Connection between Geology and Scenery | Geology

There are places in our islands where the connection between the geology of the neighborhood and its scenery is by no means obvious, but more often the one bears a very clear relation to the other.

As we know, the present form of our islands began to be fashioned in Miocene times. Since then the rain has been falling and the streams have been wearing down the land, except during the Glacial period when, however, the action of ice assisted in the formation of the Britain of to-day.

Owing to the fact that the glaciers were at times so thick that they moved in various directions regardless of the valleys which they crossed, those valleys may have been partly or wholly filled up by material carried along by the moving ice. Hence the river system of post- glacial times is in parts quite different to that which obtained during times before the ice pall settled over the greater part of this country.

The point to be noted here is that we may have the scenery of a part of a country largely due to a deposit which cannot have been placed in its present position by any agent that we can now see acting in Britain. Climate, therefore, has a profound effect on scenery, and it must always be remembered that the geology of a country is only one of the factors which affect its surface.

One of the very obvious points which affects the scenery of a country is the shape of its hills. Where the hills are very similar in outline and continuous for great distances the scenery is apt to be monotonous and not so pleasing as in places where the hills are scattered and of varying out­lines. The causes of diversity in hill contours and outlines are many.

If one goes into a room with a floor of planks which have been used for several years without any covering, the continual wear and tear which the wooden planks have experienced has resulted in the hard knots standing up higher than the softer surrounding wood.

It is just the same where the earth’s surface has been scraped by means of solid material carried along by running water, and the fragments worn off have been washed away. In the long run the softer rocks are worn away more quickly than the harder rocks, and so the latter may be left to form the tops of hills and the ridges of mountains.

Escarpments:

When the rocks lie in unbroken horizontal sheets, the one over the other, for many miles a very definite type of hill range results. A hard bed of rock is left forming a more or less horizontal top, and this is cut through by river valleys, which eat back into the hills and often leave a very abrupt edge to the hill range. Such an edge is called an Escarpment, and there are many instances of very long escarpments in our islands.

In the north of England the hard Carboniferous limestone rests on softer rocks and is constantly seen forming more or less flat-topped ranges of hills with steep, and in places vertical, sides. These walls of grey limestone rock are called “Scaurs” and are very characteristic of much Yorkshire hill scenery. When there are beds of hard limestone separated by softer beds, one sees parallel horizontal lines of small cliffs separated by intervening grass-grown layers.

Of course, if the limestone is broken by having been cracked or has been tipped up by earth movements, the lines of scaurs are neither continuous nor horizontal. When the rock is not so hard, as is the case with the oolite beds of the Cotteswolds, we get more or less horizontal- topped hill ranges whose sides are not so vertical as is the case with the Yorkshire hills.

The escarpment which looks west over the Severn valley is, however, of the same type as the Carboniferous limestone escarpment, although it sinks more gradually to the valley below.

Other great lines of escarpments run through Kent, Surrey, and Sussex, forming the north edge of the South Downs, and the south edge of the North Downs, over­looking the great central area of the Weald.

Here the more or less horizontal rock is the porous chalk, which is even softer than the Cotteswold limestone, resting on clays and sandy beds. In consequence of its softness it might be expected to yield an even less abrupt escarp­ment than the harder oolite limestone. But, owing to its porosity, water tends to soak into the chalk and not to run over its surface, and in consequence we find the edges of the Downs facing the Weald exceedingly steep.

Another part of our islands which shows flat hill-tops and abrupt scarps is the west coast of Scotland. Here, over a very wide area, there occurs a great thickness of lava, and into this lava has been injected a molten rock, which is now far less easy for the weathering agents, frost, rain, wind, and running water, to attack, than the lava amongst which it occurs.

In consequence of this it tends to form flat hill-tops, since the beds are all more or less horizontal, and round the sides of these hills run small or large cliffs formed by the harder rocks, while the more easily decomposed lava produces grass-covered slopes at the foot of every cliff.

These are, of course, only some of the grander examples of escarpments to be seen in our islands, and numerous other cases can be seen in other parts of the country.

Outliers:

A very common, and often a very striking thing to see is an isolated hill standing up at some distance from an escarpment on the plain stretching from its foot and often nearly as high as the top of the escarpment. Such a hill may sometimes be almost conical in shape, and may resemble a volcano in outline, but careful investigation shows that it is not a pile of material produced by explosions.

On the contrary, the rocks which build up the hill can be paralleled bed for bed on the escarpment some distance away.

This at one time gave rise to the idea that such a hill had cracked off from the escarpment and slid into the plain. But, again, investigation often shows that there is no evidence of any such convulsion, and we are driven to the conclusion that these hills are remnants from a former day when there was an unbroken connection between the long line of hill, whose edge now forms the escarpment, and the isolated hill which for some reason has withstood the forces of denudation.

The slow, quiet, but invincible action of rain and running water has removed the intervening material and left on the one side of the gap the escarpment of the hill range, and on the other the solitary hill or Outlier, as it is called.

Thus in the region near Gloucester the main escarpment of the Cotteswolds runs north and south some four miles to the east of that town, and is formed of fairly hard oolite limestone on the top of softer Lias clay, and this escarp­ment is slowly but surely retreating eastwards as bits of it are carried away by the streams that flow down its slope to the Severn.

The escarpment just behind Gloucester rises to about 800 feet, and between the city and this edge is a lofty hill 650 feet high, called Robin’s Wood Hill. This rises from the Lower Lias of the plain, with a more or less circular out­line, up through the Middle Lias and the Upper Lias, and is capped by a small thickness of oolite limestone.

These are just the beds seen in the hill-side some two miles away. Again to the north-east rises another somewhat conical hill, called Churchdown, which shows, -over the Lower Lias of the Vale, Middle and Upper Lias beds. The oolite, however, which no doubt existed in former times on the top of this hill, has all been washed away, and the hill-top is now only some 450 feet above the sea.

Such outliers are also seen in front of the Chalk Escarp­ments, and wherever they occur their presence does much to break an otherwise somewhat monotonous stretch of grass-covered down.

There are in places similar hills which owe their preservation to the fact that the hard bed at their top and the lower beds which it covers have all been let down by a fault or crack which occurred when the beds forming the escarp­ment and outlier were continuous.

Fig. 28. Churchdown, an outlier of the Cotteswold Hills on the Severn Plain

To the north of Cheltenham the base of the oolite lime­stone along the Cotteswold Escarpment is at a height of about 800 feet, but the same basement bed forms the top of Oxenton Hill a mile and a half away, which is only 500 feet in height, and the general slope of the beds is downwards from Oxenton Hill to the escarpment.

Here the cause of the existence of the outlier is a fault running between the hill ridge of the Cotteswolds and the outlier, the latter having been let down and so preserved from some of the denudation which wore away the Cotteswolds eastwards, for the limestone would all be removed on one side of the fault, while some hundreds of feet of the same rock remained on the other forming a hard cap.

Speaking generally, regularity of geological structure produces a regularity in the scenery, and this is perhaps most marked where the beds of rock are nearly horizontal. Where they are tipped up at an angle the hills tend to have a long flat slope called the dip slope, which is parallel to the top surface of the beds, and a steep face called the scarp which cuts across the beds. The higher the angle of dip given to the beds the sharper their outline tends to be, though this must obviously partly depend on the hardness of the rocks.

Our jagged mountain scenery is often largely due to the steep angle of dip of the rocks which make the mountains, these rocks being at the same time comparatively hard. When, however, instead of being similar over large stretches of country, the rocks are of a very varied nature, some soft, some hard, some horizontal, some vertical, then we get just the conditions which produce a varied scenery.

The soft rock gets scoured out and a stream covers it, a hard rock withstands denudation and stands up above its surround­ings. A glance at a geological map of any of our mountain districts will show how varied are the rocks in them. Round Keswick we have Skiddaw made of slates, and the sides of Derwentwater and of Borrowdale of lavas and volcanic ashes. In Snowdonia we have ashes and rocks which have been molten when forced into their present position, and many lava flows, while slates and other sedimentary rocks occur, the whole mass having been bent and fractured in many directions.

In some places a large or small mass of molten rock has been forced up amongst soft rocks and has then solidified into a hard mass. Denudation acting on the whole after­wards has left the hard mass standing out. Thus have been formed the great hill masses in South-West Scotland of the Criffel, the Cairnsmore of Fleet, of Carsphairn, and of Dee.

Thus, too, have been formed the small eminences so common in the east of Scotland called Laws, such as North Berwick Law, these being formed of the molten rock which froze and plugged up the openings through the rocks up which it had forced its way.

Landslips:

When, as in the Cotteswolds, the slope of the hard uppermost bed is away from the valley lying below the escarpment, occasional slips have occurred due to the beds becoming extra heavy after a rainy season so that the underlying clay is unable to support the pressure. The upper rock has then cracked and slithered down the greasy under-rock.

Even the clay itself after a very wet season slips in this way, and hedges are destroyed by the masses of clay that make their way down the slope for some yards. In consequence the slope of the Cotteswold Escarpment is very bumpy, and here and there large masses of limestone have slipped down for more than 100 feet over the clay below.

But the best conditions for slipping to occur are to be found where a hard or porous rock rests on a sand and the sand on a clay, and where the whole set of beds slope towards a valley or the sea. When there has been an unusually wet season the sand becomes a slippery mass, and the over­lying bed becomes extra heavy owing to the water it has absorbed, and so it may slip over the sand and clay down towards the valley floor or towards the seashore, as the case may be.

The above conditions obtain at certain spots on our south coast, and in times gone by enormous slips have occurred. On the Dorsetshire coast near Axmouth the porous chalk rests on the Upper Greensand and this on the Lias clay. Here in 1839 a huge slip took place, the chasm formed being 1000 yards long and 300 yards broad, and at some places over 200 feet in depth.

In the Isle of Wight, near Ventnor, the chalk has slipped in the same way towards the sea and has formed the picturesque under-cliff, which has made the stretch of country nearby so famous.

We know how one stream with a sharper fall may work its way backwards and capture the waters which flowed down another river. This has often led to a very pronounced effect on the scenery.

Dry Valleys:

It may leave a large valley absolutely dry, and we then get the strange sight of a valley with the absence of any apparent cause of production. At other times a small stream continues to run down the big valley and is seen to be utterly inadequate to have carved out the valley down which it wanders.

Such a case is well seen in the Northern Cotteswolds where the drainage areas of the Thames and Severn meet.

The older river, the Thames, flowing in a more or less easterly direction, carved out, in days now long gone by, large valleys in the oolite limestone, and in places cut their floors down to the impervious Lias clay. Then the Severn with a more rapid fall cut its way backwards from south to north, and denudation, lasting through an enormous period, eroded the gigantic gap, over twenty miles in width, between the Malverns and the present Cotteswold Escarpment.

As the Severn worked backwards to the north it captured one after another the streams from Wales which formerly fed the Thames, and the old Thames valleys in the present Cotteswolds became beheaded.

Not only did the Severn receive tributaries from its western side, but other streams came down the steep slope of the Cotteswold Escarpment from the east. These cut their way back eastwards and did their part in capturing water which, in former times, would have drained into the Thames.

The consequence of this is that when one goes up one of the Severn tributaries, which comes down westwards from the Cotteswolds, one finds oneself at its top not at the crest of a hill range, but at the bottom of a broad valley, which slopes slowly eastwards to the Thames.

But there are other causes which may produce either dry valleys or large valleys with small streams flowing along them. It may have been that in the past a bed of clay, which is an impervious rock, extended over a part of the country from which it was gradually removed by denuda­tion.

The rain which fell on the clay ages ago could not sink far in and formed streams; these united and made a river which carved out a valley of some size. But as the clay was removed, if it was underlain by a porous bed, the portion of the rain that formed streams which ran over the surface became less and less, while that which sank into the porous bed and came out as springs elsewhere increased in quantity. Thus the stream running down the old large valley grew smaller and smaller and eventually, if the clay bed was removed altogether, the valley may have been left dry.

Another cause which made rivers larger in the past may have been that at the end of the Glacial period the melting of the winter’s snow occasioned torrential streams in the spring-time, and so denudation would have gone on at a greater pace than now. Hence from one cause or another we find in amongst our hill countries dry valleys with their marked scenery.

River Erosion:

River erosion is not always very obvious unless one goes year after year to the same bank of a stream. One then realises that each year some portion of the bank is removed. Where the river runs through a fairly level tract it generally wanders in loops, called meanders and, as its stream is faster on the outer edge of a curve than on the inner, it is always cutting away the bank on the outer edge and depositing material on the other side. In this way a loop may eventually be cut clean through, and the river take a new course, leaving its old one as a marshy tract, which eventually becomes filled up by dead vegetation.

Rapids:

If a river flows across the junction of a hard and a soft rock it will be able to cut down into the latter more easily than into the former, and in consequence the slope of its bed will tend to become somewhat steep near the fine of separation. We then get rapids where the water pours over the hard rock, very often in a narrow channel, while, when it gets on to the softer rock it has a broader channel and a less steep slope, and so makes its way onwards more slowly.

But the soft rock which underlies the hard one may be removed so fast that a more or less vertical face of the hard rock is left, and then we get a waterfall.

Waterfalls:

This is the cause of many waterfalls in Yorkshire where the water falls over a hard layer of lime­stone down on to soft shale below, Hardraw Force in Uredale being formed in this way. On a grander scale Niagara owes its existence to exactly the same circum­stances.

Hanging Valleys:

There are, however, other ways in which a fall may be made. For instance a side-valley may lead into a main valley, and at one time the floor of the side- valley may be level with the floor of the main one. But the latter Valley carrying perhaps a large river may be deepened at a much greater rate than the smaller one, and in consequence the smaller side-stream may pour over a waterfall before it can get down to the river in the main valley which it ultimately joins.

Such a side-valley which joins a larger one above the main valley floor is called a hanging valley, and instances of such valleys are common in our mountain districts, such as those of Snowdon or of Galway.

Thus we see that water is the great agent which nature uses to carve out the surface of the land. We must not regard isolated hills as separated from neighbouring ranges by violent convulsions, nor need we imagine the steep sides of river gorges as having been torn apart and so to have provided channels for the waters which now rush between them.

Rather must we regard the hills as bumps left after the surrounding country has been washed away, and the gorges as having been sawn out by the materials washed down by the streams.

Neither must we look on the formation of hills and gorges as a thing of the past; the causes which formed them still exist, and the effects which were hills and valleys in the past will still be hills and valleys now.

Lakes:

Besides being affected by the presence of hills and valleys the scenery of a country may owe much to the presence of lakes. Any hollow capable of holding a con­siderable expanse of water may contain a lake, and since such a hollow may be formed in different ways, lakes show many different features.

When the necessary hollow exists it is very obvious that climate has much to say in the formation of a lake. If the region is a desert one no rain falls, and there will be no streams to run into the hollow; if the rainfall is small only a small accumulation of water results, and this may soon vanish owing to evaporation; but if the rainfall is sufficient the hollow will be filled up to the lowest part of its encircling bank, and a stream will flow out over the rim.

The necessary hollow may be formed in several ways— either by-the erection of a dam across a valley, or by the sinking in of a portion of the earth’s crust, or by the piling up of erupted material round a volcanic crater, or the hollow may be scooped out.

As we have seen, during the Glacial period much material was deposited over most of our islands by glacial action. When this became uncovered by the melting of the ice, the surface of the deposit was often very irregular and the hollows in the glacial drift, as the deposit is termed, often fill with water and give us those small tarns so fre­quently very effective as a foreground in our mountain views.

Again material either deposited at the lower end of a glacier, or under a glacier which moved across a pre­existing valley, has often produced a dam across the valley which has led to the subsequent formation of a lake. This is a cause which has been very active in producing lakes in Cumberland, but it may be seen equally well amongst the Galway Hills or in the south-west of Ireland.

But the dam across a valley may be caused by landslips from the valley’s sides, or by a lateral stream bringing down such a quantity of material that the main stream cannot move it away fast enough to prevent it reaching right across the main valley. It is by this latter cause that the well-known Sty Head Tarn above Wastdale has been caused.

But the lake-dam formed by glacial drift is responsible for very many of our best known lakes. Such dams are to be found at the lower ends of Windermere, Ullswater, and Bassenthwaite in Cumberland, and of Llanberis Lake and Gwynant Lake in North Wales.

There is one rather peculiar type of lake which often gives the finest scenery. A hollow in a mountain-side may during Glacial times have been packed with snow and ice which rose in a slope to the precipices which surrounded it. As blocks became dislodged from those precipices by the action of frost and thaw they would fall on to the snow slope and shoot down it for some distance, and so pile up a mass of material.

When the snow and ice melted a hollow would be left between the precipitous cliffs and the ring of fallen material, and this, in several cases, has given rise to a lake. The combination of a rugged dark cliff behind and the dark waters of the tarn below gives sometimes the grandest picture imaginable.

Though there are examples of lakes, such as the Dead Sea, which owe their existence to a sinking of the earth’s crust, this cause does not seem to have produced any lakes in our islands. The lake in a volcanic crater also, though, well known in Italy, in the Auvergne, and elsewhere, is unknown in Britain and in Ireland.

When we come to the possibility of a lake basin having been scooped out we meet with very great differences of opinion with regard to the agent by which that scooping- out may have been done.

It was as long ago as 1859 that the geologist, Sir Andrew Ramsay, put forward the idea that the Alpine lakes were in basins that had been dug out by glacial action.

In certain cases a very careful set of soundings have been made in lakes, and it has been proved that they exist over a rocky floor and that in some parts this floor descends to a considerable depth and then rises towards the lake’s lower end which is formed of bare rock.

The puzzle which confronts one is—If the hollowing-out is due to glacier action, how is it that this action went on more at one spot in the valley than at another? One cannot say that the greatest depths are in softer rock than is found elsewhere in the valley, for this is not the case. If the glacier did scoop out the valley, then it has scooped out at a spot only part way down the valley far deeper than nearer to its lower end.

It has been noticed that in some lakes the sides of the valley approach closer together just opposite the deep place in its floor than elsewhere. We may then picture a glacier moving down the valley and scraping on the floor with a certain pressure and producing a gentle declivity sloping down from the upper part of the valley.

When, however, it reached the constriction the ice would be wedged up in order to pass between the narrowed passage, and so there would be a greater quantity of ice above the floor between the sides of the valley where they came nearer to one another than elsewhere. This would produce a greater pressure, and so the floor would be scooped out more at that spot. After passing the constriction the ice would once more spread out laterally, and so the effect on the rocky floor would be less lower down the valley.

Besides the lakes which have been formed in the various ways, there are some in England formed by quite a different method. These are the Norfolk Broads.

There is good reason to believe that the rivers of East Anglia once opened into large estuaries, but owing to the tides sediment accumulated across the mouths of these rivers, which formed breakwaters and checked the pace of the river currents. Such a bank of shingle can be seen across the mouth of the Aide, a river which comes close down to the sea at Aldborough and then turns southwards between the mainland and a narrow shingle bank for miles before actually entering the sea at Orfordness.

The rivers, which came down in the past against these single accumulations, deposited their silt on the landward side, and so the land gradually was built up from east to west. As this land worked backwards it would cut across the places where side-branches ran into the main stream, thus clamming them up, and converting them into lakes.

So we see that various processes have been at work preparing for the formation of sheets of water which, accord­ing to their size, are termed tarns or lakes, and it is seldom that these sheets of water do not have a profound effect on the scenery of the district where they occur.

Coast Erosion:

When we regard coast scenery the effect of water is far more obvious than elsewhere. The cliffs are battered by the waves, and the hurling of boulders against them during storms assist the efforts of the water to tear pieces away. The harder rocks stick out as headlands, while the softer are eaten away and so lead to the formation of coves and bays.

Even the hard ones are worn away in time, and in some cases portions are left standing as sea- stacks and as islands, while the coast retreats behind them. Where the rocks are hard, as at the Lizard or the Land’s End, there the rate of erosion, or eating away, is very slow. Where they are of soft sand or gravel, as in Yorkshire, Norfolk, and Suffolk, there the rate of erosion is very much quicker.

This cutting back of the land’s edge produces the most marvellously different types of coast scenery. The hard rocks of Cornwall and Wales give cliffs which are only slowly eaten away by the sea, and in consequence they usually do not rise straight up from the shore to the cliff’s top, but are worn back more quickly by the rain and wind at the top.

The softer chalk is, on the other hand, often cut back at its base by the sea faster than wind and rain can remove it above, and so a very different outline is formed from that which we see in harder rocks. One result of this under­cutting of a chalk cliff is that after a time an extensive fall of rock may occur and a vertical cliff may thus be temporarily formed.

If the rock is soft it is rapidly re­moved, and thus when a band of more or less vertical hard rock in front of a softer one is once breached by the sea, the waters soon scour away the softer rock behind and a cove is formed. In this way the celebrated Lulworth Cove has been formed on the coast of Dorsetshire.

Rocks which were once sediments have in the course of time dried and, in so doing, contracted, and in consequence of this they have often developed a set of cracks called Joints. Igneous rocks when they’ have consolidated and cooled have also often developed joints. When the sea gets to work on a rock it generally splits off fragments along these joint planes. In fact the joints form lines of weak­ness, of which the sea makes use in breaking up the rock.

Hence we often see caves formed by the removal of the rock which occurs between two joint planes. On the other hand, when the rock between two joint planes is harder than elsewhere it is left as a projection into the sea, and then may become a solitary mass called a Stack. Such stacks are common round certain parts of our sea-coast. The Red Sandstone of Ireland gives rise to some fine examples, and the same rock makes sea-stacks off the South Wales coast.

The Needles, at the west end of the Isle of Wight, are examples of huge masses of a rock being left after the surrounding softer beds have been removed.

Here the chalk is almost vertical and forms a high down through the central part of the island. It has sands and clays to the north and to the south of it, and where it projects into the sea it forms lofty, almost vertical, cliffs.

The actual end of this mass of chalk has been cut through, probably along joints, and so several large masses of chalk have been left standing surrounded by sea, but year by year they diminish in size and must ultimately vanish altogether.

Scenery affected by Vegetation:

But besides depending on the relative heights of its parts, on the abruptness or slowly shelving nature of its hills, and on the outline of its higher land-masses, the scenery of a country may be markedly affected by the vegetation which grows on its hills and plains or in its valleys.

Now one of the great causes of difference of vegetation is the character of the soil. On a clay, one gets oaks and elms; on a limestone, beeches; on a sand, pines. Hence, for instance, as one traverses the south of England one finds a very marked difference in the general type of tree which is prevalent at different places.

On the London clay in Surrey are oak- woods; on the Bagshot sands, which overlie it towards the western end of the London basin, are masses of pine woods; while on the chalk of the Chilterns are masses of beech. The same things can be seen farther to the west. On the Lias clay of the Severn valley one meets with elms and oaks, while on the Cotteswold Hills, formed of limestone, beeches are far the, most common trees.

This great variation in the type of tree is one of the most charming points in the scenery of the south of England.