Southern England

FIG 32. Diagram illustrating the movement processes of plates (not to scale).

UNDERSTANDING SURFACE MOVEMENTS

We have been considering the large movement systems that originate within the Earth. There are also more local movement systems operating on the Earth’s surface, which are linked to a very variable degree to the large-scale movements of plate tectonics. To explore this complex linkage further, it will be helpful to look now at different processes that may combine to cause particular local movements.

Horizontal movements as part of convergence, divergence or lateral transfer

Tectonic plates are recognised by their rigidity, so there is relatively little horizontal movement between points within the same plate compared to the deformation seen in plate boundary zones. This extreme deformation may involve folding and fracturing of the rock materials, addition of new material from below, or absorption of material into the interior during subduction.

Nonetheless, deformation is not restricted solely to plate boundaries, and does occur to a lesser extent within the plates. In some cases, major structures that originally formed along a plate boundary can become incorporated into the interior of a plate when prolonged collision causes two plates to join. Southern England includes the remains of a former convergent plate boundary and contains many examples of structures of this sort (particularly around Dorset and the Isle of Wight). These structures have often been reactivated long after they first formed in order to accommodate forces along the new plate boundary via deformation within the plate. Conversely, changes of internal stress patterns can sometimes lead to the splitting of a plate into two, forming a new, initially divergent plate boundary. Many of the oil- and gas-containing features of the North Sea floor originated when a belt of divergent rift faults formed across a previously intact plate.

It needs to be stressed that the patterns of deformation (fracturing and folding) due to these plate motions occur at a wide range of different scales, from centimetres to thousands of kilometres. Sometimes they are visible at the scale of an entire plate boundary, such as the enormous Himalayan mountain chain that marks the collision of India with Asia.

The effects of features as large as plate boundaries on landscapes persist over hundreds of millions of years, long after the most active movement has ceased. For example, parts of southwestern England, Wales and the Scottish Highlands are underlain by bedrocks that were formed in convergent boundary zones of the past. The tin and lead mines of Cornwall owe their existence to a 300-million-year-old convergent plate boundary, where an ocean was destroyed as two plates converged and continents collided. The convergence released molten rock that rose in the crust and gradually cooled to form granite, while metals were precipitated in the surrounding crust as ‘lodes’ containing tin and lead (see Chapter 4).

Mapping the patterns of bedrock exposed at the surface often reveals folds and faults that provide key information about the movements that have taken place during the past. Figure 33 provides a key to some of the terms commonly used to classify these structures as a step towards understanding the sorts of movement patterns that they represent. In broad terms, folds tend to indicate some form of local convergent movement, though they may be the result of larger movement patterns of a different kind. Normal faults tend to indicate divergent movements, at least locally, whereas reverse and strike-slip faults tend to indicate convergence. Two broad types of fold are distinguished: synclines are u-shaped downfolds, while anticlines are the opposite – n-shaped upfolds.

Further mapping of folds and faults often reveals complex patterns of changing movements. In the example shown in Figure 34, divergent movements in an area of crust produce plastic deformation in the warmer lower crust, and faulting into a number of discrete blocks in the colder, more brittle, upper crust. This is then followed by an episode of convergent movement that results in closing up the upper crustal blocks and further flow in the plastic lower crust, causing crustal thickening and mountain building at the surface.

Vertical crustal movements as part of other crustal movements

The movement of lithospheric plates is the main cause of convergent and divergent movements affecting thousands of kilometres of the Earth’s surface. As shown in Figures 33 and 34, these horizontal movements are generally accompanied by vertical movements that can produce very large scenic features, such as a mountain belt or a rift valley. In this book we are primarily concerned with scenic features at a more local scale, so we now consider various other processes that may be important in creating vertical crustal movements without contributions from large-scale plate interactions.

FIG 33. The most important types of folds and faults, and the local patterns of forces responsible.

Vertical changes by erosion or deposition

Addition or subtraction of material to the surface of the Earth is happening all the time as sediment is deposited or solid material is eroded. The field of sedimentology is concerned with the wide range of different processes that are involved in the erosion, transport and deposition of material, whether the primary agent of movement is water, ice, mud or wind. An important point is that few of these sedimentary processes relate directly to the large tectonic movements of the Earth’s crust that we have discussed above. Scenery is often produced by erosion of thick deposits that formed in sedimentary basins where material eroded from the surrounding uplands accumulated. One of the characteristic features of these thick deposits is their layered appearance, which is often visible in the scenery. Layering varies from millimetre-scale laminations produced by very small fluctuations in depositional processes, to sheets hundreds of metres thick that extend across an entire sedimentary basin. These thicker sheets are often so distinctive that they are named and mapped as separate geological units representing significant changes in the local environment at the time they were deposited.

FIG 34. Example of a cross-section through the crust, showing how a divergent movement pattern (A) may be modified by later convergent movements (B and C).

Vertical crustal movements resulting from loading or unloading

In addition to the direct raising or lowering of the surface by erosion or deposition, there is a secondary effect due to the unloading or loading of the crust that may take some thousands of years to produce significant effects. As mentioned above, we can visualise the lithosphere as ‘floating’ on the asthenosphere like a boat floating in water. Loading or unloading the surface of the Earth by deposition or erosion will therefore lower or raise the scenery, just as a boat will sit lower or higher in the water depending on its load.

An example of this is the lowering of the area around the Mississippi Delta, loaded by sediment eroded from much of the area of the USA. The Delta region, including New Orleans, is doomed to sink continually as the Mississippi river deposits sediment around its mouth, increasing the crustal load there.

A second example of such loading is provided by the build-up of ice sheets during the Ice Age. The weight of these build-ups depressed the Earth’s surface in the areas involved, and raised beaches in western Scotland provide evidence of the high local sea-levels due partly to this lowering of the crustal surface.

Unloading of the Earth’s surface will cause it to rise. Recent theoretical work on the River Severn suggests that unloading of the crust by erosion may have played a role in raising the Cotswold Hills to the east and an equivalent range of hills in the Welsh Borders (see Chapter 6, Area 9). In western Scotland, as the ice has melted the Earth’s surface has been rising again.

Vertical movements by expansion or contraction

Changing the temperature of the crust and lithosphere is an inevitable result of many of the processes active within the Earth, because they often involve the transfer of heat. In particular, rising plumes of hot material in the Earth’s mantle, often independent of the plate boundaries, are now widely recognised as an explanation for various areas of intense volcanic activity (for example beneath Iceland today). These plumes are often referred to as ‘hot spots’ (see Fig. 32). Heating and cooling leads to expansion or contraction of the lithosphere and can cause the surface to rise or sink, at least locally.

An example of this is the way that Southern England was tilted downwards to the east about 60 million years ago. At about this time, eastern North America moved away from western Europe as the North American and Eurasian plates diverged. The divergence resulted in large volumes of hot material from deep within the Earth being brought to the surface and added to the crust of western Southern England. It is believed that the heating and expansion of the crustal rocks in the west has elevated them above the rocks to the east, giving an eastward tilt to the rock layers and exposing the oldest rocks in the west and the youngest ones in the east. This sequence has important implications for the scenery of England’s south coast (see Chapter 5).

HOW CAN LOCAL SURFACE MOVEMENTS BE DETECTED?

Having just reviewed some of the processes that cause vertical movements of the Earth’s surface, it is useful to consider the practical difficulties of how such movements are measured.

For present-day applications, it seems natural to regard sea level as a datum against which vertical landscape movements can be measured, as long as we remember to allow for tidal and storm variations. However, much work has demonstrated that global sea level has changed rapidly and frequently through time, due to climate fluctuations affecting the size of the polar icecaps and changing the total amount of liquid water present in the oceans and seas. It has also been shown that plate tectonic movements have an important effect on global sea level by changing the size and shape of ocean basins.

Attempts have been made to develop charts showing how sea level, generalised for the whole world, has varied through time. However, it has proved very difficult to distinguish a worldwide signal from local variations, and the dating of the changes is often too uncertain to allow confident correlation between areas.

In sedimentary basins, successful estimates of vertical movements have been made using the thicknesses of sediment layers accumulating over different time intervals in different depths of water. In areas of mountain building, amounts of vertical uplift have been estimated using certain indicator minerals that show the rates of cooling that rocks have experienced as they were brought up to the surface. However, both these approaches are only really possible in areas that have been subjected to movements of the Earth’s crust that are large and continuous enough to completely dominate other possible sources of error.

Local horizontal movements are also difficult to estimate, although fold and/or fault patterns may allow a simple measure in some cases. Movement of sediment across the Earth’s surface by rivers or sea currents can be estimated if mineral grains in the sediment can be tracked back to the areas from which they have come. In the detailed consideration of landscapes in this book, we have to rely on using the widest possible range of types of evidence, carefully distinguishing the times and scales involved. Even then, we are often left with probable movement suggestions rather than certainties.

CHAPTER 4 The Southwest Region

GENERAL INTRODUCTION

MOST OF THE BEDROCK near the surface in the Southwest Region (Fig. 35) is distinctly older than the near-surface bedrock in the rest of Southern England. It therefore provides us with information about earlier episodes, and this is all the more interesting because these episodes involved movements of the crust that created a mountain belt, the only one fully represented in the bedrock story of Southern England. Not only does this add greatly to the interest of the Southwest, but it has resulted in the presence of valuable minerals that have strongly influenced the human history in the Region.

Bedrock foundations and early history

Sedimentation and surface movement before the mountain building

The Southwest Region consists predominantly of bedrock formed between about 415 and 300 million years ago, during Devonian and Carboniferous times. This bedrock records an episode during which some areas of the Earth’s crust rose while others sank, as part of a general buckling of the crust that is the first indication of compression and mountain building (Figs 36 and 37). As the rising areas became significantly elevated they were eroded, shedding sediment into the neighbouring sinking areas that became sedimentary basins. It is these basins that preserve most of the evidence of these events (Fig. 38).

In material that has been further and later deformed, it is difficult to work out the shape of the sinking areas, but many of them were probably elongated or trough-shaped, with the troughs separated by rising ridges that ran roughly east-west, parallel to the general trend of the later mountain belt. The troughs and ridges were caused in the early stages of mountain building by the compression and buckling of the crust. The troughs were generally flooded by the sea, or on the margins of relatively narrow seaways. Muds were the commonest materials to accumulate, although sands were also in plentiful supply. Lime-rich sediments, sometimes with corals and other shelly marine animals, were locally important. There were also periodic episodes of igneous activity that contributed volcanic lavas to the sedimentary successions.

FIG 35. The Southwest Region, showing Areas 1 to 3.

During these episodes of Devonian and Carboniferous basin and ridge activity, the Southwest Region was just one small part of a larger belt of similar activity that extended to the west into Ireland and Canada. Canada was then very much closer, because the Atlantic Ocean is a younger feature that only started to grow (at this latitude) about 100 million years ago, as divergence and spreading occurred along the mid-Atlantic plate boundary. To the south and east, the same belt of activity continued across northern France and into Germany. This broadly east-west trending belt later became the Variscan mountain belt.

Crustal convergence that created the mountain belt

The subsidence and sedimentation were sometimes interrupted by, and generally followed by, episodes of convergence of the Earth’s crust. This was caused by compression or squeezing, broadly in a north-south direction, so that areas of bedrock were folded and pushed closer together, making the east-west trending Variscan belt narrower, as if between the jaws of a vice. The map (Fig. 39) and cross-section (Fig. 40) show how the folds and faults of the region vary locally in their pattern, but can be explained generally by convergent movements in this north-south direction. These continued over at least 100 million years, and occurred along thousands of kilometres of the belt. Mountain-building events such as this occur when tectonic plates collide (as described in Chapter 3) and always have a profound effect upon the scenery in the vicinity of the collision. The Variscan mountain belt is just one of the great mountain-building episodes that have occurred periodically, throughout the Earth’s history.

FIG 36. Timeline diagram showing bedrock deposition and emplacement events in the Southwest Region.

Some of the best evidence for the horizontal crustal shortening comes from examining folds that can be seen in the bedrock at many localities (Figs 41 and 42). Folding of the originally flat layers of the bedrock is a spectacular feature of many southwestern sea cliffs, and the direction of the folding gives a clear indication of the direction of the shortening that resulted. Fractures (faults) also frequently cut the bedrock, and careful mapping makes it possible to recognise that, although some of them are very local features, others turn out to have been flat-lying fractures across which many kilometres of movement have taken place.

FIG 37. Simplified geological map of the Southwest Region.

FIG 38. Typical Devonian sedimentary basin in the Southwest Region.

FIG 39. The major bedrock structures of Southwest England and South Wales.

Another feature of the mountain building is that muddy material – the most abundant sediment in the Southwest Region – was often converted into slates that can typically be split into thin sheets and are said to possess a ‘slatey cleavage’. These rocks are locally referred to as killas, to distinguish them from other rocks with no cleavage, particularly the granites. The conversion into slates took place during the folding and fracturing of the mountain building, when the original muds, rich in clay minerals, were buried deeply below other sediments and then compressed to produce a new layering (or cleavage).

One large feature visible in the bedrock is the ‘Culm fold belt’ or ‘synclinorium’, a large and complex downfold representing horizontal crustal convergence (Figs 39 and 40). The centre of this feature is a belt of bedrock of Carboniferous age that extends between Bude and Exeter running across the centre and north of the Southwest Region. To the north and south of this, older (Devonian) rocks occur at the bedrock surface, forming the margins of the large downfold or syncline (Fig. 37). Culm is an old term much used by miners and European geologists for Carboniferous sediment, and synclinorium is a name for a downfold (or syncline) which contains numerous smaller folds.

In the Lizard area, much of the bedrock consists of a distinctive group of igneous rocks (Figs 39 and 40). These rocks cooled and solidified earlier than the main mountain building, and were mostly formed by intrusion of hot molten rock in a way that is typical of the floor of an ocean basin. The Lizard area provides one of the best examples now visible on land in Britain of material formed originally as ocean-floor crust. In Late Devonian times, as a result of early Variscan convergence, this large area of oceanic crust was forced northwards over and against sedimentary rocks lying just to the north. This shows how, in a large mountain belt, an area of crust (tens of kilometres across), with a distinctive history as the floor of an ocean basin, can be uplifted and incorporated into a mountain belt as its margins are squeezed together.

FIG 40. Schematic cross-section representing major structures of the Variscan mountain belt. The deep structure shown is speculative but shows how crustal shortening seen at the surface may be related to deeper, flat-lying fractures (faults). Located on Figure 39.

FIG 41. Zigzag folding due to horizontal convergence during the Variscan mountain building is spectacularly exposed at Hartland Quay (Area 3). (Copyright Will Brett/www.lastrefuge.co.uk)

FIG 42. Zigzag folding due to horizontal convergence during the Variscan mountain building, this time at Bude (Area 2). (Copyright Dae Sasitorn & Adrian Warren/www.lastrefuge.co.uk)

Some of the bedrock of this Lizard Complex is called serpentinite, after the common occurrence of the green mineral serpentine in sinuous cracks and veins. This gives the rocks an attractive colour patterning, and the absence of quartz makes them surprisingly easy to work with steel tools, giving rise to a local industry of carving serpentinite ornaments.

Granites and valuable minerals

The granites of Southwest England are an important later feature of the Variscan mountain belt. Granites are igneous rocks with coarse (millimetre across) crystals that have grown, interlocking with each other, as the molten material cooled slowly and solidified at some depth in the Earth’s crust. The minerals of the granite are most commonly quartz (typically about 30 per cent) and feldspar, generally with some other minerals such as mica (Fig. 43). The granite liquid (often called magma) formed as a result of melting deep within the outer layers of the Earth and was then forced upwards, sometimes pushing aside the overlying bedrock and sometimes replacing it by melting. The granites solidified at depths of several hundred metres or more below the surface and are now at, or near, the surface because of landscape erosion. The main granite areas include the Isles of Scilly in the west, followed by Land’s End, Carnmenellis, St Austell, Bodmin Moor and Dartmoor in succession to the east. These distinct granite areas at the surface can be visualised as the tops of fingers extending upwards from a single continuous granite body detectable by gravity surveys at greater depth under the spine of Southwest England (Fig. 44). The deep body extends for some 200 km along the length of the mountain belt.

FIG 43. Polished slab cut in the Dartmoor granite showing typical granite texture. Crystals of quartz (light grey), feldspar (white) and biotite (black) have interlocked as the magma (molten rock) solidified on cooling. (Copyright Landform Slides – Ken Gardner)

FIG 44. Diagram showing the large granite body below the bedrock of Cornwall and Devon, and the way the granite bosses now visible at the surface are upward extensions of this larger body.