface of Britain Že.g., Brown, 1979; Goudie, 1990., accounts of mobile areas such as the Great Escarpment of southwestern Africa ŽGilchrist and Summerfield, 1990., or the Western Ghats of India ŽWiddowson, 1997. provide evidence of continuous uplift in response to steady erosion. In Britain, the evidence for isostatic adjustment to the ice load of the Devensian glaciation, and of recovery with its wastage prior to the Windermere Interstadial ŽShennan, 1989; Lambeck, 1991. shows that the crust reacts to changing loads with little delay. Thus, the reduction in load as rivers and glaciers have cut into the land mass and transported sediment to the surrounding seas has surely been matched by isostatic uplift. The concept is supported by Small and Anderson Ž1995., who argue that the result of denudation in the Californian Sierra Nevada over several million years has been to reduce mean elevation and increase summit elevations as a result of isostatic response to unloading. Apatite fission track analysis provides evidence of thermal cooling, and of the gradual unroofing of deeply-buried rocks by erosion. Using this approach, the removal of one or more kilometres of rock during the Tertiary has been described for the East Midlands ŽGreen, 1986, 1989. and up to 3 km for Northern England ŽLewis et al., 1992.. Although there have been challenges to the assumptions made in using the apatite fission track evidence, and allowance must be made for variations in the assumed palaeogeothermal gradient ŽBrown et al., 1994., the evidence for considerable Tertiary denudation is strong. It is supported by data using sediment compaction offshore ŽHillis, 1991. and the constraint is more usually the problem of inferring adequate thicknesses for the cover sediment ŽHolliday, 1993. or credible rates of deposition ŽJapsen, 1997., than the implied rates of denudation which are low, given the long timescale of the Tertiary period. However, the significance of these papers is that they demonstrate the reality and magnitude of denudational unloading. Commenting on the removal of post-Palaeozoic sediment from parts of northern England, Lewis et al. Ž1992., p. 140, stated, ‘‘it is therefore important to recognise that the apparent ’uplift‘ indicated by the removal of 3 km of overburden is not all tectonically induced, but that the majority occurs in response to isostatic rebound’’.

2. The development of British relief as denudation has dissected the land mass The aim of this paper is to use the essentially empirical approach to analysis of this British database already utilised in Papers I and II of this series to explore in more detail the British landform. Numerical relationships which might throw light on the history and origins of the British landform are sought, constraining these as little as possible by an imposed theoretical background. The intention is to devote a later paper to a comprehensive explanatory approach to British relief: this paper explores numerical links without imposing a theoretical context. Nevertheless, it is necessary to clarify the concepts involved in neotectonic movements which are central to the approach adopted here. Section 1, following Paper I, has noted the significance of isostatic uplift consequent upon denudational unloading. Yet as England and Molnar Ž1990. point out, while the development of relative relief and increases in summit elevations ŽSmall and Anderson, 1995. are a consequence of isostatic response to denudational unloading, mean elevation will not be increased. Indeed, because the full extent of isostatic recovery involves removal of rocks with lower density than those deeper in the crust, only some 5r6ths Ž0.82. of the mean denudation is matched by uplift ŽGilchrist et al., 1994, p. 964.. Thus, while the dissection of the British landmass has resulted in increases in summit elevations and isostatic recovery of the land which has further emphasised the persistent downcutting achieved by all our larger rivers, it cannot be the cause of the values of mean elevation across Britain. These were andror are the result of true tectonic uplift which created the uplifted landmass on which the persistent and effective process of denudation have for so long been at work. For the moment we take that uplift for granted, seeking only to try and establish how far it might still be proceeding, rather than exist as a fossil relic of past tectonic activity. 3. The contribution of local base-level to mean elevation Despite the disparity in elevation of the land and adjacent sea floor noted in Paper I Žand to be further

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explored in Paper IV., all the major rivers of Britain flow smoothly to present-day sea level. For much of the Quaternary, they have flowed further offshore before reaching the coastline on the continental shelf, mean sea level over the last half million years having been at about y55 m ŽShackleton, 1987; Porter, 1989, fig. 9., with lowest sea levels in glacial periods up to 140 m below present-day sea level. In detail, large rivers are able to adjust to varying length and local slope by adjusting their behaviour Žchanging channel dimensions, by incision or by aggradation. as must have happened over the 5–6000 years since sea-level reached its present stand. Newson mapped the overall slope of rivers as part of the analysis of their flood behaviour, indeed his maps of stream frequency and channel slope ŽNewson, 1978, p. 289. are useful adjuncts to the data held on our digital data base. The slope of major rivers, draining the upland water partings, is adjusted to liquid discharge and the calibre and quantity of sediment being carried, itself a function of rock type and valleyside slopes. The need to evacuate eroded sediment if denudation is to persist, requires channel gradients which cannot easily be reduced, at least while uplift continues in response to erosion. Steep slopes on resistant rocks will require the highest channel slopes and together with the distance to the coast Žtypically 100 km or more—see fig. 1 in Paper I. will give relatively high local base levels within our uplands. These channel Žvalley-floor. elevations can be plotted by mapping the lowest elevation within a moving window, sampling the low-point values for each square. Given the typical separation of the main rivers within our uplands, a moving window 11 = 11 km was chosen as appropriate, so identifying the lowest valley floor within 11 km of any point ŽLIP11 —see list of abbreviations, Table 1.. The smoothed Ž25 = 25 km to match other maps. map of these values, essentially a map of local base-levels, is shown at Fig. 2. The overall resemblance between this map and a relief map of Britain is clear, though values are low as this maps the local base-level. The Highland blocks are shown, with values reaching over 100 m in Dartmoor, the Peak District of the southern Pennines, and the Highlands north of the Great Glen, and just failing to reach 100 m in Exmoor and the

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Table 1 List of abbreviations for variables derived from the database LP HP MP RD LIP11 HIP11

Lowest elevation of each 1 km2 Highest elevation of each 1 km2 Mean elevation of each square, the average of the LP and HP values for each square River distance from the coast for each square Žkilometres. The lowest included point in an area of 11=11 km2 The highest included point in an area of 11=11 km2

North York Moors Žfor all locations mentioned in the text, see Fig. 1.. Values over 150 m occur in Central Wales, the Askrigg Block and the Southern Uplands. The highest values are found in the Alston Block Žjust exceeding 200 m. and the Grampian Highlands with values reaching over 300 m in the Cairngorms and over 250 m in the Mondhliath Mountains. The Wessex Chalk just touches 100 m. Thus, for most of Highland Britain, areas near watersheds generally have local base-levels in the range 100–200 m, only in the Grampians are values above 200 m at all widespread.

4. The spatial pattern of isostatic adjustment to denudational unloading Fig. 3 is based on the square by square values of a measure of denudational unloading Žand thus a measure of isostatic uplift following denudation. represented by ŽHIP11-MP. = 0.82. HIP11 ŽTable 1. is a measure of a reconstructed upper surface below which the current landscape has been developed through erosion by rivers and glaciers, it involves the same level of generalisation as the LIP11 values used for Fig. 2. MP is the average elevation of each square, thus, the difference is the mean volume eroded below the inferred summit surface. The adjustment factor of 0.82 is based on Gilchrist et al. Ž1994, p. 964. and allows for the contrast in density between the eroded rocks and the crustal rocks involved in isostatic adjustment. Thus, Fig. 3 is designed to map uplift consequent upon dissection below the 11 = 11 km summit surface. As is the case for all the maps in this series of papers, it has been generalised by a 25 = 25 km window.

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Fig. 1. Places and features named in the text.

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Fig. 2. Local base-level—as mapped from the lowest point within each 11=11 km2 ŽLIP11., generalised by 25=25 window.

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Fig. 3. Map of isostatic uplift resulting from denudational unloading below a reconstructed summit surface defined by HIP11; ŽHIP11-MP. )0.82. For explanation of the basis of the calculation, see text.

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Although a map of predicted isostatic uplift, Fig. 3 also bears a strong resemblance to the pattern of a relief map of Britain. The lowlandrhighland division is dominant and emphasised by the colour switch between blue and green at 150 m. Within lowland Britain, the main elements of the cuesta and vale relief can be discerned, including the Lincolnshire Wolds, the Chilterns and the western margin of the Weald where the North and South Downs converge. Within Highland Britain the main upland masses can be seen separated by the intervening lowlands. From south to north, Dartmoor, Exmoor, the Welsh Upland, Lake District, Pennines, North York Moors, Southern Uplands, and the Scottish Highlands are all readily recognised, while the Central Lowlands of Scotland, the lowlands at the head of the Solway Firth and the LancashirerCheshire Plain are also obvious. Another feature of Highland Britain is the overall slope from west to east with highest values occurring in Snowdonia, the Lake District, the SW part of the Southern Uplands Žcentred on the Rhinns of Kells. and the western margin of the Scottish Highlands from Ben Nevis, through to Ben More Assynt. In terms of overall shape, this map of isostatic uplift following denudational unloading matches very closely David Linton’s careful reconstruction of the sub-Cenomanian surface utilising the highest summits ŽLinton, 1951, fig. 2, p. 70..

5. The relationship between mean relief and isostatic uplift We may compare the denudational isostatic uplift mapped in Fig. 3 with mean elevation by subtracting the calculated uplift from the average elevation. The overall Žgeneralised. pattern is mapped in Fig. 4. Quantitatively the difference reflects the fact that the uplift consequent upon unloading predicted by Fig. 2

is equivalent to only about half of the actual mean elevation of Britain. Thus, the broad pattern of differences mapped in Fig. 4 again reflects in a subdued way the overall pattern of relief in Britain, though there are some important exceptions. The areas of broad agreement with the map of mean relief Žfig. 6, Paper I of this series; Clayton and Shamoon, 1998a. will be noted first, they are emphasised by the widespread pale colours indicating modest differences in elevation between the two maps. The LowlandrHighland boundary is reasonably clear, though the Cotswolds just touch a residual value of 100 m, while the North York Moors do not. The uplands of Dartmoor, Exmoor, Wales, the Pennines, the Southern Uplands and the Grampian Highlands all have relatively high residual values. However, Snowdonia and the Lake District have negative values; indeed negative values fringe the eastern side of the Irish Sea from Aberystwyth and Lleyn to Conway and around the Solway Firth. Western Scotland, from Kintyre to Sutherland is everywhere strongly negative. These areas where the calculated uplift exceeds current mean relief are not easy to explain. Like other maps in these papers, questions are raised by the evidence of the map, but answers must be sought elsewhere. There is likely to be a tectonic influence and the close association with the Irish Sea and the west coast of Highland Scotland implies this. Down-warping andror down-faulting of the western margin of Highland Britain is a concept that has been raised elsewhere ŽLinton, 1951; Cope, 1994, 1995. and this map suggests it should be taken seriously; it could explain the relative elevation of these areas. Part of the lack of fit will be accounted for by the fact that the summit surface selected is generalised over a relatively small distance, i.e., a 11 = 11 km window. We must also expect some lack of fit due to

Fig. 4. Contribution of isostatic response to denudational unloading to elevation attributed to relative resistance to erosion. ŽMean altitude Žfig. 6, Paper I. less presumed isostatic uplift resulting from denudational unloading Žas shown in Fig. 3. —computed from MP— wŽHIP11-MP. )0.82x. Fig. 6. The residuals when the predicted mean altitudes using the six regression equations for each relative rock resistance class Žtable 8 and fig. 3b, Paper II. are subtracted from actual square by square values of MP; the calculated square by square values are smoothed by a 25 = 25 km window.

K. Clayton, N. Shamoonr Geomorphology 27 (1999) 173–189

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the fact that relatively large areas of mixed rock type will move isostatically—there are no bounding faults or flexures around individual outcrops. Indeed, England and Molnar Ž1990, p. 1173. suggest that an area of at least 10 3 to 10 4 km2 is the minimum that will be affected by tectonic processes, including the isostatic adjustment to erosion. Thus, an outcrop of weak rock within an area of more resistant rocks will

be dominated by the movement as the more resistant rocks are dissected, and our statistical approach to classification will have included this fact as part of the statistical ‘noise’ in the data we are handling. Another match that deserves comment is with a measure of glacial erosion in the deeply dissected Žglaciated. landscapes of westernmost Scotland, the Lake District and Snowdonia. These areas are not

Fig. 5. Sample upland areas analysed in Table 2 and structural divisions utilised in Table 3.

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only deeply dissected, which has the effect of maximising the contribution from HIP11-MP and reducing the value of MP, but have been dissected by ice, which by producing wider, more open valleys than upland river erosion, has further reduced the value of MP. It should be noted that all these areas have previously been mapped on morphological grounds as Zone IV, the most intense style of glacial erosion ŽLinton, 1963; Clayton and Linton, 1964; Clayton, 1974.. In the western Highlands where divide elimination has proceeded furthest, the effect is most marked. It appears that these areas fail to reflect in their mean relief the deep dissection and thus potential isostatic recovery achieved by successive glaciations over the last half million years or more. At this stage of analysis, a possible explanation for the anomalous areas of severe glacial erosion is that they are too small to respond fully to the stresses induced by denudational unloading and have moved as part of larger blocks which are in general less deeply dissected. However, the differential stresses must be considerable, especially in western Scotland and some isostatic adjustment must be expected, both in the past and for the future. In particular, it may be significant that the highest peaks of England, of Wales and of Scotland lie within these areas.

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6. The combined effects of local base-level and isostatic uplift induced by denudation Both Figs. 2 and 3 account for about half the mean altitude mapped in fig. 6, Paper I of this series. The highest elevations coincide on both maps and are generally very similar in value. This is true, not only of the general overall pattern Žwhich mimics the relief map of Britain., but there is also a remarkably close match of detail, such as the Cotswolds which just attain 100 m on both maps, the highest values of Central Wales Žjust over 200 m in each case. and the unusually high values for the Cross Fell ŽAlston Block. area of the northern Pennines. In Scotland, the highest Southern Upland value is similar on both maps, while in the Cairngorms and the Monadliath Mountains ŽEastern Grampians., both the pattern and the highest values are very similar. These observations suggest that local base-level together with the measure of isostatic uplift should match a large part of the mean altitude of our uplands. To test this, a series of 12 samples of areas of about 80–110 km2 was taken to cover the main upland areas, including two samples from Lowland Britain. For each headwater area selected, a sample area with the highest values for the 11 = 11 km

Table 2 Comparison of predicted mean elevation Žfrom the sum of mean denudational unloading and local base-level. and actual mean elevation for sample upland areas Žfor locations see Fig. 5. Column 1 Sample location

2 Area Žkm2 .

3 Mean elevation Žm.

4 Mean LIP11 Žm.

5 Unload ŽHIP11-MP. =0.8 2 Žm.

6 Cols. 4 q 5 Žm.

7 Col. 6 as % Col. 3 Ž%.

Cairngorm Rannoch Moor Central S. Uplands Cheviots Cross Fell Askrigg Block N. Peak District S. Peak Mid-Wales S. Wales Cotswolds Wessex Chalk Mean all sites

83 90 81 84 108 69 89 72 77 90 96 174 93

869.5 635.5 447.6 396.7 554.7 468.6 424.8 376.2 383.5 469.6 198.1 171.0 428.3

423.0 348.3 231.7 192.7 323.9 200.1 200.6 209.2 228.1 209.3 119.4 109.0 216.5

297.7 666.1 292.2 136.2 195.2 191.1 155.7 138.0 130.8 281.7 70.8 85.0 182.8

720.7 482.7 523.9 328.9 519.1 391.2 356.3 347.2 358.9 491.0 190.2 194.0 407.4

82.9 104.8 117.0 82.9 93.6 83.5 83.9 92.3 93.6 104.6 96.0 113.5 95.1

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low-point calculations was selected, these varied in shape, but met the sample area required for a reliable estimate of mean relief and mean depth of dissection below the reconstructed ŽHIP11. summit surface. The sample areas are shown on Fig. 5, and the resulting values are set out in Table 2. It will be seen that the combined values for local base-level ŽLIP11. and uplift consequent upon denudational unloading Žhere represented by wHIP11-MPx = 0.82. explain over 80% of the value for mean elevation for most of these samples. That two variables, each justified by our understanding of the behaviour of an eroding surface, can account for mean elevation so consistently is very encouraging. It is not in itself an explanation of present mean relief Žhow has the elevation of the valley floors been achieved, for example?. but it goes a long way in suggesting the direction of future enquiry. Another way to look at the values in Table 2 is to consider that we start with two elements of the current relief, the reconstructed summit surface ŽHIP11. and the mean elevation ŽMP.. The values of HIP11 are far too large to be accounted for by our measure of isostatic uplift, even when the local base-level is allowed for. Explanation of these summit levels will require further work, but in the meantime it is interesting that mean elevation is quite closely matched by the sum of local base-level and our measure of isostatic uplift. If we deduct local base-level plus uplift from mean altitude, we usually have a residual mean elevation to account for. This is a maximum of 17% of the mean elevation for the Cairngorms, the Cheviots, the Askrigg Block and the Northern Peak District, and 4–8% for Cross Fell, the Southern Peak District, mid-Wales and the Cotswolds. For the remaining four cases we have higher values than the mean elevation, very much more in the case of the Southern Uplands. The possibility that regional tectonic movement might play a part in these differences is discussed in Section 9 below.

7. Further consideration of the link between relative rock resistance and relief It is important to remember that the isostatic uplift resulting from incision and the resulting denuda-

tional unloading is included in the measure of relative rock resistance Žfor details see Paper II of this series.. It is not an additional variable, even though it may be assessed by HIP11-MP or some similar measure of erosion below a summit surface. Indeed, it is clear, that when this issue is considered, an important element in creating areas of high ground on relatively resistant rocks is not the direct resistance of the rocks to erosion, but rather their relative ability to withstand deep dissection Žwith steep valley slopes. and thus to contribute above-average unloading which in turn drives higher values of isostatic uplift. Tough rocks form high ground because they can be deeply dissected, not because they resist dissection. Weaker rocks may be readily incised, if slopes to base-level allow, but the relatively gentle valley-side slopes mean that interfluves will be attacked and lowered relatively easily. In these cases, overall uplift in response to denudation is not fully recorded because the interfluves have been lowered as well as the valleys. Weak rocks contribute finer material to rivers, allowing more gentle down-valley slopes and thus creating lower local base-levels in the upper reaches, such as characterise the clay vales of the scarplands of southeastern England. In contrast, the steep slopes associated with tough rocks will contribute larger-sized material to the rivers, so requiring them to retain steeper down-valley profiles and so limiting the rate at which local base-level in their upper reaches may be lowered. Where rivers cross tough rocks with high local relief, areas upstream may be protected from base-level lowering, slowing down the rate at which their mean altitude can be reduced. All these situations will have been included in our generalised approach to relative rock resistance. In Paper II of this series, it was noted that the intercepts of regressions between height and river distance correlated well with other variables designed to measure relative elevation, as did computed altitudes at river distance between 20 and 80 km. However, in all cases the regression slopes did not produce similar rank orders to the other variables. The reason for this is linked with the fact that, in contrast to the regression slopes, the intercepts do correlate very well with the other variables; the more resistant rocks are more deeply dissected whenever

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they outcrop near the coast and as a consequence have been isostatically uplifted to produce a higher freeboard at the coast. The result is that such rocks often show a low regression slope as river distance ŽRD. values increase, while others have been uplifted less near the coast and rise more steeply inland. Thus, while absolute values of altitude both near the coast and inland correlate well with the other variables, regression slope does not, nor do the regression slopes for HP, MP and LP show any common pattern.

8. Departures from the rock r relief relationship already described We can eliminate the average effect of rock resistance on average altitude ŽMP., by plotting residuals from the statistical relationship for each of the six classes. The starting point is the set of regression equations between MP and RD Ždefined in Table 1. for each of the rock resistance classes Žtable 8, Paper II.. For every kilometre square for which we have a rock resistance class, mean elevation ŽMP. is calculated from the RD value. These values were plotted in fig. 3a of Paper II. These values are then compared with the actual square by square values of MP Žshown in generalised form as fig. 6 of Paper I. by subtracting the predicted value from actual MP and again the results may be plotted as a map. The resulting map is inevitably dominated by ridge and valley contrasts, since at any small range in RD values, the valley bottoms will be lower and the ridges higher than the predicted mean altitude. Thus, generalisation is required, with the smoothing window covering a larger area than a typical spread from ridge to ridge or valley to valley. In practice, smoothing by an 11 = 11 km window works very well, though for consistency with other maps in these papers, a 25 = 25 km window is used here ŽFig. 6.. The most significant feature of Fig. 6 deserving comment is the absence of any discontinuities along the boundaries of the relative resistance classes Žsee fig. 1, Paper II.. EÕen before smoothing, the map compiled from the separate calculation of departures for six different rock resistance classes, each with its own regression Žtable 8, Paper II., shows no discon-

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tinuities along the class boundaries. This strongly supports the reality of the departures plotted and smoothed in Fig. 6. Fig. 6 shows that the full range of residuals is from y141 m to q321 m. Comparison with a map of average height Žfig. 6, Paper I. shows close overall agreement of pattern. The effect of local base-level as mapped in Fig. 2 is also clearly apparent. Thus, the zero metre residual contour from Flamborough Head to the Suffolk coast lies close to the 50 m contour of average height. On higher ground, the differences mount; thus, the residual for Dartmoor only just exceeds 100 m, while mean height there is over 300 m. The damped relationship at higher altitudes is confirmed by comparisons in the uplands of Wales, northern England and Scotland; thus, the South Wales uplands show over 200 m positive residual, with mean height over 300 m; Cross Fell shows a residual of just over 300 m, and a mean height just exceeding 500 m, while Cairngorm has residuals also just over 300 m, with a mean altitude of 6–700 m. This map of residuals from the general relationship between altitude and river distance for each of the six rock resistance classes ŽFig. 4. eliminates those characteristics of the map of mean height which may be explained by variations in rock resistance. Thus, typically across the ridge and vale landscape of southern Britain, variations in mean altitudes between 0 and just over 200 m are largely accounted for by the relative resistance of the local geology, apparently little affected by disturbance in the spatial pattern which might be linked with local neotectonic movement. There is a hint of gentle subsidence along the North Sea coast from Flamborough Head to southern Suffolk, and the inland extension of the negative values for residuals from the geologically-controlled pattern is best explained by the impact of higher rates of glacial erosion in the Vales of York and Belvoir and in the FensrWash embayment. Cornwall shows negative departures on Fig. 6, though the average altitude is over 50 m Žfig. 6, Paper I.. A similar effect is seen in southwestern Wales, and it is of interest that both coasts are deeply indented by rias, implying drowning of a formerly uplifted landmass. One hypothesis that is worth testing is that these peninsulas, because they have rela-

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tively deep water on both sides, have been more strongly affected by offshore water loading as the Flandrian transgression proceeded, leading to local, and very recent, downwarping. The same argument may apply to the peninsulas in southwestern Scotland, but the deep glacial erosion of the fiord-like embayments complicates the relationship. The positive anomalies of Fig. 6 are more readily accounted for than the largest negative departures and will be considered first. The majority are linked to Žthough not explained by. the pattern of local base-level as mapped in Fig. 2, so that positive anomalies occur consistently along the major water partings of the uplands of Britain; a north south line throughout Wales, along the Pennine axis from the Peak District in the south to Cross Fell in the north, and in the Scottish Highlands, both north and Žpredominantly. south of the Great Glen, though with a strong cut-off from the Dornoch Firth to Ullapool. In the case of the Alston Block ŽCross Fell., a major negative gravity anomaly over the buried Weardale granite supports the case for neotectonic movement and for a buoyancy that is not yet spent. Exposed granites may be buoyant, but in these cases they will contribute to the average elevation of all granites Žrelative to river distance. and thus to their apparent relative resistance to erosion. In Cross Fell, the elevation expected for the Carboniferous rocks at the surface is greatly exceeded, hence the suggestion of uplift which is supported by the gravity data. There are, of course, buried granites which do not display such anomalies, and the unusual buoyancy of the Weardale granite Žneatly portrayed by the negative Bouguer anomaly of the published British Geological Survey Ž1977. gravity map. deserves further investigation. There is a similar link for the Askrigg Block, though the gravity and relief anomalies are both smaller. The positive anomalies in the Grampian Highlands also coincide with a considerable area with negative gravity values and are also to be regarded as areas of additional neotectonic uplift, as is an area to the north of Ben Nevis, just north of the Great Glen. Lesser areas with a similar match include the Lake District and the Dartmoor granite; a ridge along the Scottish border from the Larristone Fells towards Cheviot has a weaker relationship with the gravity map as the highest anomaly lies well NE of the highest relief residual.

The problem in explaining the negative anomalies of Fig. 6 is they could be caused by unusually severe local erosion Žin practice most probably by ice., or by subsidence, or a combination of both. Where there is evidence for severe erosion, as in the Central Lowlands of Scotland, the Wirral, or the Fens of East Anglia, this is the most likely explanation. Where positive gravity anomalies coincide with the negative values and severe glacial erosion seems unlikely, neotectonic subsidence seems the most likely cause—the obvious case is Buchan. Some other areas must remain uncertain as to cause. Thus, with no supporting gravity data, Strath Naver in northernmost Scotland seems a candidate for glacial erosion, as do the areas north of the Solway Firth and the Tweed basin. The large negative anomaly in part of the Wye valley with Hereford at its southern edge also has no matching gravity anomaly, so despite the lack of any suggestion hitherto, ice must be suspected. This is also true of the smaller area on the eastern edge of the Welsh Mountains north of Newtown. But as already noted, such an explanation cannot apply to Buchan, where although there is local evidence of erosion by active ice streams, the landscape north of Aberdeen is notable for the apparent survival of deeply weathered granites and ‘Pliocene’ gravels ŽHall and Sugden, 1987.; so much so in fact that it has been a matter of discussion whether or not the area was glaciated at the Devensian stage. Thus, here the most attractive hypothesis must be neotectonic subsidence, and the map of gravity anomalies gives support for this idea, as there are positive anomalies in this region. The suggestion is consistent with the preservation of Pliocene deposits and weathered regolith in an area that seems to have been too low to be eroded rapidly. Yet again, we note that maps such as Fig. 6 do not explain anything. They quantify values and show them as patterns which demand rational explanation. All that can be done at this early stage is to point to possible hypotheses, and so trigger new avenues of exploration for the origins of the British landform. They can also contain statistical artefacts. Areas such as the Somerset levels appear as negative anomalies because they consist of virtually level stretches of alluvium, across which river distance necessarily rise inland without any corresponding increase in altitude. On the other hand, the apparently curious

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anomaly across the Chalk escarpment on the NorfolkrSuffolk border is real enough and neatly plots the zone of most severe breaching of the Chalk escarpment by the advancing Anglian ice ŽClayton, in press a; Linton, 1963.. 9. Regional tectonic contrasts It was noted in Paper II that by using aspects of relief as a basis of rock classification, tectonic influences, both regional and local, have been included with the influence of rock strength. From the work just described, it appears that while local neotectonic effects may be detected, signs of exceptional uplift are limited to just a few locations if the evidence here has been interpreted correctly. Certainly neotectonic uplift Žother than the ubiquitous response to denudational unloading. is of far less importance than the effects of differences in rock resistance. It should also be understood that as denudational unloading is itself linked to varying depths of dissection on different rocks, it varies from one part of the country to another as is clear from Fig. 3. Nonetheless, we might also expect differences in response between the major structural units of Britain, and this has been tackled in the following way. Britain has been divided into eight structural regions ŽFig. 5., and the elevation predicted by the generalised relationships between river distance and elevation Žmean height. which form the basis for Fig. 6 have been extracted for all squares in each of these

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regions. The mean of these predicted values is then compared with the actual mean elevation of all the kilometre squares in each region. If there has been no differential uplift between these regions, we would expect the values for positive and negative departures to balance out; if one region has been uplifted Žor depressed. relative to others, then it should appear with a preponderance of positive Žor negative. values. The mean values for each structural region are set out in Table 3. The zero value has no particular significance; at present, we have no way of establishing anything other than the relative movement between tectonic blocks Žsee final column of Table 3.. In comparison with the scale of isostatic adjustment to denudation, or to the contrast between rocks of varying resistance to erosion, the values plotted in Table 3 imply relatively small regional differences, especially when the identified neotectonic sites are allowed for. It is of interest that the values for Northern Scotland, Wales, the Southwest Peninsula and Southern England are essentially the same, and notable as an unexpected result. The low value for the Central Lowlands of Scotland is significant and implies relative movement young enough to influence the present average relief which is even lower than the relatively weaker rocks of this area would predict. To a lesser extent, the Southern Uplands also seem depressed and this deserves further comment, especially as they also proved anomalously low in Table 2. Table 3 gives an overall depression of just over 20 m when the South-

Table 3 Relative heights of eight structural regions, based on the actual mean altitude less the predicted mean altitude using the MPrRD relationship for each rock resistance class Žthe equations of table 7, Paper II a .. For boundaries of these units see Fig. 5 Region

Mean height Žm.

Predicted height

Mean deviation actual height Žm.

Percentage deviation

Relative to lowest unit

I. II. III. IV. V. VI. VII. VIII.

269.9 139.3 221.7 202.8 204.7 144.4 64.6 80.8

254.6 151.6 231.4 157.6 190.5 134.7 70.6 66.6

q15.3 y12.3 y9.7 q45.2 q14.2 q9.7 y6.0 q14.2

q5.7 y8.8 y4.4 q22.3 q6.9 q6.8 y9.3 q17.6

q27.6 0.0 q2.6 q57.5 q26.5 q22.0 q6.3 q26.5

a

Scottish Highlands Central Lowlands Southern Uplands Pennines and Lake District Wales Southwest Peninsula Midlands and East Anglia Southern England

Clayton and Shamoon, 1998b.

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ern Uplands are compared with Wales, the Scottish Highlands and Southern England. Search of the literature reveals no field evidence of late Tertiary or Quaternary movement on the boundary fault between the Central Lowlands and the Scottish Highlands, but there is evidence of Quaternary movement on the Southern Uplands Boundary Fault. While mapping the area for the Geological Survey, Lumsden and Davies Ž1965. found that the River Nith Ž10 km NW of Sanquhar. has a buried channel filled with Devensian deposits. The channel floor was found to be about 50 m lower north of the line of the fault between the Southern Uplands and the Central Lowlands than to the south, implying a pre-Devensian downthrow to the north. The Doon valley further southwest shows a very similar relationship. Since the overall contrast with the Central Lowlands is 3 m in the other direction, it may be that the Southern Uplands, while uplifted in relation to the Central Lowlands, may be tilted down in relation to the Pennines and Lake District to the south, giving a block dominated by the relative lowering of the southern part. Certainly possible evidence of neotectonic movement on the faults north of the Alston Block Žnotably the Stublick fault. would seem worth investigation. However, even the Central Lowlands case is minor Ži.e., an overall contrast in mean elevation of 25 m when compared with the Highlands after allowance has been made for the geological contrasts., and in general we infer that there has been no major differential tectonic influence Žother than that linked with differences in the depth of dissection on rocks of varying resistance to erosion. on the elevation of each of the structural regions of Britain. Regional differences in isostatic uplift related to local variations in unloading, which in turn are linked with the effects of contrasts in rock resistance, are of greater importance. On the other hand, it is important to point out that regional vertical movements will be offset over erosional time by the effect they have on relative base-level. Uplifted areas will erode more rapidly, subsiding regions will be dissected more slowly. Thus small differences in average relief may conceal much larger relative tectonic movements, especially if these are late Tertiary in age, rather than Quaternary.

10. The origin of the British relief We have noted six contributions to the differentiation of the relief of Britain. Ž1. The varying relative resistance of the various rocks to erosion. Ž2. The effect of steeper and longer river slopes draining areas of higher relief on more resistant rocks on local base-level and thus on overall elevation. Ž3. The ubiquitous isostatic uplift consequent upon denudational unloading. Ž4. The local impact of neotectonic movements distinct from the impact of unloading or the growth and decay of ice sheets; this includes evidence of considerable uplift above the buried Weardale granite in the Alston Block, and the possible local relative depression in Buchan. Ž5. The local effect of severe glacial erosion on the depth of dissection which appears to be undercompensated by local isostatic recovery. Ž6. Finally, what appears to be a relatively small contribution to the overall relative elevation associated with each of the major structural regions of Britain, though one of some significance for the Southern Uplands. To take these in turn, Paper II ŽClayton and Shamoon, 1998b. quantified the differences between the main lithologies. Fig. 2 shows the effect of river slope on local base-level. Fig. 3 maps a measure of denudational unloading, Fig. 4 quantifies the impact of isostatic response to denudational unloading in relation to mean altitude. Fig. 6 emphasises the effects of exceptional neotectonic uplift or subsidence, as well as unusually effective regional glacial erosion by eliminating the considerable variations in relief which are directly linked to variations in rock resistance to erosion. One or two of these contributions deserve a little more comment, especially where they have received limited attention in the literature. Partly because even in areas of severe glacial erosion some parts of the landscape seem to remain almost unaffected by ice, authors have been deeply divided over the efficacy of ice as an erosional agent. Fig. 4 supports the view ŽClayton, 1996. that locally, ice has bitten deeply into the land on a scale of hundreds of cubic kilome-

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tres. In the western uplands, the effect of trough widening, areal scouring and divide narrowing on average relief values of heavily-glaciated uplands is very great, with average values of mean elevation falling 100–250 m below those in the less intensely glaciated uplands of eastern Scotland, the Southern Uplands and Wales. This can be observed in such areas as Western Scotland, Galloway, the Lake District and Snowdonia. Areas visible on Fig. 6 include the Central Lowlands of Scotland southwest of Edinburgh, the Tweed Basin, the Lancashire–Cheshire Plain, the eastern margin of the Welsh Uplands, and similarly the eastern margin of the Pennines, as well as the area around Hereford and the well-known eroded basins of the Vale of Belvoir and the Fens and The Wash. The last appears to have lost at least 500 km3 in the Anglian glaciation ŽClayton, in press a.. We have already reviewed the growing understanding of the significance of isostatic response to denudational unloading in Section 1. So far as neotectonic effects are concerned, with the significant exception of glacioisostasy, there is relatively little in the published geomorphological literature on Britain relevant to these inferences, apart from the review by Embleton Ž1993. which concludes that undoubtedly neotectonic activity has affected Britain, but that it is difficult to find proof that movement has occurred. A recent extended review of the published evidence ŽClayton, in press b. comes to the same conclusion and finds the evidence increasingly convincing. Work which may not easily be ignored includes many studies by French researchers Že.g., Battiau-Queney, 1980, 1989a,b; Coque-Delhuille, 1987, 1991.. In terms of the relative buoyancy of the Welsh Uplands, the unexpected discovery in the Mochras borehole, that 525 m of Neogene Beds lie unconformably on the Lias close to the Welsh coast ŽWoodland, 1971. establishes that huge relative movements must have characterised the western margin of that upland block in the Tertiary, though there is no direct evidence that they continue today—or even that they persisted into the early Quaternary. C.A.M. King, after a visit to New Zealand, wrote a perceptive account of the northern Pennines in neotectonic terms ŽKing, 1962., and returned to the neotectonic theme in her essay for the 1977 INQUA Congress in Birmingham ŽKing, 1977.. Wheeler Ž1979. in his discussion of the longi-

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tudinal profiles of rivers cites several cases where he thought there was evidence of steepening linked to neotectonic displacements, including the River Greta at the crossing of the Craven Faults at Ingleton. We have already noted the evidence of late Quaternary movement on the Southern Uplands Boundary Fault ŽLumsden and Davies, 1965.. In a recently published paper, Maddy Ž1997. has estimated the uplift of the Upper Thames Basin as about 70 mmrka on the assumption that incision Žproducing the stairway of terraces. has matched uplift. Another recent paper by Ringrose and Migon ´ Ž1997. utilises the closelyspaced altitudinal values available from the Ordnance Survey to plot altitude frequency graphs similar to the approach of Hollingworth Ž1938.. They interpret higher frequencies as upland surfaces and by correlating them on transects across the Grampian Highlands, show evidence of neotectonic movement on two faults, the fault along the line of the Great Glen and the Ericht–Laidon fault which runs roughly parallel some 35 km to the southeast and separates the Monadliath Mountains from the Cairngorms ŽFig. 1..

11. Conclusion The detailed calculations that may readily be made with this digital database and the relatively complex maps that can be produced allow the problem of the development of the relief of Britain to be explored in new ways. It seems that something like half of the mean elevation of Britain, and much more in deeply dissected areas such as Snowdonia and the Lake District, matches the modelled isostatic adjustment to denudational unloading. The bulk of the remaining average elevation may be attributed to the necessary slope of the main rivers, given the relatively rugged relief they drain in Highland Britain. This in turn can be traced back to the relative resistance of the rocks in these uplands which has allowed deep dissection and steep slopes. Local differences in average altitude of up to 400 m may readily be explained by the effect of contrasts in rock resistance to erosion. More local Žregional. signs of neotectonics are limited to the possibility of subsidence in the Buchan area and more generally in the Southern Uplands, and the probability of considerable local uplift of the

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Alston Block above the underlying Weardale granite, of South Wales Žcentred on Fforest Fawr. and of the Cairngorms. Rather surprisingly, differential movement of our major structural regions seems to have little differential effect on the overall relief Žwith the exception of the Southern Uplands.; the obvious differences between them are the result of contrasts in rock resistance, the indirect result of their geological history. The varying uplift as a result of varying depths of erosion ŽFig. 3. is usually of greater importance, as is the local effect of the isolated examples of neotectonic movements we have established. Current accounts of the origin of the relief of Britain pay virtually no attention to the direct and indirect effects of isostatic response to denudational unloading or regional neotectonic movements Žsee Embleton, 1993 for an exception.; they require considerable amendment. We may also reword Mackinder’s explanation of the contrast between Highland and Lowland Britain, already quoted in Paper II ŽMackinder, 1904, p. 63.. He considered that the weaker rocks of the southeast had been reduced to lowlands, while the harder rocks had so far withstood erosion to form uplands and peaks. It seems that erosion and consequent uplift has in fact been greatest on the more resistant rocks, since they allow deeper dissection without losing the elevation of the intervening ridges. We have also suggested that the need to maintain river slopes to transport eroded sediment from such rugged uplands is a further factor as Fig. 1 suggests. At a more fundamental level, the origin of the HighlandrLowland Britain contrast lies in the Tertiary history of the two areas. Highland Britain had a thinner cover of younger rocks, and was uplifted more, so that the underlying Palaeozoic rocks were reached by late Tertiary erosion. The greater depth of the younger cover over the Palaeozoic rocks of Lowland Britain and the lesser amount of uplift means that erosion has so far exposed very little of the buried basement. As the thinner and perhaps incomplete cover of Highland Britain has been removed, dissection of the more resistant older rocks unloaded the area, so leading to further uplift and the shaping of the uplands we see today. There is, of course, much more to explain, but this digital database offers new ways of tackling old problems.

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