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A new approach to the relief of Great Britain
Geomorphology 25 Ž1998. 31–42
A new approach to the relief of Great Britain I. The machine-readable database Keith Clayton ) , Nadhim Shamoon School of EnÕironmental Sciences, UniÕersity of East Anglia, Norwich NR4 7TJ, UK Received 12 March 1997; revised 19 February 1998; accepted 11 March 1998
Abstract We describe a simple, if fairly large, database of relief and geology for Great Britain and the surrounding shelf based on the kilometre squares of the National Grid. The area covered is 560,000 km2 , 319,490 km2 classified as sea and 240,510 km2 as land Žsquares which included the coastline were classed as land.. The database affords a new approach to establishing the importance of rock resistance to erosion as a factor affecting the form of the present land surface, the legacy of older tectonic movements, and the role of neotectonics. It can be used to test existing hypotheses, or propose new ones, which can then be tested against other evidence. Initial data on landrshelf contrasts are presented. The current coastal zone is found to be a sector of relatively abrupt transition from shelf depths between y60 and y70 m, to land elevations exceeding an average of over 100 m, even within a few kilometres of the coast. This transition zone is attributed to the isostatic buoyancy of a land area steadily unloaded by denudation and attacked by the transgressing sea as water levels rise after each glacial stage. The database is a novel and powerful way to tackle some fundamental aspects of the evolution of the relief of Great Britain and further papers will explore issues such as relative rock resistance and evidence for neotectonic movement. q 1998 Elsevier Science B.V. All rights reserved. Keywords: relief; geology; Great Britain; computer map; land uplift; continental shelf
1. Introduction The published literature on the landforms of the British Isles provides considerable information on the evolution of our relief, though even in Southeast England, where relatively young rocks constrain the history, differing views are held on the age of the landforms and the role of tectonic movement in the evolution of the present relief ŽJones, 1980.. We also have a considerable literature on the close links )
Corresponding author.
between geological structure and relief Že.g., Whittow, 1992; Goudie, 1990, Chapter 4.. However, analyses of the influence of geology on relief are limited to qualitative statements, with a general awareness of local contrasts in the resistance of rocks to denudation Žnotably in the cuesta and vale landscapes of lowland Britain., but with no precision over just how resistant various rock types are, nor any way in which we might place our geological outcrops in order of resistance to erosion. The ability to acquire data on geology and relief in machine-readable form offers a chance to find
0169-555Xr98r$19.00 – see frontmatter q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 5 5 5 X Ž 9 8 . 0 0 0 3 7 - 3
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K. Clayton, N. Shamoonr Geomorphology 25 (1998) 31–42
ways round some of these problems, with the potential that we might unravel some of the relationships which have so far eluded us and which should help us understand more fully the controls over our large-scale landforms. Existing hypotheses may be tested against the new data, and new hypotheses can be developed which may then be tested against other evidence. This first article in a planned series describes the simple database we have built up and makes initial use of it to clarify the very large-scale patterns of the relief and geology of Great Britain and the adjacent continental shelf. Later articles will tackle the relative resistance of the many strata that make up our complex geology, and approach such problems as the role of neotectonics in the present elevation of the land surface.
2. The database The decision was taken to base values on the kilometre squares of the National Grid. The variables had to be of potential value in the investigation of the relationships between geological outcrops and structure, erosional history and present relief, relatively easy to acquire, and as objective as possible, since it was inevitable that several different assistants would be involved. In addition to the location ŽNational Grid coordinates for each square., the initial values collected were: Ž1. HP, highest altitude on land Žm. Žfrom manual interpretation of OS 1:25,000 maps: latest edition.; Ž2. LP, lowest altitude on land Žm., also from OS 1:25,000 series.; Ž3. A–Q, age of dominant Žsolid. rocks Ž19 divisions based on BGS 1:50,000 maps on land Žwhere published. and utilising the 1:250,000 series offshore.; Ž4. 1–99, lithology of dominant Žsolid. rocks Žinitially 86 classes— sources as for geological age.; Ž5. RD, distance by river and up valleyside slopes from tidewater Žkm. Žbased on the OS 1:50,000 series for land.; Ž6. Bouguer gravity anomalies, acquired by digitising the contours of published 1:625,000 maps and, as for offshore relief, using a computer program Ždetails below. to assign values to each kilometre square; Ž7. offshore, it was only possible to acquire one value for altitude Žfrom digitising the bathymetrical contours., while the direct distance offshore replaced river distance. These distances were entered as nega-
tive values, in order to complement the sequence of positive values on land; Ž8. for convenience in analysis, the database was also divided, square by square, into two classes: sea and land. For most of these variables, values for individual squares are reasonably precise—many squares, for example, have numbered spot heights which denote the highest point, and along river valleys, with low gradients and contours every 10 or 15 m Žthe two most common intervals on the maps consulted. estimates will be accurate within 2 m. On steeper slopes and summits without spot heights, errors of 5 m can occur for both HP and LP, but these become irrelevant once many squares are combined in an analysis, as is always the case. There are inevitably problems in standardising lithology from the published geological maps, some of which are very old, while even in this century, both terminology and practice have evolved over time Žsee below.. It was also found that some subjectivity could creep into river distances, especially in the way that rivers traversing grid squares obliquely were handled, and also in the extent to which major meanders should be taken into account Ži.e., channel cf. valley distance.. For this reason, particular care was exercised in establishing a consistent approach which involved following valley meanders, but ignoring channel meanders. Over small distances the differences which might occur are very small, but up-river cumulative differences of up to 10 km could have occurred had not care been taken to standardise the treatment of major river meanders. All values for HP, LP and RD are in whole numbers. For sea areas, while the broad age divisions used in classifying solid geology are fairly readily available for offshore areas now the BGS 1:250,000 maps are being published, lithological information is less widely available and is certainly far less detailed than the information available onshore. Thus, the accuracy of both the relief and geological information is necessarily far less than onshore. Although this contrast should be kept in mind, the differences do not seem to cause problems when combining data from both onshore and offshore areas. For both land and sea, lithology presents many problems, given the very great range in age of our published geological maps. The initial classes followed the wording of the published sheets, and only
K. Clayton, N. Shamoonr Geomorphology 25 (1998) 31–42
later were decisions made to reclassify some of the more awkward Žand usually small in area. classes. It is not possible to avoid mixed lithologies Žas for example in the Yoredale series., nor is it easy to deal with cases where the lithology changes gradually over long distances Žas in some of the Eocene strata in the southern North Sea.. However, it was thought best not to oversimplify the lithological classification, as larger groups can always be put together by placing different classes in the same larger group, while the disaggregation cannot easily be restored, once a class is renumbered Žreclassified. on the database. The coastline was digitized separately from the 1:625,000 maps and can be overlaid on any dataset, or other computed values, when portrayed as a map. The 100 km squares of the National Grid can also be overlaid if required.
3. Areal coverage The database holds relief, river distance, gravity, and geological information for all of mainland United Kingdom together with the eastern half of Northern Ireland. The Inner Hebrides, Lewis, and the southernmost tip of the Orkney Islands are also included. The Shetland Islands and the Isles of Scilly are excluded. In the figures, the term Great Britain is used for the area covered, as these divergencies are relatively small. The area covered is based on the Ordnance Survey National Grid system and consequently forms a rectangular box with its corners at the following positions: South–West: Easting 100, Northing 0; South–East: Easting 670, Northing 0; North–West: Easting 100, Northing 1000; North–East: Easting 670, Northing 1000. The national grid was extended westwards to facilitate the inclusion of part of Northern Ireland. The small portion of Eire falling within the grid boundaries is not covered due to a lack of published maps comparable to those in the UK. Where an area of land above high water mark occurs in a square, that square is always classified and mapped as land. There were 16,045 such squares, about twice as many as those with value 1 or y1 to landward and seaward. Squares are referenced by their southwest
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corners, using the National Grid in the conventional way Žtwo letters and four numbers..
4. Data entry Manual recording of numerical values was used for River Distance, Relief Žhigh and low points., Age and Lithology on land and offshore. It entails the recording of values for each square kilometre and storing the keyboarded data in files relating to 10 km = 10 km blocks. Computerised digitising was used for gravity and bathymetric data only, where the scale of the maps and the type of data made it impossible to produce two data sets of high and low points, as used in the manual style of digitising. The Toscanelli 1.4 ŽTOSCA. digitising program, for the GIS package IDRISI, was used to acquire and convert these values. The data are held in 100 km = 100 km files, named by grid square letters, e.g., NY. This package was rather error prone during digitising Že.g., not responding to some buttons on the puck and freezing the program on others. but was very reliable in file saving and management. Occasionally, the file memory was too small Žtoo many coordinate pairs used.; thus, two files had to be created and joined later in IDRISI. The vector files produced by digitising the data using TOSCA were easily rasterized and displayed using IDRISI. This gave values for kilometre squares and allowed interpretation of values between the bathymetric or gravity contours.
5. Software A large part of the analyses used standard statistical and graphical packages such as SPSS, IDRISI and ERDAS. A small number of routines were written in Fortran 77 to allow for particular calculations not provided in the standard packages. These procedures include the extraction and reclassification of particular classes of data Že.g., the combination of age and lithology into a single ‘Geology’ variable.. Many analyses, e.g., those dividing pre-Permian and post-Carboniferous rocks, also require regrouping of the basic data.
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K. Clayton, N. Shamoonr Geomorphology 25 (1998) 31–42
In-house programs were also written for using moving windows to generalise values across an area or to replace square by square values with the lowest or highest altitude found in surrounding squares within a window of any size. Fuller details of these programs and the packages utilised are in Appendix A. 6. Initial analysis of the complete database Obviously, maps of relief and geology can be produced from the database, but these are already published and well known. The map of river distance on land has not been seen before and is thus of greater interest ŽFig. 1.. The highest value is at Plynlimon at the source of the River Severn, though we should recall that this value has arisen relatively recently with the diversion of the Severn during the Devensian glaciation through the Ironbridge gorge from its earlier route through the Cheshire Plain. In general, it will be seen that high values are relatively limited and generally found in the headwaters of the Severn, Thames, Trent and other eastward-flowing English rivers. The frequency diagram of river distance values ŽFig. 2. shows a rapid falling off with distance so that for river distance values on land in excess of 103 km, there are fewer than 500 km2 ; offshore, at distances of over 173 km, areas again fall below 500 km2 . A cumulative distribution plot of mean height for both seafloor and land has been constructed ŽFig. 3.. This shows a smooth pattern Žthe minor irregularity just offshore from the present coastline is an artefact of the decision to treat all squares including the coastline as land. and, in comparison with graphs based on river distance, shows the effect of large areas close to the coast, compared to the smaller areas at greater distances both out to sea and inland, as already seen in Fig. 2. There is no universal relationship between river distance and altitude, especially on land. This is shown in the graph ŽFig. 4a. linking river distance and mean height Žthe average of highest and lowest values for each kilometre square.. This is a result of the limited maximum river distances in the major areas of high relief, especially in Scotland and the Pennines. Thus, for river distances greater than some 100 km, mean height varies between a maximum of
270 m at 107 km and 133 m at 154 km, rising again to 286 m at 196 km. The subsequent low point is 167 m at 221 km, with the very small number of cases at higher distances reaching a maximum value of 500 m at 246 km. However, as Fig. 4b shows, there is a good relationship for river distances between 0 and 80 km, with a correlation coefficient, admittedly based on average height for each river distance, of r 2 s 0.92. What is most apparent is the intercept, with a value of 103.5 m at the coast. Offshore ŽFig. 4c., the overall relationship is far better, with an r 2 value Žagain based on the average height for each offshore distance. of 0.86, and again with an intercept at the coastline of y58.7 m. As the graph of values shows, there is in fact a considerable levelling off at a little less than y100 m, with values between y90 and y99 m from 152 to 270 km offshore, and no lower than y101 m to the maximum distance of 295 km. The other feature of the offshore graph is the persistence of values in the range y63 to y69 m between 25 and 96 km offshore. When the land and sea values for mean height for each kilometre of RD are linked ŽFig. 4a and b., a remarkable pattern results. The extremely smooth continental shelf is dominated by altitudes between y60 and y100 m, the land area generally lies above 130 m; indeed, only river distances 0–14 have averages below this value. There is thus an overall difference in level between the submerged shelf and the land of almost 200 m. A similar figure results if we link the two regression lines for the whole shelf and the land area with river distances from 0 to 80 km ŽFig. 5.. With intercepts of y58.7 and q103.5 m, respectively, we have a discrepancy at the present coast of 162.2 m. Although distance offshore is a simpler concept than the distance downslope to tidewater that underlies the definition of river distance onshore, there is nothing in this Žnecessary. distinction that might explain the lack of match of the calculated regression lines at the coast. The sea-floor within just a few kilometres of the coast is almost 60 m lower than present day sea level, while the land within 5 km of tidewater is on average more than 100 m above present day sea level. Before considering this issue further, the relief change at the edge of the landmass is further investigated by separating the values for Highland and
K. Clayton, N. Shamoonr Geomorphology 25 (1998) 31–42
Fig. 1. River distance Žland only.. Fig. 6. Great Britain: average height smoothed by 25=25 km window. 35
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K. Clayton, N. Shamoonr Geomorphology 25 (1998) 31–42
Fig. 2. Frequency diagram ‘river distance’ Žonshore and offshore. by kilometre classes. River distance is measured up rivers and their tributaries on land; at sea, it is a measure of the direct distance from the coast.
Lowland Britain ŽMackinder, 1902.. The simplest way to do this is to plot the same graphs, based on average height values for river distances, for pre-Per-
mian rocks ŽHighland Britain. and post-Carboniferous strata ŽLowland Britain.. The adjacent sea floor is divided on the basis of the geology of the nearby
Fig. 3. Hypsometric curve, mean height, sea and land.
K. Clayton, N. Shamoonr Geomorphology 25 (1998) 31–42
land as a geological division is not appropriate for the continental shelf. To save space, the graphs are not reproduced here, but the values are listed in Table 1. The two additional intercepts for bathymetry in Table 1 were calculated for the area of sea off Highland Britain and also for the sea area of the southern North Sea and the English Channel for Lowland Britain. Overall, there is a consistent relationship with the older and no doubt tougher rocks standing higher, the younger rocks lying at a lower level, though still well above the adjacent sea floor. The reason for this ‘freeboard’ around the British coast is discussed briefly below and more fully in Paper III of this series. In any discussion of this ‘freeboard’ by which the land stands above the adjacent sea floor, consideration of several issues is required. We first need to note the exceptional altitude of present day sea level when considered across the span of the Quaternary —or even over the last 100,000 years. Estimates of low sea level at the peak of the Devensian glaciation Žca. 18,000 years BP. are between 120 and 140 m below OD. An estimate of the pattern of past sea levels based on past ice volumes has been made for the period covered by the ice volume curve published by Shackleton and Opdyke Ž1973. for the last 140,000 years. This suggests an average sea level over the last glacialrinterglacial cycle of about y55 m. Data are not readily available in the same form for earlier glacialrinterglacial cycles, but following Porter Ž1989., we may expect repetition with each 100,000 year cycle. Thus, it is this low sea level of some y55 m that is likely to be linked to the Quaternary evolution of the land surface, rather than the abnormally high sea level of the current interglacial. It will be noted from Fig. 4b that the depth of 60 m is reached on average at a distance offshore of no more than 10–15 km, though the actual figure varies around different parts of the British coast. Noting these fluctuations of sea level, the difference between the altitude of the sea floor offshore and the adjacent land is of considerable interest. It could be the result of three influences: Ø the tendency of a land subject to continuous erosion to rise isostatically through denudational unloading; Ø the recurrent impact of marine erosion on the
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margin of the rising land as sea level has risen after each glacial period Ža transgressing sea having particularly effective erosional power.; Ø the effect of water loading during interglacial periods and the longer-term effect of offshore sedimentation progressively adding load to the continental shelf. Of course, the eroded nearshore zone will have had something of the same tendency to rise, but this will have been offset by both repeated planation and the water and sediment loading noted above. Indeed, even the recent, exceptionally rapid, sedimentation brought about by glaciation ŽClayton, 1996. has not only built out the edge of the continental shelf with a prograding wedge, but has also downwarped the original shelf surface as a result of the depositional loading Žsee Clayton, 1996, Fig. 2, p. 127.. That the unloading due to denudation is continuously reflected in consequential uplift is argued convincingly by Gilchrist and Summerfield Ž1991. and they note the role of denudation at the margins of continents in producing significant marginal upwarps. In terms of scale, the whole of Britain may be considered as a fragment of a European marginal upwarp. The contribution of uplift consequent upon denudational unloading to the present elevation of Great Britain is discussed in more detail in Paper III of this series.
7. Contrasts between the land area and the continental shelf We have already noted the overall contrast between the shape of the land and shelf areas as related to distance from the coast. Except in the near coastal zone, shelf slopes are very low when compared with those on the land. There is, in addition, a considerable geological contrast between land and shelf which is summarised in Table 2. The values in Table 2 show that, with minor irregularities, there is a consistent sequence in the ratios by area of the rocks when arranged in age order; older rocks are disproportionately common on land, younger rocks disproportionately common on the submerged shelf. The contrast between shelf and land in the two divisions of the Tertiary is particu-
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K. Clayton, N. Shamoonr Geomorphology 25 (1998) 31–42
Fig. 4. Ža. Mean height Žaltitude plotted against river distance for all values of river distance on land, and regression line; Žb. mean height Žaltitude. plotted against river distance for values of river distance of 0–80 km on land, and regression line; Žc. mean sea depth plotted against distance offshore, with regression line.
K. Clayton, N. Shamoonr Geomorphology 25 (1998) 31–42
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Fig. 5. Plot of mean height Žaltitude. against river distance for land Žriver distance values 0–110 km. and depth offshore Žfor all river distance values.. Underline: the regression for the values plotted here is shown, and the two regressions for offshore and land values are also plotted. The intercepts of these at the coastline show the lack of fit Ž‘freeboard’. at the coast; the values are y58.7 m and q118.6 m, a difference of 177.3 m.
larly great. The contrast in the proportion of the Tertiary rocks is largely due to lack of deposition across the present land area, though there will also be a contribution from preferential erosion on land. The Cretaceous Chalk was probably deposited across almost the whole British land mass; it is rather more widespread Ževen by outcrop. on the shelf, and since it is generally present beneath the offshore Tertiary beds, the contrast is far greater than suggested by the outcrop values recorded here. If we return to the hypothesis that the sea floor lying between y70 m and the present day coast has been eroded, largely no doubt by transgressive wave action, we would expect to see a further contrast between the outcrop geology of the deeper and the shallower shelf. At the same time, we might expect the higher land to reflect the operation of erosional processes over a longer time, and perhaps at a more rapid rate, than lower areas. This is tested by dividing the land area at the level of 200 m and the sea at the y70 m level. The results are set out in Table 3. From Table 3, it will be seen that older rocks of all ages up to the Cretaceous are more common at
shallower depths than on the deeper parts of the shelf. It seems, therefore, that the shallower shelf has been eroded differentially, partly by successive ice sheets as well as by the transgressing sea following each glacial stage. On land, the division at the 200 m level shows a somewhat similar contrast with older rocks more common at higher altitudes than low. This is attributed mainly to differential erosion, though with the contributory element that these older rocks are more resistant to denudation and thus more likely to form high ground. However, Palaeozoic rocks cannot form high ground if they do not occur at high levels prior to exhumation, as the buried London Platform Table 1 General elevation of the land towards the coast, Highland and Lowland Britain
All Britain Post-Carboniferous Pre-Permian
Land intercept
Sea floor intercept
Difference Žm.
118.6 50.3 153.2
58.7 43.5 76.5
177.3 93.8 229.7
K. Clayton, N. Shamoonr Geomorphology 25 (1998) 31–42
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Table 2 Areas Žkm2 . by geological age, land and shelf Žpre-Quaternary rocks only. Geological Age
All areas
Shelf
Land
ShelfrLand
Shelf Ž% of all.
Land Ž% of all.
Pre-Cambrian L. Palaeozoic U. Palaeozoic Permo-Trias Jurassic Cretaceous Palaeogene Neogene Totals
36,867 52,165 97,214 78,586 61,121 86,476 85,148 62,423 560,000
8552 7056 33,038 51,781 34,615 55,840 66,228 62,380 319,490
28,315 45,109 64,176 26,805 26,506 30,636 18,920 43 240,510
0.30 0.16 0.51 1.93 1.31 1.82 3.50 1451 1.33
1.53 1.26 5.90 9.25 6.18 9.97 11.83 11.14 57.05
5.06 8.06 11.46 4.79 4.73 5.47 3.38 0.01 42.95
Post-Jurassic Post-Palaeozoic All Tertiary
234,047 373,754 147,571
184,448 270,844 128,608
49,599 102,910 18,963
3.72 2.63 6.78
32.94 48.37 22.97
8.86 18.38 3.39
demonstrates. Equally, they can lie low as a result of prolonged erosion at some past stage, as in Anglesey where the planation could be as old as the Permo-
Trias and where faults separate the island from the uplift of the Welsh Massif and the Snowdon dome in particular ŽNeedham and Morgan, 1997..
Table 3 Areas Žkm2 . by geological age Žpre-Quaternary only., land and shelf, both divided into two altitudinal groups Geological age
Below y70 m
y69 to 0 m
0 to q200 m
Over 201 m
(a) By area Pre-Cambrian L. Palaeozoic U. Palaeozoic Permo-Trias Jurassic Cretaceous Palaeogene Neogene Totals
2742 1871 9764 20,638 11,044 17,631 34,211 61,093 158,994
5810 5185 23,274 31,143 23,571 38,209 32,017 1287 160,496
11,430 20,935 41,461 25,657 25,346 30,114 17,350 43 172,336
16,885 24,174 22,715 1148 1160 522 1570 0 68,174
Post-Jurassic Post-Palaeozoic
112,935 144,617
71,513 126,227
47,507 98,510
2092 4400
(b) Percentages (area) Pre-Cambrian L. Palaeozoic U. Palaeozoic Permo-Trias Jurassic Cretaceous Palaeogene Neogene Totals
1.73 1.18 6.14 12.98 6.95 11.09 21.52 38.43 100
3.62 3.23 14.50 19.40 14.69 23.81 19.95 0.80 100
6.63 12.15 24.06 14.89 14.71 17.47 10.07 0.03 100
24.77 35.46 33.32 1.68 1.70 0.77 2.30 0.00 100
Post-Jurassic Post-Palaeozoic
71.03 90.96
44.56 78.65
27.57 57.16
3.07 6.45
K. Clayton, N. Shamoonr Geomorphology 25 (1998) 31–42
In summary, the table shows that older, pre-Permian rocks are dominant above 200 m, that Permian to Cretaceous rocks are concentrated between y69 and q200 m, while Tertiary strata are dominant at depths of y70 m and above.
8. The broad pattern of relief We can smooth the detailed square by square data by moving windows of varying size, assigning to each individual square the average elevation of a wider area. A map Žnot reproduced here. of generalised Žsmoothed. relief produced by a moving 11 = 11 km window, i.e., the average elevation for each point is the average of the 121 km2 centred on it. The result is a map that picks out the level of detail of a typical small atlas map, with the higher Chalk and Jurassic limestone cuestas of southern England showing up well, with Bodmin Moor, Dartmoor and Exmoor separately identified in the Southwest Peninsula, and so on. Offshore, deep water in trough-like form within the Irish Sea and east of the Outer Hebrides is matched by smaller areas within the northern part of the North Sea, with a wide area of deeper water off northeastern Scotland. Bolder smoothing ŽFig. 6. using a 25 = 25 km window Ži.e., each square represents the average height of 625 km2 centred on it. picks up the essentials of the British relief, with depths of over 100 m limited to the northernmost part of the North Sea and to the centre line of the Irish Sea. The form of the Irish Sea depressions, including the North Channel, strongly suggests incision by ice. On land, a major area of high ground in the Scottish Highlands, mostly south of the Great Glen, is unmatched elsewhere, though the northern Pennines reach over 500 m, higher than the Southern Uplands, the Lake District, the whole of the Welsh Massif, and the rest of the Pennines. Dartmoor, the southern Pennines and much of Wales exceed 300 m, while Exmoor and the North York Moors exceed 200 m. The highest areas of the cuestas and plateaus of the Jurassic limestones and the Chalk fall between 100 and 200 m at this scale of generalisation. Eastern England from the Vale of York to east of the Chilterns and the whole of Kent is everywhere below an average elevation of 100 m.
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9. Conclusion This initial analysis of the digital map data conveys a picture of the Great Britain landmass as created by past modest uplift, probably of Tertiary age, and developing in the very late Tertiary and the Quaternary period by erosional incision of rivers and glaciersrice-caps, leading to a pattern of denudational unloading which has led to isostatic adjustment. This has given a coastal ‘freeboard’ of at least 120 m above present sea level, even more above the general level of the continental shelf. This offshore bench is attributed in part to marine erosion during transgression after each glaciation, while its present elevation reflects the effects of water loading as a result of the unusually high sea level experienced in interglacial periods such as the present. The contrasts in both geology and relief between land and sea are very great, and although these are not new discoveries, they are here quantified for the first time. The challenge to be faced in further interpretation of this database is to explain the overall form of our relief in terms of geological history Žpast tectonics., rock resistance, neotectonics and possible regional contrasts in denudation. This will be pursued in later papers in this series.
Acknowledgements The considerable task of compiling the database underlying this work was funded initially, and predominantly, by UK Nirex, as part of a contract designed to provide a better understanding of the past evolution of the British land surface, as a more secure basis on which to predict future change. The data covering Southern England and Wales were acquired using an Emeritus Award from the Leverhulme Foundation. This large database, although it may be used on a desktop PC, has made considerable demands in terms of data management, the utilisation of packages such as IDRISI, ERDAS, SPSS, and associated special programming to perform particular tasks. This has been handled from the first by Dr. Nadhim Shamoon. Keith Clayton is grateful to Dr. Tim Atkinson for several valuable discussions on the utilisation of the database, to Dr. Paul Burton for discussion of geophysical issues, and to Dr. Mike
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K. Clayton, N. Shamoonr Geomorphology 25 (1998) 31–42
Thorne who has consistently supported this approach towards a better understanding of the evolution of the British landform.
Appendix A. A note on computing programs and packages The complete database can be handled efficiently on a PC with a PENTIUM 133 MHz processor, 32 MB RAM, and 1.6 GB hard disk. It is helpful to have a 20 in. SVGA monitor, as this allows more data to be presented on a single screen. In particular, the whole UK raster image can be displayed at one time at a size sufficient to easily resolve data by eye. The larger screen also enables ERDAS Žon DECALPHA. GIS and Image Processing software to be used effectively as many windows are required to be open simultaneously. The data were entered directly into an INGRESrSQL database using a FORTRAN program with embedded SQL commands. The main database is called ELKMC holding geology-tab and height-tab. INPUTNX.FOR is a FORTRANrSQL program which sends data to INGRES and allows the data to be displayed on screen for the purpose of quality control. Most of the work utilises existing packages, but a few routines had to be written Žmostly in FORTRAN 77. to manipulate the data efficiently. These include: NIREX.FOR, a program principally designed to generate frequency tables and extract statistical data from the project database. Another is MASK.FOR, which is used to extract variables based on their river
distance from the coast Žin steps of 1, 2, 3, etc.. this can be based on age, lithology, geology. Once extracted or computed, the output file formats are such that data can be readily plotted Žin SPSS or Minitab. and further statistical work done. The third program frequently used is WINDOW.FOR, which provides a smoothing function based on a moving window. The size is specified and average, lowest or highest values can then be placed in the centre of the window.
References Clayton, K.M., 1996. Quantification of the impact of glacial erosion on the British Isles. Trans. Inst. British Geogr. NS 21, 124–140. Gilchrist, A.R., Summerfield, M.A., 1991. Denudation, isostasy and landscape evolution. Earth Surf. Process. Landforms 16, 558–562. Goudie, A., 1990. The landforms of England and Wales. Blackwell, Oxford, 394 pp. Jones, D.K.C., 1980. The Tertiary evolution of south–east England with particular reference to the Weald. In: Jones, D.K.C. ŽEd.., The Shaping of Southern England. Academic Press, London, pp. 13–47. Mackinder, H.J., 1902. Britain and the British Seas. Heinemann, London, 377 pp. Needham, T., Morgan, R., 1997. East Irish Sea and adjacent basins: new faults or old?. J. Geol. Soc. London 154, 145–150. Porter, S.C., 1989. Some geological implications of average Quaternary glacial conditions. Quat. Res. 32, 245–261. Shackleton, N.J., Opdyke, N.D., 1973. Oxygen isotope and palaeomagnetic stratigraphy of Equatorial Pacific core V28238: oxygen isotope temperatures and ice volumes on a 10 5 and 10 6 year scale. Quat. Res. 3, 39–55. Whittow, J. 1992. Geology and Scenery in Britain. Chapman & Hall, London, 478 pp.
A new approach to the relief of Great Britain I. The machine-readable database Keith Clayton ) , Nadhim Shamoon School of EnÕironmental Sciences, UniÕersity of East Anglia, Norwich NR4 7TJ, UK Received 12 March 1997; revised 19 February 1998; accepted 11 March 1998
Abstract We describe a simple, if fairly large, database of relief and geology for Great Britain and the surrounding shelf based on the kilometre squares of the National Grid. The area covered is 560,000 km2 , 319,490 km2 classified as sea and 240,510 km2 as land Žsquares which included the coastline were classed as land.. The database affords a new approach to establishing the importance of rock resistance to erosion as a factor affecting the form of the present land surface, the legacy of older tectonic movements, and the role of neotectonics. It can be used to test existing hypotheses, or propose new ones, which can then be tested against other evidence. Initial data on landrshelf contrasts are presented. The current coastal zone is found to be a sector of relatively abrupt transition from shelf depths between y60 and y70 m, to land elevations exceeding an average of over 100 m, even within a few kilometres of the coast. This transition zone is attributed to the isostatic buoyancy of a land area steadily unloaded by denudation and attacked by the transgressing sea as water levels rise after each glacial stage. The database is a novel and powerful way to tackle some fundamental aspects of the evolution of the relief of Great Britain and further papers will explore issues such as relative rock resistance and evidence for neotectonic movement. q 1998 Elsevier Science B.V. All rights reserved. Keywords: relief; geology; Great Britain; computer map; land uplift; continental shelf
1. Introduction The published literature on the landforms of the British Isles provides considerable information on the evolution of our relief, though even in Southeast England, where relatively young rocks constrain the history, differing views are held on the age of the landforms and the role of tectonic movement in the evolution of the present relief ŽJones, 1980.. We also have a considerable literature on the close links )
Corresponding author.
between geological structure and relief Že.g., Whittow, 1992; Goudie, 1990, Chapter 4.. However, analyses of the influence of geology on relief are limited to qualitative statements, with a general awareness of local contrasts in the resistance of rocks to denudation Žnotably in the cuesta and vale landscapes of lowland Britain., but with no precision over just how resistant various rock types are, nor any way in which we might place our geological outcrops in order of resistance to erosion. The ability to acquire data on geology and relief in machine-readable form offers a chance to find
0169-555Xr98r$19.00 – see frontmatter q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 5 5 5 X Ž 9 8 . 0 0 0 3 7 - 3
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K. Clayton, N. Shamoonr Geomorphology 25 (1998) 31–42
ways round some of these problems, with the potential that we might unravel some of the relationships which have so far eluded us and which should help us understand more fully the controls over our large-scale landforms. Existing hypotheses may be tested against the new data, and new hypotheses can be developed which may then be tested against other evidence. This first article in a planned series describes the simple database we have built up and makes initial use of it to clarify the very large-scale patterns of the relief and geology of Great Britain and the adjacent continental shelf. Later articles will tackle the relative resistance of the many strata that make up our complex geology, and approach such problems as the role of neotectonics in the present elevation of the land surface.
2. The database The decision was taken to base values on the kilometre squares of the National Grid. The variables had to be of potential value in the investigation of the relationships between geological outcrops and structure, erosional history and present relief, relatively easy to acquire, and as objective as possible, since it was inevitable that several different assistants would be involved. In addition to the location ŽNational Grid coordinates for each square., the initial values collected were: Ž1. HP, highest altitude on land Žm. Žfrom manual interpretation of OS 1:25,000 maps: latest edition.; Ž2. LP, lowest altitude on land Žm., also from OS 1:25,000 series.; Ž3. A–Q, age of dominant Žsolid. rocks Ž19 divisions based on BGS 1:50,000 maps on land Žwhere published. and utilising the 1:250,000 series offshore.; Ž4. 1–99, lithology of dominant Žsolid. rocks Žinitially 86 classes— sources as for geological age.; Ž5. RD, distance by river and up valleyside slopes from tidewater Žkm. Žbased on the OS 1:50,000 series for land.; Ž6. Bouguer gravity anomalies, acquired by digitising the contours of published 1:625,000 maps and, as for offshore relief, using a computer program Ždetails below. to assign values to each kilometre square; Ž7. offshore, it was only possible to acquire one value for altitude Žfrom digitising the bathymetrical contours., while the direct distance offshore replaced river distance. These distances were entered as nega-
tive values, in order to complement the sequence of positive values on land; Ž8. for convenience in analysis, the database was also divided, square by square, into two classes: sea and land. For most of these variables, values for individual squares are reasonably precise—many squares, for example, have numbered spot heights which denote the highest point, and along river valleys, with low gradients and contours every 10 or 15 m Žthe two most common intervals on the maps consulted. estimates will be accurate within 2 m. On steeper slopes and summits without spot heights, errors of 5 m can occur for both HP and LP, but these become irrelevant once many squares are combined in an analysis, as is always the case. There are inevitably problems in standardising lithology from the published geological maps, some of which are very old, while even in this century, both terminology and practice have evolved over time Žsee below.. It was also found that some subjectivity could creep into river distances, especially in the way that rivers traversing grid squares obliquely were handled, and also in the extent to which major meanders should be taken into account Ži.e., channel cf. valley distance.. For this reason, particular care was exercised in establishing a consistent approach which involved following valley meanders, but ignoring channel meanders. Over small distances the differences which might occur are very small, but up-river cumulative differences of up to 10 km could have occurred had not care been taken to standardise the treatment of major river meanders. All values for HP, LP and RD are in whole numbers. For sea areas, while the broad age divisions used in classifying solid geology are fairly readily available for offshore areas now the BGS 1:250,000 maps are being published, lithological information is less widely available and is certainly far less detailed than the information available onshore. Thus, the accuracy of both the relief and geological information is necessarily far less than onshore. Although this contrast should be kept in mind, the differences do not seem to cause problems when combining data from both onshore and offshore areas. For both land and sea, lithology presents many problems, given the very great range in age of our published geological maps. The initial classes followed the wording of the published sheets, and only
K. Clayton, N. Shamoonr Geomorphology 25 (1998) 31–42
later were decisions made to reclassify some of the more awkward Žand usually small in area. classes. It is not possible to avoid mixed lithologies Žas for example in the Yoredale series., nor is it easy to deal with cases where the lithology changes gradually over long distances Žas in some of the Eocene strata in the southern North Sea.. However, it was thought best not to oversimplify the lithological classification, as larger groups can always be put together by placing different classes in the same larger group, while the disaggregation cannot easily be restored, once a class is renumbered Žreclassified. on the database. The coastline was digitized separately from the 1:625,000 maps and can be overlaid on any dataset, or other computed values, when portrayed as a map. The 100 km squares of the National Grid can also be overlaid if required.
3. Areal coverage The database holds relief, river distance, gravity, and geological information for all of mainland United Kingdom together with the eastern half of Northern Ireland. The Inner Hebrides, Lewis, and the southernmost tip of the Orkney Islands are also included. The Shetland Islands and the Isles of Scilly are excluded. In the figures, the term Great Britain is used for the area covered, as these divergencies are relatively small. The area covered is based on the Ordnance Survey National Grid system and consequently forms a rectangular box with its corners at the following positions: South–West: Easting 100, Northing 0; South–East: Easting 670, Northing 0; North–West: Easting 100, Northing 1000; North–East: Easting 670, Northing 1000. The national grid was extended westwards to facilitate the inclusion of part of Northern Ireland. The small portion of Eire falling within the grid boundaries is not covered due to a lack of published maps comparable to those in the UK. Where an area of land above high water mark occurs in a square, that square is always classified and mapped as land. There were 16,045 such squares, about twice as many as those with value 1 or y1 to landward and seaward. Squares are referenced by their southwest
33
corners, using the National Grid in the conventional way Žtwo letters and four numbers..
4. Data entry Manual recording of numerical values was used for River Distance, Relief Žhigh and low points., Age and Lithology on land and offshore. It entails the recording of values for each square kilometre and storing the keyboarded data in files relating to 10 km = 10 km blocks. Computerised digitising was used for gravity and bathymetric data only, where the scale of the maps and the type of data made it impossible to produce two data sets of high and low points, as used in the manual style of digitising. The Toscanelli 1.4 ŽTOSCA. digitising program, for the GIS package IDRISI, was used to acquire and convert these values. The data are held in 100 km = 100 km files, named by grid square letters, e.g., NY. This package was rather error prone during digitising Že.g., not responding to some buttons on the puck and freezing the program on others. but was very reliable in file saving and management. Occasionally, the file memory was too small Žtoo many coordinate pairs used.; thus, two files had to be created and joined later in IDRISI. The vector files produced by digitising the data using TOSCA were easily rasterized and displayed using IDRISI. This gave values for kilometre squares and allowed interpretation of values between the bathymetric or gravity contours.
5. Software A large part of the analyses used standard statistical and graphical packages such as SPSS, IDRISI and ERDAS. A small number of routines were written in Fortran 77 to allow for particular calculations not provided in the standard packages. These procedures include the extraction and reclassification of particular classes of data Že.g., the combination of age and lithology into a single ‘Geology’ variable.. Many analyses, e.g., those dividing pre-Permian and post-Carboniferous rocks, also require regrouping of the basic data.
34
K. Clayton, N. Shamoonr Geomorphology 25 (1998) 31–42
In-house programs were also written for using moving windows to generalise values across an area or to replace square by square values with the lowest or highest altitude found in surrounding squares within a window of any size. Fuller details of these programs and the packages utilised are in Appendix A. 6. Initial analysis of the complete database Obviously, maps of relief and geology can be produced from the database, but these are already published and well known. The map of river distance on land has not been seen before and is thus of greater interest ŽFig. 1.. The highest value is at Plynlimon at the source of the River Severn, though we should recall that this value has arisen relatively recently with the diversion of the Severn during the Devensian glaciation through the Ironbridge gorge from its earlier route through the Cheshire Plain. In general, it will be seen that high values are relatively limited and generally found in the headwaters of the Severn, Thames, Trent and other eastward-flowing English rivers. The frequency diagram of river distance values ŽFig. 2. shows a rapid falling off with distance so that for river distance values on land in excess of 103 km, there are fewer than 500 km2 ; offshore, at distances of over 173 km, areas again fall below 500 km2 . A cumulative distribution plot of mean height for both seafloor and land has been constructed ŽFig. 3.. This shows a smooth pattern Žthe minor irregularity just offshore from the present coastline is an artefact of the decision to treat all squares including the coastline as land. and, in comparison with graphs based on river distance, shows the effect of large areas close to the coast, compared to the smaller areas at greater distances both out to sea and inland, as already seen in Fig. 2. There is no universal relationship between river distance and altitude, especially on land. This is shown in the graph ŽFig. 4a. linking river distance and mean height Žthe average of highest and lowest values for each kilometre square.. This is a result of the limited maximum river distances in the major areas of high relief, especially in Scotland and the Pennines. Thus, for river distances greater than some 100 km, mean height varies between a maximum of
270 m at 107 km and 133 m at 154 km, rising again to 286 m at 196 km. The subsequent low point is 167 m at 221 km, with the very small number of cases at higher distances reaching a maximum value of 500 m at 246 km. However, as Fig. 4b shows, there is a good relationship for river distances between 0 and 80 km, with a correlation coefficient, admittedly based on average height for each river distance, of r 2 s 0.92. What is most apparent is the intercept, with a value of 103.5 m at the coast. Offshore ŽFig. 4c., the overall relationship is far better, with an r 2 value Žagain based on the average height for each offshore distance. of 0.86, and again with an intercept at the coastline of y58.7 m. As the graph of values shows, there is in fact a considerable levelling off at a little less than y100 m, with values between y90 and y99 m from 152 to 270 km offshore, and no lower than y101 m to the maximum distance of 295 km. The other feature of the offshore graph is the persistence of values in the range y63 to y69 m between 25 and 96 km offshore. When the land and sea values for mean height for each kilometre of RD are linked ŽFig. 4a and b., a remarkable pattern results. The extremely smooth continental shelf is dominated by altitudes between y60 and y100 m, the land area generally lies above 130 m; indeed, only river distances 0–14 have averages below this value. There is thus an overall difference in level between the submerged shelf and the land of almost 200 m. A similar figure results if we link the two regression lines for the whole shelf and the land area with river distances from 0 to 80 km ŽFig. 5.. With intercepts of y58.7 and q103.5 m, respectively, we have a discrepancy at the present coast of 162.2 m. Although distance offshore is a simpler concept than the distance downslope to tidewater that underlies the definition of river distance onshore, there is nothing in this Žnecessary. distinction that might explain the lack of match of the calculated regression lines at the coast. The sea-floor within just a few kilometres of the coast is almost 60 m lower than present day sea level, while the land within 5 km of tidewater is on average more than 100 m above present day sea level. Before considering this issue further, the relief change at the edge of the landmass is further investigated by separating the values for Highland and
K. Clayton, N. Shamoonr Geomorphology 25 (1998) 31–42
Fig. 1. River distance Žland only.. Fig. 6. Great Britain: average height smoothed by 25=25 km window. 35
36
K. Clayton, N. Shamoonr Geomorphology 25 (1998) 31–42
Fig. 2. Frequency diagram ‘river distance’ Žonshore and offshore. by kilometre classes. River distance is measured up rivers and their tributaries on land; at sea, it is a measure of the direct distance from the coast.
Lowland Britain ŽMackinder, 1902.. The simplest way to do this is to plot the same graphs, based on average height values for river distances, for pre-Per-
mian rocks ŽHighland Britain. and post-Carboniferous strata ŽLowland Britain.. The adjacent sea floor is divided on the basis of the geology of the nearby
Fig. 3. Hypsometric curve, mean height, sea and land.
K. Clayton, N. Shamoonr Geomorphology 25 (1998) 31–42
land as a geological division is not appropriate for the continental shelf. To save space, the graphs are not reproduced here, but the values are listed in Table 1. The two additional intercepts for bathymetry in Table 1 were calculated for the area of sea off Highland Britain and also for the sea area of the southern North Sea and the English Channel for Lowland Britain. Overall, there is a consistent relationship with the older and no doubt tougher rocks standing higher, the younger rocks lying at a lower level, though still well above the adjacent sea floor. The reason for this ‘freeboard’ around the British coast is discussed briefly below and more fully in Paper III of this series. In any discussion of this ‘freeboard’ by which the land stands above the adjacent sea floor, consideration of several issues is required. We first need to note the exceptional altitude of present day sea level when considered across the span of the Quaternary —or even over the last 100,000 years. Estimates of low sea level at the peak of the Devensian glaciation Žca. 18,000 years BP. are between 120 and 140 m below OD. An estimate of the pattern of past sea levels based on past ice volumes has been made for the period covered by the ice volume curve published by Shackleton and Opdyke Ž1973. for the last 140,000 years. This suggests an average sea level over the last glacialrinterglacial cycle of about y55 m. Data are not readily available in the same form for earlier glacialrinterglacial cycles, but following Porter Ž1989., we may expect repetition with each 100,000 year cycle. Thus, it is this low sea level of some y55 m that is likely to be linked to the Quaternary evolution of the land surface, rather than the abnormally high sea level of the current interglacial. It will be noted from Fig. 4b that the depth of 60 m is reached on average at a distance offshore of no more than 10–15 km, though the actual figure varies around different parts of the British coast. Noting these fluctuations of sea level, the difference between the altitude of the sea floor offshore and the adjacent land is of considerable interest. It could be the result of three influences: Ø the tendency of a land subject to continuous erosion to rise isostatically through denudational unloading; Ø the recurrent impact of marine erosion on the
37
margin of the rising land as sea level has risen after each glacial period Ža transgressing sea having particularly effective erosional power.; Ø the effect of water loading during interglacial periods and the longer-term effect of offshore sedimentation progressively adding load to the continental shelf. Of course, the eroded nearshore zone will have had something of the same tendency to rise, but this will have been offset by both repeated planation and the water and sediment loading noted above. Indeed, even the recent, exceptionally rapid, sedimentation brought about by glaciation ŽClayton, 1996. has not only built out the edge of the continental shelf with a prograding wedge, but has also downwarped the original shelf surface as a result of the depositional loading Žsee Clayton, 1996, Fig. 2, p. 127.. That the unloading due to denudation is continuously reflected in consequential uplift is argued convincingly by Gilchrist and Summerfield Ž1991. and they note the role of denudation at the margins of continents in producing significant marginal upwarps. In terms of scale, the whole of Britain may be considered as a fragment of a European marginal upwarp. The contribution of uplift consequent upon denudational unloading to the present elevation of Great Britain is discussed in more detail in Paper III of this series.
7. Contrasts between the land area and the continental shelf We have already noted the overall contrast between the shape of the land and shelf areas as related to distance from the coast. Except in the near coastal zone, shelf slopes are very low when compared with those on the land. There is, in addition, a considerable geological contrast between land and shelf which is summarised in Table 2. The values in Table 2 show that, with minor irregularities, there is a consistent sequence in the ratios by area of the rocks when arranged in age order; older rocks are disproportionately common on land, younger rocks disproportionately common on the submerged shelf. The contrast between shelf and land in the two divisions of the Tertiary is particu-
38
K. Clayton, N. Shamoonr Geomorphology 25 (1998) 31–42
Fig. 4. Ža. Mean height Žaltitude plotted against river distance for all values of river distance on land, and regression line; Žb. mean height Žaltitude. plotted against river distance for values of river distance of 0–80 km on land, and regression line; Žc. mean sea depth plotted against distance offshore, with regression line.
K. Clayton, N. Shamoonr Geomorphology 25 (1998) 31–42
39
Fig. 5. Plot of mean height Žaltitude. against river distance for land Žriver distance values 0–110 km. and depth offshore Žfor all river distance values.. Underline: the regression for the values plotted here is shown, and the two regressions for offshore and land values are also plotted. The intercepts of these at the coastline show the lack of fit Ž‘freeboard’. at the coast; the values are y58.7 m and q118.6 m, a difference of 177.3 m.
larly great. The contrast in the proportion of the Tertiary rocks is largely due to lack of deposition across the present land area, though there will also be a contribution from preferential erosion on land. The Cretaceous Chalk was probably deposited across almost the whole British land mass; it is rather more widespread Ževen by outcrop. on the shelf, and since it is generally present beneath the offshore Tertiary beds, the contrast is far greater than suggested by the outcrop values recorded here. If we return to the hypothesis that the sea floor lying between y70 m and the present day coast has been eroded, largely no doubt by transgressive wave action, we would expect to see a further contrast between the outcrop geology of the deeper and the shallower shelf. At the same time, we might expect the higher land to reflect the operation of erosional processes over a longer time, and perhaps at a more rapid rate, than lower areas. This is tested by dividing the land area at the level of 200 m and the sea at the y70 m level. The results are set out in Table 3. From Table 3, it will be seen that older rocks of all ages up to the Cretaceous are more common at
shallower depths than on the deeper parts of the shelf. It seems, therefore, that the shallower shelf has been eroded differentially, partly by successive ice sheets as well as by the transgressing sea following each glacial stage. On land, the division at the 200 m level shows a somewhat similar contrast with older rocks more common at higher altitudes than low. This is attributed mainly to differential erosion, though with the contributory element that these older rocks are more resistant to denudation and thus more likely to form high ground. However, Palaeozoic rocks cannot form high ground if they do not occur at high levels prior to exhumation, as the buried London Platform Table 1 General elevation of the land towards the coast, Highland and Lowland Britain
All Britain Post-Carboniferous Pre-Permian
Land intercept
Sea floor intercept
Difference Žm.
118.6 50.3 153.2
58.7 43.5 76.5
177.3 93.8 229.7
K. Clayton, N. Shamoonr Geomorphology 25 (1998) 31–42
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Table 2 Areas Žkm2 . by geological age, land and shelf Žpre-Quaternary rocks only. Geological Age
All areas
Shelf
Land
ShelfrLand
Shelf Ž% of all.
Land Ž% of all.
Pre-Cambrian L. Palaeozoic U. Palaeozoic Permo-Trias Jurassic Cretaceous Palaeogene Neogene Totals
36,867 52,165 97,214 78,586 61,121 86,476 85,148 62,423 560,000
8552 7056 33,038 51,781 34,615 55,840 66,228 62,380 319,490
28,315 45,109 64,176 26,805 26,506 30,636 18,920 43 240,510
0.30 0.16 0.51 1.93 1.31 1.82 3.50 1451 1.33
1.53 1.26 5.90 9.25 6.18 9.97 11.83 11.14 57.05
5.06 8.06 11.46 4.79 4.73 5.47 3.38 0.01 42.95
Post-Jurassic Post-Palaeozoic All Tertiary
234,047 373,754 147,571
184,448 270,844 128,608
49,599 102,910 18,963
3.72 2.63 6.78
32.94 48.37 22.97
8.86 18.38 3.39
demonstrates. Equally, they can lie low as a result of prolonged erosion at some past stage, as in Anglesey where the planation could be as old as the Permo-
Trias and where faults separate the island from the uplift of the Welsh Massif and the Snowdon dome in particular ŽNeedham and Morgan, 1997..
Table 3 Areas Žkm2 . by geological age Žpre-Quaternary only., land and shelf, both divided into two altitudinal groups Geological age
Below y70 m
y69 to 0 m
0 to q200 m
Over 201 m
(a) By area Pre-Cambrian L. Palaeozoic U. Palaeozoic Permo-Trias Jurassic Cretaceous Palaeogene Neogene Totals
2742 1871 9764 20,638 11,044 17,631 34,211 61,093 158,994
5810 5185 23,274 31,143 23,571 38,209 32,017 1287 160,496
11,430 20,935 41,461 25,657 25,346 30,114 17,350 43 172,336
16,885 24,174 22,715 1148 1160 522 1570 0 68,174
Post-Jurassic Post-Palaeozoic
112,935 144,617
71,513 126,227
47,507 98,510
2092 4400
(b) Percentages (area) Pre-Cambrian L. Palaeozoic U. Palaeozoic Permo-Trias Jurassic Cretaceous Palaeogene Neogene Totals
1.73 1.18 6.14 12.98 6.95 11.09 21.52 38.43 100
3.62 3.23 14.50 19.40 14.69 23.81 19.95 0.80 100
6.63 12.15 24.06 14.89 14.71 17.47 10.07 0.03 100
24.77 35.46 33.32 1.68 1.70 0.77 2.30 0.00 100
Post-Jurassic Post-Palaeozoic
71.03 90.96
44.56 78.65
27.57 57.16
3.07 6.45
K. Clayton, N. Shamoonr Geomorphology 25 (1998) 31–42
In summary, the table shows that older, pre-Permian rocks are dominant above 200 m, that Permian to Cretaceous rocks are concentrated between y69 and q200 m, while Tertiary strata are dominant at depths of y70 m and above.
8. The broad pattern of relief We can smooth the detailed square by square data by moving windows of varying size, assigning to each individual square the average elevation of a wider area. A map Žnot reproduced here. of generalised Žsmoothed. relief produced by a moving 11 = 11 km window, i.e., the average elevation for each point is the average of the 121 km2 centred on it. The result is a map that picks out the level of detail of a typical small atlas map, with the higher Chalk and Jurassic limestone cuestas of southern England showing up well, with Bodmin Moor, Dartmoor and Exmoor separately identified in the Southwest Peninsula, and so on. Offshore, deep water in trough-like form within the Irish Sea and east of the Outer Hebrides is matched by smaller areas within the northern part of the North Sea, with a wide area of deeper water off northeastern Scotland. Bolder smoothing ŽFig. 6. using a 25 = 25 km window Ži.e., each square represents the average height of 625 km2 centred on it. picks up the essentials of the British relief, with depths of over 100 m limited to the northernmost part of the North Sea and to the centre line of the Irish Sea. The form of the Irish Sea depressions, including the North Channel, strongly suggests incision by ice. On land, a major area of high ground in the Scottish Highlands, mostly south of the Great Glen, is unmatched elsewhere, though the northern Pennines reach over 500 m, higher than the Southern Uplands, the Lake District, the whole of the Welsh Massif, and the rest of the Pennines. Dartmoor, the southern Pennines and much of Wales exceed 300 m, while Exmoor and the North York Moors exceed 200 m. The highest areas of the cuestas and plateaus of the Jurassic limestones and the Chalk fall between 100 and 200 m at this scale of generalisation. Eastern England from the Vale of York to east of the Chilterns and the whole of Kent is everywhere below an average elevation of 100 m.
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9. Conclusion This initial analysis of the digital map data conveys a picture of the Great Britain landmass as created by past modest uplift, probably of Tertiary age, and developing in the very late Tertiary and the Quaternary period by erosional incision of rivers and glaciersrice-caps, leading to a pattern of denudational unloading which has led to isostatic adjustment. This has given a coastal ‘freeboard’ of at least 120 m above present sea level, even more above the general level of the continental shelf. This offshore bench is attributed in part to marine erosion during transgression after each glaciation, while its present elevation reflects the effects of water loading as a result of the unusually high sea level experienced in interglacial periods such as the present. The contrasts in both geology and relief between land and sea are very great, and although these are not new discoveries, they are here quantified for the first time. The challenge to be faced in further interpretation of this database is to explain the overall form of our relief in terms of geological history Žpast tectonics., rock resistance, neotectonics and possible regional contrasts in denudation. This will be pursued in later papers in this series.
Acknowledgements The considerable task of compiling the database underlying this work was funded initially, and predominantly, by UK Nirex, as part of a contract designed to provide a better understanding of the past evolution of the British land surface, as a more secure basis on which to predict future change. The data covering Southern England and Wales were acquired using an Emeritus Award from the Leverhulme Foundation. This large database, although it may be used on a desktop PC, has made considerable demands in terms of data management, the utilisation of packages such as IDRISI, ERDAS, SPSS, and associated special programming to perform particular tasks. This has been handled from the first by Dr. Nadhim Shamoon. Keith Clayton is grateful to Dr. Tim Atkinson for several valuable discussions on the utilisation of the database, to Dr. Paul Burton for discussion of geophysical issues, and to Dr. Mike
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Thorne who has consistently supported this approach towards a better understanding of the evolution of the British landform.
Appendix A. A note on computing programs and packages The complete database can be handled efficiently on a PC with a PENTIUM 133 MHz processor, 32 MB RAM, and 1.6 GB hard disk. It is helpful to have a 20 in. SVGA monitor, as this allows more data to be presented on a single screen. In particular, the whole UK raster image can be displayed at one time at a size sufficient to easily resolve data by eye. The larger screen also enables ERDAS Žon DECALPHA. GIS and Image Processing software to be used effectively as many windows are required to be open simultaneously. The data were entered directly into an INGRESrSQL database using a FORTRAN program with embedded SQL commands. The main database is called ELKMC holding geology-tab and height-tab. INPUTNX.FOR is a FORTRANrSQL program which sends data to INGRES and allows the data to be displayed on screen for the purpose of quality control. Most of the work utilises existing packages, but a few routines had to be written Žmostly in FORTRAN 77. to manipulate the data efficiently. These include: NIREX.FOR, a program principally designed to generate frequency tables and extract statistical data from the project database. Another is MASK.FOR, which is used to extract variables based on their river
distance from the coast Žin steps of 1, 2, 3, etc.. this can be based on age, lithology, geology. Once extracted or computed, the output file formats are such that data can be readily plotted Žin SPSS or Minitab. and further statistical work done. The third program frequently used is WINDOW.FOR, which provides a smoothing function based on a moving window. The size is specified and average, lowest or highest values can then be placed in the centre of the window.
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