Читать книгу Britain’s Structure and Scenery - L. Stamp Dudley - Страница 11

THE GEOLOGICAL MAP

DURING the war an important series of maps, designed to supply information basic to any work of national planning, was initiated by the Research Division of the Ministry of Town and Country Planning and published by the Ordnance Survey. The main series is on the scale of 1: 625,000 or approximately 10 miles to the inch and the series to be issued on that scale, covering the island of Britain in two large sheets, includes many of the maps which were proposed in the scheme for a National Atlas drawn up by the National Atlas Committee of the British Association for the Advancement of Science. Amongst the maps actually prepared and available, those of the Relief of the Land, Land Utilisation, Types of Vegetation, Types of Farming and Land Classification are very relative to the study of the natural history of this country, but fundamental to any study are the geological maps. Following the practice of the Geological Survey in some of their detailed maps, there are to be two maps—one to show the history of this country, while fundamental to any study are the geological maps. Following the practice of the Geological Survey in some of their detailed maps, there are to be two maps—one to show the “solid” geology as it would appear if superficial deposits such as boulder clay, glacial sands and gravels and clay-with-flints were removed and the other to show the “drift” geology with all those surface deposits indicated.¹ It has already been pointed out that many geologists are interested primarily in the older rocks and in the structure of the earth’s crust and the superficial deposits are to them simply a nuisance; from the economic point of view in the investigation of mineral resources the same is true and so the drift deposits and their mapping have been relatively neglected—notwithstanding their supreme importance in agriculture.

The Geological Survey came into existence in 1835 as an offshoot of the Ordnance Survey and its immediate task was the mapping of the geology of the country on the scale of one inch to one mile. This work was vigorously prosecuted, especially under the energetic guidance of Sir Roderick Impey Murchison who became Director-General in 1855, and the results were published in the form of hand-coloured one-inch maps, the base maps used being the original one-inch sheets of the Ordnance Survey. These original one-inch maps were “solid” maps though indications of the presence of drift is conveyed by words printed across the map in appropriate places. Its preliminary work finished, the Geological Survey then set to work to carry out a detailed revision or second survey. The field work was done on the six-inch or even, in cases, a larger scale. Attention was concentrated on the coalfields and on areas around populous centres, together with selected tracts in different parts of the country. For purposes of publication the “small-sheet series” of one-inch Ordnance Survey maps was chosen and these have been kept up to date for use as Geological Survey base maps long after they have been superseded by other editions for ordinary uses. They cover an area of 18 miles from east to west and 12 miles from north to south, and have the advantage of showing contours in black as part of the base map. Some of the small sheets series are hand coloured but the technical advances in colour printing has meant that all the later maps have been printed in colours. Each sheet is accompanied normally by a detailed explanatory memoir. In those areas where drift deposits are widespread or important it is usual to publish two editions of the map, the Solid and the Drift Editions, which are in fact two distinct maps. After more than half a century of work only about a third of the country has been re-surveyed and the maps published, so that for the rest reliance still has to be placed on the original survey and hand-coloured maps of a century ago. There is thus an obvious difficulty in issuing generalised maps of the whole country in that the detail available is so varied from one part to another, and it explains why, in a country where superficial deposits are of such tremendous importance in the study of soils, vegetation and agriculture, there is no generalised map to show their distribution. Pending the publication of the maps on the 10-mile scale just mentioned there is a useful map of the solid geology of the British Isles on the scale of 25 miles to one inch, published by the Ordnance Survey as one sheet at the modest price of two shillings.

This map is of the greatest value in giving a general picture of the distribution in Britain of the rocks of each of the systems. In the third edition, which is dated 1939, very considerable revisions and additions were made as a result of incorporating recent work.

Speaking very generally,. in geological terms the oldest part of Britain is the north-west and on the whole the rocks become steadily younger in geological age as one goes towards the south and east so that the major stretches of the Tertiary rocks are to be found in the London and Hampshire Basins.

The great mass of the Highlands of Scotland is made up of a complex of ancient metamorphic rocks with numerous large intrusions of granite. As described more fully in Chapter 23, one must picture the whole as the worn-down stumps of the great Caledonian fold mountains and there is little to-day in the relatively tame, rolling relief of much of the Scottish moorland to suggest the wild contortions exhibited by the underlying rocks—structures the interpretation of which has long baffled and continues to baffle the most expert of geologists. The oldest rocks of all are probably the Lewisian gneisses in the Outer Hebrides but the relative ages of the different pre-Cambrian or Archaean rocks is still uncertain. Along the coastlands of the North-West Highlands is a considerable stretch of Torridonian Sandstones—still pre-Cambrian but unmetamorphosed sediments, obviously much younger than the main bulk of the Highland rocks (see Plate IB). There is a narrow belt of Cambrian also in the north-west of the Highlands but the main masses of Old Red Sandstone—reminders of the great lake basins created by the Caledonian upheaval—lie on the east. Tiny patches of Jurassic rocks both in the west and along the east coast in the far north are reminders that Jurassic seas must have stretched far north but have left only small traces, and there is little evidence of the detailed geological history of Scotland over vast periods of time. Though the Alpine earth movements failed to fold the rigid old mass of the Highlands there is abundant evidence of the way in which great cracks were formed through which poured masses of molten lava. These make the great red splashes on the map in Skye and Mull and many of the smaller Hebridean islands.

A glance at the distribution of the deep purple colour used to indicate the Cambrian and the mauves used for the Ordovician and Silurian serves to demonstrate that it is the Older Palaeozoic rocks which make up the Caledonian mountain ranges of the Southern Uplands of Scotland, the English Lake District and the Isle of Man, as well as the whole of north and central Wales. That the Old Red Sandstone occupied basins is not only clear from the Highlands of Scotland: there are broad belts in the great tectonic depression of the Central Lowlands of Scotland between the Highlands and the Southern Uplands. In the south the main stretch of the Old Red Sandstone is in the Welsh Borderland whilst marine Devonian rocks of the same age cover Exmoor and much of South Devon and Cornwall.

Three colours are used for the rocks of the Carboniferous system—blue for the Carboniferous Limestone and the Scottish rocks (with sandstones, shales and coals) of the same age; sage green for the Millstone Grit and barren Culm Measures of Devon and Cornwall, and slate-grey for the Coal Measures. In passing it must be noted that the outcrops of Coal Measures shown on the geological map are not co-extensive with the coalfields because much of the most valuable parts of the coalfields are hidden beneath younger rocks. The Carboniferous rocks, with many areas of lava and other reminders of volcanic activity, fill in the remainder of the central lowland of Scotland whilst in England one is struck at once by the great north-south stretch of Carboniferous rocks which makes up the Pennines—the so-called backbone of England. This north-south alignment is a new one: it is a reminder that the rocks were deposited long after the Caledonian folding and that after their deposition were subject to the Armorican movements. These created north-south folds such as the Pennines and Malverns, folded as it were against the older blocks to the north and west, as well as the more common east-west folds so well seen in the alignments of the beds in South Wales and in the South-western Peninsula. Where the north-south and the east-west folds crossed, the creation of basins and dome-shaped uplifts is obvious and can be clearly seen from the map. Thus the South Wales Coalfield and the Forest of Dean are examples of basins and the Mendips are an example of the uplifts.

The way in which the bright blue streak of the Magnesian Limestone (Permian) cuts across different beds of the Carboniferous shows that the latter had already been folded and denuded before the deposition of the Permian.

The remainder of the map relates to Lowland Britain. The Midlands of England show clearly the stretch of the red Triassic deposits and the islands of older rocks which appear from beneath this cover.

FIG. 11.—A Simplified Geological Map of the British Isles

Then follow the successive belts of the Scarplands (see Chapter 18), sweeping across England from north-east to south-west—brown for the Liassic clays, yellow for the Oolitic sequence, dark green for the lower Cretaceous rocks, and light blue-green for the Chalk follow one another in sequence of ever decreasing age as one goes towards the south-east. It is in the south-east that the orderly sequence is interrupted and this is a reminder that the south and south-east of Britain lay on the fringes of the great Alpine storm and that some of the rocks there were folded by the Alpine movements. The uplift of the Weald, roughly east and west in its main axis, separates the two main Tertiary basins of London and Hampshire ; the sharpest folds of Alpine date are those in the extreme south—across the Isle of Wight and the Isle of Purbeck. The location of the main stretch of Pliocene rocks in the coastal parts of Norfolk and Suffolk is suggestive, and rightly so, that by that period the geography of Britain had acquired something of its present form—only certain parts, on the whole near the present coasts, came under Pliocene seas.

There the geological story shown by the general map we have been discussing ceases. It tells us nothing of the stupendous events of the Great Ice Age. For that we must turn to the detailed drift maps and from them try to piece together what is one of the most fascinating and important, and yet, despite its recent occurrence in terms of geological time, one of the most difficult episodes to reconstruct in the geological story of the building of Britain. Before we deal with this, however, it is essential to consider how many of the major surface features of Britain have evolved.