Chapter 4 The Geological Conservation Review

In a country such as Britain, with a relatively small land area and a population of over 55 million, there are many demands on land that may conflict with site conservation. There is a demand for hard rock and sand and gravel to meet the requirements of the construction industry, as well as clay for bricks, limestone for cement, and landfill sites for waste disposal. Just as it is impracticable to conserve every rock exposure, it is essential to conserve those that make a unique contribution to Britain's Earth heritage. The identification of those sites that need to be conserved has been the purpose of the Geological Conservation Review.

From the late 1940s until the late 1970s, the identification of the most important Earth heritage sites was undertaken by the Nature Conservancy (1949–1973) and then the Nature Conservancy Council (1973–1991) on the basis of available information and the advice of Earth scientists.

The launch of the Geological Conservation Review in 1977 marked a more systematic approach to site selection. The objective of the review was to identify those sites needed to show all the key scientific elements of the Earth heritage of Britain. Site selection was undertaken between 1977 and 1990 by the Nature Conservancy Council. It covered both the geology and geomorphology of Britain and involved several hundred scientists from higher education, government, industry and the voluntary sector. The sites selected were called Geological Conservation Review sites and they form the basis of statutory Earth heritage conservation in Britain. The current responsibility for site selection and deselection rests with the statutory nature conservation agencies: the Countryside Council for Wales, English Nature and Scottish Natural Heritage.

The results of the Geological Conservation Review programme are being published in a series of 42 volumes, each of which provides a public record of the evaluation of each Geological Conservation Review site placed in a national and, where appropriate, international context.

Principles of site selection

Aims of the Geological Conservation Review

From the outset, the Geological Conservation Review used the highest scientific standards to identify systematically the key Earth science sites in Britain. The site series would reflect the range and diversity of Great Britain's Earth heritage, and each site would ultimately satisfy the legal requirements for notification as a Site of Special Scientific Interest by reason of its 'geology or physiography' ('physiography' is synonymous with 'geomorphology'). The notification of Sites of Special Scientific Interest under the National Parks and Access to the Countryside Act 1949 and subsequently under the Wildlife and Countryside Act 1981, is the main mechanism of legal protection in Great Britain (see information box on page 46).

To achieve these aims, criteria and guidelines were developed. These can be encapsulated in three distinct, but complementary, components:

1. sites of importance to the international community of Earth scientists
2. sites that are scientifically important because they contain exceptional features
3. sites that are nationally important because they are representative of an Earth science feature, event or process which is fundamental to Britain's Earth history.

'Nationally important' in the context of Geological Conservation Review site selection refers to importance to Great Britain as a whole, and means that any site chosen for the Geological Conservation Review has been assessed, wherever possible, against comparable features, where they exist, across the whole of Great Britain.

Each site selected for the Geological Conservation Review is of at least national importance for Earth heritage conservation, and many of the sites are of international importance.

Sites of Special Scientific Interest (SSSIs)

The first component — international importance

The first component for Geological Conservation Review site selection ensures that geological and geomorphological sites of international importance are included so that our international responsibilities are met. Five main types of internationally important Geological Conservation Review site can be recognised:

• time interval or boundary stratotypes; for example, the boundary stratotype at Pitch Coppice, Herefordshire ((Figure 41); see also (Figure 2))
• type localities for biozones (rock strata which are characterised by a closely defined fossil content, usually a fossil species) and chronozones (rock strata formed during the time-span of the relevant stratotypes) (Figure 42)
• internationally significant type localities for particular rock types, mineral or fossil species (e.g. the fossil reptile Megalosaurus from Stonesfield in the Cotswolds, (Figure 43), and outstanding landform examples such as Chesil Beach (see (Figure 45))
• historically important type localities where rock or time units were first described or characterised, or where great advances in geological theory were first made (e.g. Hutton's Unconformity at Siccar Point, Berwickshire (see (Figure 37))
• important localities where geological or geomorphological phenomena were first recognised and described, or where a principle or concept was first conceived or demonstrated (e.g. cauldron subsidence at Glencoe; (Figure 44)).

The second component — exceptional features

Many sites have unique, rare or special features (Figure 45). For example, at Rhynie in Scotland, a mineralised peat, in the form of a siliceous rock called chert (Figure 46), of Devonian age, preserves a detailed record of an early land ecosystem. The exceptional microscopic detail preserved in the Rhynie fossils makes this site quite exceptional. No other sites are known in Britain to contain such well-preserved material of this age. This makes Rhynie irreplaceable. The inclusion of exceptional sites ensures that the highlights of British geology and geomorphology are conserved.

Exceptional sites may be visually striking and can contribute dramatically to the character of the landscape, for instance, Haytor Rocks on Dartmoor (see (Figure 1)), Cheddar Gorge, Somerset, and Chesil Beach, Dorset. In contrast, the unremarkable appearance of East Kirkton Quarry belies the extraordinary character of the fossil assemblage it contains (see (Figure 27)).

The third component — representativeness

Important though international and exceptional sites are, they cannot provide the basis for a systematic approach to the selection of sites to cover the essential features of the Earth heritage of Britain. This is provided by the selection of sites representative of features, events and processes that are fundamental to our understanding of the geological history of Britain. The starting point of the review was to create subject 'blocks' which provided an overall structure for site selection (the list of blocks can be found at the end of this chapter). This ensured that the different themes of Earth science would receive comparable treatment. The second stage in this approach was to consider the characteristic features of each block and to select representative sites. Within individual blocks, sites fall into natural groupings based upon geological features or scenarios. These groups are now referred to as networks and there may be one or more networks in any block.

Geological conservation review blocks

Many of the Geological Conservation Review blocks correspond to the standard divisions events of geological time or to major events within those periods. They can be grouped into seven broad categories:

• stratigraphy (35 blocks)
• palaeontology (16 blocks)
• Quaternary geology (16 blocks)
• geomorphology: the landforms and processes that form the current landscape (10 blocks)
• igneous petrology (6 blocks)
• structural and metamorphic geology (10 blocks)
• mineralogy (7 blocks)

Stratigraphy blocks

For the most part, these blocks are classified according either to their stratigraphical age (stage, period) or to a range of stratigraphical ages (e.g. Caradoc–Ashgill Block). Blocks for some stratigraphical ages, however, were defined not purely by age, but also by geographical area or environmental setting where there were significant variations in the rocks across Britain formed at the same time. This is why there are two blocks for the Devonian Period, one for marine rocks and one for non-marine rocks.

It is not possible in every case to define blocks by stratigraphical age. For example, where fossils are rare or absent it is difficult to locate the boundary between different geological ages. Such stratigraphical units are named after the geographical localities where they were defined, for example, the Wealden Group consists of mudstone, shale and sandstone that only occur in south-east England.

Most invertebrate fossils (e.g. trilobites, echinoderms, ammonites and other molluscs) are also addressed within the stratigraphy blocks, since these fossils are widely used in correlating rock strata. Because of the relative rarity of fossils such as reptiles, fish, mammals, birds, terrestrial plants, insects and other arthropods (excluding trilobites), these are covered in separate palaeontology blocks.

Palaeontology blocks

These blocks address the evolution and diversity of significant animal and plant groups not included in the stratigraphy blocks (see above) and therefore have independent block status. Geological time is used as the basis to define some blocks, for example, Jurassic–Cretaceous Reptilia.

Quaternary blocks

The Quaternary blocks are classified on a regional basis, although the subdivision of time (usually stratigraphical age) is an important factor. Sites were selected to represent the stratigraphy of Quaternary successions and the development of landforms.

During the Quaternary Period, northern Britain was covered by a succession of ice sheets, while southernmost Britain was not glaciated, although frozen ground conditions were experienced. The wide variation of stratigraphical units and geomorphological features across Britain, and the large number of sites available for study, required that a topic classification for networks within these blocks was adopted. Three principal themes form the basis of the Geological Conservation Review Quaternary site selection:

• environmental history and change based on the stratigraphy at different localities, age and fossil content, e.g. glacial–interglacial history, sea-level changes
• processes and patterns of landscape evolution, e.g. glaciation, periglaciation
• the history and development of the flora and fauna, e.g. vegetation history, evolution of vertebrates.

Comparisons were made between the regional Quaternary blocks to ensure that certain categories of site were not over-represented.

Geomorphology blocks

Geomorphology blocks cover the history and development of landforms and geomorphological processes active today, for example, rivers, coasts and landslides. Unlike geological sites where processes can only be inferred, active geomorphological sites provide open-air laboratories where processes can be studied.

Because geomorphology influences landscape and habitat, there is great potential for integrating the physical and biological components of nature conservation in geomorphological sites.

Igneous petrology and structural and metamorphic geology blocks

The igneous petrology and structural and metamorphic geology blocks relate to the effects of mountain building activity, such as the Caledonian Orogeny.

Major episodes of igneous activity form the basis of six igneous Geological Conservation Review blocks, and these are linked to mountain building (e.g. South-west England Igneous), and the opening of oceans (e.g. Tertiary Igneous).

Structural blocks relate to the deformation processes during three major mountain building episodes (e.g. the Caledonian, Variscan and Alpine orogenies) and their variation across Britain. These blocks include geological features such as folds and faults, and other effects resulting from compressional and tensional forces acting within the crust of the Earth.

Four blocks relate to Precambrian rocks in Scotland: the Lewisian, Torridonian, Moine and Dalradian Blocks. Three of these, the Lewisian, Moine and Dalradian Blocks, have been deformed and metamorphosed during mountain building.

Mineralogy blocks

These blocks address the minerals produced as the result of igneous, metamorphic or sedimentary processes.

Geological conservation review networks

Because the concept of the network is fundamental to the methods of the Geological Conservation Review, it is illustrated in some detail in this chapter by describing the actual networks adopted in Geological Conservation Review blocks. To aid understanding, the networks described here have been simplified to some degree, but they indicate the overall approach taken.

Marine Permian geological conservation review block

Durham Province network: the western edge of an inland sea

(Figure 47)a is an artist's impression of a satellite view of the 'British Isles' as they would have looked some 250 million years ago, towards the end of the Permian Period. The climate was arid, much like the Sahara and Arabian Gulf regions of the present day. Mountainous areas in what is now southern England and Wales had only recently (in geological terms) been uplifted during the Variscan Orogeny, and remnants of the earlier Caledonian mountains persisted in Scotland. Much of the present-day area of the North Sea was occupied by an inland sea (the Zechstein Sea, named after thick Permian rocks deposited in this sea to the east in northern Germany and Poland) which was probably linked to an open ocean to the north through a narrow sea-way between 'Norway' and 'Greenland' (Figure 47)b. Sediments accumulated on the western margin of this sea, and those in an even smaller inland sea to the west of what is now the Pennines are referred to as the Marine Permian. One volume of the Geological Conservation Review is devoted to them (Smith, 1995).

When the level of the Zechstein Sea was relatively high, reefs and shallow-water sand banks fringed its margins (Figure 48)a. The reefs and banks formed limestone deposits (calcium carbonate), but were altered to dolomite (calcium-magnesium carbonate) soon after deposition. Their outcrop forms a north–south strip across County Durham, Yorkshire and Nottinghamshire, with small outcrops in Cumbria. When the sea level dropped, it seems that the link with the open ocean to the north was severed or reduced, so that evaporation caused the salinity of the Zechstein Sea to increase. This resulted in the deposition of evaporite minerals such as halite (sodium chloride — common salt) and anhydrite (calcium sulphate), seaward of the earlier limestone deposits (Figure 48)b.

Five episodes of sea-level highs and lows have been recognised in deposits laid down beneath the Zechstein Sea. These are referred to as Zechstein Cycles, and can be traced from north-east England and under the North Sea into Germany and Poland. Only evidence for the first two and the beginning of the third is exposed at the surface in the County Durham area. (Figure 48)c is a west to east cross-section of the area showing the rock units deposited during cycles 1 and 2 and the base of 3. Note that the rock formations composed of evaporite minerals are only known from boreholes to the east of strata exposed at the surface, and are represented at the surface only by the thin residues left after the soluble minerals were dissolved away. The Ford Formation within the first Zechstein Cycle (Figure 48)d is particularly important, as it contains evidence for the development of a reef that fringed the Zechstein Sea. This was not a coral reef. The most abundant fossils found within it are bryozoans (small colonial organisms that build calcareous skeletons) that acted as sediment baffles which trapped the muddy limestone sediments to build the reef edifice. The sediments probably became rapidly hardened (lithified) due to the action of cyanobacteria and the formation of inorganic carbonate minerals which cemented them, and so would have been able to withstand wave action. However, storm waves would have broken up the reef edge, so that a slope of reef debris built up in front of the reef. An idealised section across the reef and the location of the reef crest are shown in (Figure 48)d.

(Figure 48)c and (Figure 48)d encapsulate the theoretical framework for which sites were selected that illustrate the key geological features of the marine Permian of County Durham. Ideally, sites illustrating all the environmental settings shown in Figures 48a and 48b should be selected for the network. However, it is impossible to find sites showing sediments composed of the evaporite minerals anhydrite and halite at the surface because these minerals are dissolved away. However, sites showing the residues left behind have been selected. The development of a reef of a type very different from those known today adds an extra palaeontological dimension to the theoretical framework, requiring the selection of sites showing successive stages in the growth of the structure. The stratigraphical locations of the sites selected for the Marine Permian Durham Province network are shown in (Figure 48)c and (Figure 48)d. Their geographical locations are shown in (Figure 49).

Yorkshire Province network

The Yorkshire Province lies to the south of the Cleveland High. The rocks exposed here are the shallow-water shelf deposits grading eastwards (over a distance of about 30 kilometres) into finer-grained carbonates. The Yorkshire Province differs from the Durham Province in that the barrier carbonate rocks which were formed near the western edge of the Zechstein Sea during the second main depositional cycle lie too far east to outcrop onshore. The sites selected for the network link together the characteristic types of sedimentation seen in this lagoonal backreef environment of the late Permian in Yorkshire. The outcrops in this province are characterised by lagoonal limestone rocks (which were later converted to dolomite), together with bryozoan–algal patch-reefs. In the north of the Province, examples of features not seen in the Durham Province occur, such as marine sabkha deposits (a sabkha is a wide area of coastal f lats bordering a lagoon where evaporite minerals are formed) and a rare exposure of marine evaporites.

Igneous rocks of South-west England GCR Block

There have been many periods of igneous activity during Britain's geological history, including several during the Precambrian, and four main episodes since then, during the Ordovician, Devonian, Carboniferous/Permian and Tertiary periods. Such activity is commonly linked to large-scale movements of the Earth's crust and the resultant plate tectonic activity, as described in Chapter 3.

In those cases where the igneous processes are associated with large-scale crustal movements culminating in the growth of a mountain chain, at least three phases of activity can commonly be identified (Figure 50). The earliest events are associated with volcanic activity and shallow intrusions of molten rock in and around basins where sediments accumulate (Figure 50)a. The resulting igneous rocks then become affected by mountain-building processes, and are deformed and occasionally metamorphosed. The second phase of activity involves the generation of large volumes of granitic magma deep in the crust, which then rise through the core of the mountain chain to be intruded at higher levels in the crust (Figure 50)b. The final phase of igneous activity occurs after the main tectonic events are over, and generally involves the development of volcanic activity and minor igneous intrusions (Figure 50)c.

The igneous rocks of south-west England (Figure 51) illustrate the range of complex relationships which occurred during the three phases described above of the Variscan Orogeny. This Orogeny occurred about 300 million years ago when south-west England was on the northern margin of the mountain-building area (see (Figure 17)). The core of the mountain chain lay to the south in the Massif Central of France. The sediments which accumulated in south-west England in Devonian and Carboniferous times include volcanic rocks, which now provide the basis of the pre-orogenic volcanic Geological Conservation Review network. Although these rocks were intensely folded and deformed during the orogeny, the amount of metamorphism they underwent was low and the rocks have retained much of their original structure. After the rocks had been folded they were intruded by a large granitic magma, the characteristics of which are used to create the Cornubian granite batholith Geological Conservation Review network. The granite cut through earlier igneous rocks and fold structures, and so was intruded after the tectonic event. It can be distinguished from other granites which occur to the south, in Brittany and the Massif Central, whose formation was intimately associated with the mountain-building process. This is an example of where a potential network exists, but it is absent from Britain. Following the orogeny, there was a period of minor volcanic activity represented by the post-orogenic volcanic network. South-west England also includes the metamorphosed igneous rock complexes of the Lizard and Start peninsulas. These areas are not directly linked geologically to the networks outlined previously, but as they form a geographical unit are included as the Lizard and Start complexes network.

Pre-orogenic volcanic network

This network comprises those igneous and volcanic rocks which accumulated within the Devonian and Carboniferous sediments prior to the Variscan Orogeny ((Figure 51); compare with (Figure 50)a). The network includes the following geological situations:

• different forms of igneous intrusive and extrusive process, such as sills and pillow lavas, and the development of areas characterised by different rock types
• variations within individual igneous bodies, including gradations from intrusive to extrusive, and internal zonation in igneous bodies
• different relationships between the igneous rock and adjacent sediments
• effects of subsequent deformation and metamorphism
• representative examples of different ages of igneous activity.

Cornubian granite batholith network

This network illustrates the variations within the Cornubian batholith which outcrops in a series of distinct masses such as Dartmoor, Bodmin Moor and Land's End (Figure 51). However, these masses are connected deep within the crust into a large east–west-trending granite batholith. The network encompasses the following geological situations:

• variations in the mineral composition and texture of different parts of the individual granite masses reflecting differences in the original magma composition, age, crystallisation history, mode of intrusion and late-stage changes
• relationships between the granite and the country
• rocks, including normal contacts, roof pendants (country rock which roofs the batholith) and xenoliths (fragments of country rock contained within the batholith)
• the nature of late-stage processes associated with hydrothermal activity, such as the development of china clay, greisen (granitic rock affected by the action of chemically rich vapours) and mineral veins (the metallogenesis associated with these granites will be covered in a separate Geological Conservation Review volume) and
• the occurrence of minor intrusions, such as granite cupolas (dome-like protuberances from the main body of the batholith), and aplites (a sugary-textured igneous rock) and pegmatites (a coarsely crystalline igneous rock with crystals three or more centimetres long).

The sites selected for this network are tabulated on page 60.

The network is further illustrated by (Figure 52) which places each of the sites on a generic granite batholith diagram. The figure demonstrates how relatively few sites have been identified to characterise one of the major granitic bodies in Britain for the Geological Conservation Review.

Post-orogenic volcanic network

This network illustrates the different types of lava which erupted into the desert environment that existed after the close of the Variscan Orogeny, in late Carboniferous and Permian times, when the granite masses formed high ground that was undergoing active erosion.

The network illustrates the following geological situations:

• the considerable variations in mineralogy and rock type between different localities, some rock types being relatively unusual
• the nature of the relationships between the igneous rock and the adjacent sediments.

Lizard and Start complexes network

These complexes represent the remnant of an ancient sea floor which has been significantly metamorphosed and deformed to the point that interpretation is difficult and can be controversial. The network illustrates the complicated geological history to which these rocks have been subjected, encompassing the following geological situations:

• variations in ocean-crust rocks such as basalt, gabbro and peridotite
• the presence of gneiss and schist associated
• with multiple deformation phases
• the range of occurrence of igneous bodies such as pillow lavas and dykes
• complex contacts between rock types of different age and origin
• the influence of metamorphism.

Palaeozoic palaeobotany Geological Conservation Review block

At the beginning of the Palaeozoic Era, single-cell plants had existed for over 3000 million years but had probably been restricted to aqueous, mainly marine, environments. Plants had evolved in the seas and were adapted to an environment which supported their tissues, bathed them in nutrients, contained dissolved gases for respiration and photosynthesis, and allowed, in those plants which employed sexual reproduction, the male gametes to swim to the ova. For such plants the land was a hostile environment. 'Soils' were sparse and poor in nutrients. Photosynthesis and respiration had to take place in the air where the environment was dry. The air provided no support for above-ground vegetation and, away from damp ground, the male gametes were unable to swim to the ova. But, by the end of the Palaeozoic Era, plants had developed features that would enable them to overcome all these problems.

To succeed on land, plants required (1) water-resistant surface tissues (cuticle) and spores, (2) strengthening mechanisms to support the weight of their aerial tissues, (3) a means of conveying water and nutrients from the soil to the aerial tissues, as well as oxygen and the products of photosynthesis back to the roots, and (4) the means of fertilising ova in dry conditions.

By the end of the Silurian Period, the first vascular plants had developed which combined moisture-resistant surface tissues and spores with the development of strengthening and water-conducting structures within the stems. These developments continued in the Devonian Period with the appearance of secondary wood and an increasing sophistication of the other structures. By the late Devonian, the first seed plants (gymnosperms) appeared, together with tree-like roots. In the Carboniferous, pteridophyte forests (tree ferns and giant club-mosses up to 40 metres high) dominated the equatorial delta areas, and the main classes of seed plants evolved. In 'Europe' and 'North America', the drier conditions of the Permian saw the pteridophyte forests in decline and the proliferation of the seed plants.

These changes took place during the Palaeozoic Era over a period of 200 million years. The Geological Conservation Review treats the topic by selecting sites on the basis of five networks: Silurian, Devonian, Lower Carboniferous, Upper Carboniferous and Permian. Evolutionary networks are an example of networks where there may be gaps in the site record. Plants existed, and may have been fossilised, which show all the stages in the evolutionary development, but some of these have yet to be found. An example of a network in the Palaeozoic Palaeobotany Block is the Silurian network.

Silurian network

This network represents the different types of fossil plant from the Silurian Period (ranging from 445 to 395 million years ago). Britain has the most complete known record of Silurian land plants in the world. Most of the sites are in Wales and the Welsh Borders. The climate in which these plants grew was tropical and the conditions humid, warm and equable — highly suited to the colonisation of the land by plants.

Marine algae (seaweeds) existed throughout the Palaeozoic Era and vascular plants are believed to have evolved from the green algae group, perhaps from an ancestor shared with the stoneworts, a group characteristic of freshwater habitats. A spectacular early non-vascular terrestrial plant was Prototaxites which had thick 'trunks' that lay prostrate on the land surface and were constructed of a mass of tubes which may have functioned like vascular tissue. Another land plant, Nematothallus, was probably an encrusting plant, where the tissue was not differentiated into leaves, stems or roots. However, the most significant of the Silurian plants were small with simple stems branching into two, known as rhyniophytoids, including Cooksonia and Steganotheca. These are generally considered to have included the earliest vascular plants, and examples of slender axes (albeit without the characteristic spore-bearing bodies) with true vascular tissue are reported from the Ludlow Series. Gas and water exchange was possible through surface cells called stomata, which opened or closed to control this exchange.

The network can be illustrated diagrammatically on the basis of the presence of adaptive structures for life on land. (Figure 53) demonstrates this concept, showing where the selected sites fit into the network.

During the succeeding periods of the Palaeozoic, the evolution of features adapted to life on land continued and all the main classes of plants, except the flowering plants appeared. For most of this period the climate continued to be tropical and, in the diverse swamp forest communities, plants competed for space and light, encouraging the development of tall stems and frond-like foliage. Towards the end of the Palaeozoic era, drier conditions favoured adaptations less dependent on moist conditions. The features of the four remaining networks are summarised below.

Devonian network

By the end of the Devonian all the major groups of vascular plants were present except the flowering plants. Important characteristics of the Devonian network are:

• the evolution of plant structures, including laminate foliage, seeds, secondary wood and tree-like forms up to 20 metres high
• the development of major plant groups,
• including clubmosses, horsetails, fern-like plants and seed plants
• colonisation of upland areas away from wet basins
• ecologically important plant assemblages reflecting various environmental conditions.

Lower Carboniferous network

The Lower Carboniferous saw few really significant structural developments in plants; it was a time of consolidation of the developments that had taken place in the Devonian Period. However, seed plants diversified and a group called pteridosperms, which had fern-like fronds, produced seeds and often grew to tree size. Important characteristics of the Lower Carboniferous network were:

• the diversification of early ferns and fern-like plants, clubmosses, horsetails and seed plants
• the development of the earliest extensive forest habitats
• ecologically important plant assemblages reflecting various environmental conditions.

Upper Carboniferous network

While evolutionary advances took place in the tropical uplands (e.g. the appearance of conifers) and at higher latitudes, the plant fossils of the tropical lowland habitats are of interest primarily because they reflect the high point of forest vegetation dominated by clubmosses up to 40 metres high and, towards the end of the period, by tree-ferns, pteridosperms and cordaites, a conifer-like group. The productivity of these forests is reflected in the fact that this was the main period of deposition or organic material that became coal. Important characteristics of the Upper Carboniferous network are:

• the diversification of earlier forms, including ferns and seed plants
• the development of adaptations assisting growth in drier areas, including seed plants
• diversity of the plant communities of the lowland swamp forests
• ecologically important plant assemblages reflecting various environmental conditions.

Permian network

The increasingly arid conditions during the Permian Period in 'Britain' caused the extinction of the Carboniferous swamp-forest vegetation dominated by clubmosses and horsetails, and their replacement by ferns and seed plants, particularly conifers. There are few good Permian sites in Britain that contain plant fossils. The network's important characteristic is to demonstrate the changing nature of forest communities facing conditions of increasing aridity.

Quaternary Geological Conservation Review Networks: English lowland valley rivers

The geological history of the Quaternary Period in the major valleys of southern England is the same as their geomorphological history. The evidence consists of old river gravels preserved at different elevations in the valleys as river terraces. They increase in age with increasing height above the present floodplains. As such, they form a 'staircase', each step consisting of a gravel terrace, the top of which approximately represents the remains of an ancient floodplain surface. They have been preserved as a series of steps because the rivers have cut vertically downwards in response to climatic change and uplift of the land.

(Table 2) Each row represents a period of time, where stages with odd numbers correspond to interglacials, and even numbers to ice ages.

This unique relationship between geological deposit and landform is an invaluable opportunity to explore and understand the geological and geomorphological development of lowland England and the way in which it responded to climatic changes. Its importance is enhanced further because the later stages of terrace development coincided with the earliest human occupation of the British Isles. Hand axes fashioned by prehistoric people are often found within the old river gravels. Thus the geological history, landform (geomorphological) history, and prehistory and archaeology are related by the contents of the Quaternary sediments and the landforms they comprise.

Geological Conservation Review networks in the Quaternary of the Thames Block

• The Upper Thames Basin
• The Middle Thames
• The Lower Thames (worked example given)
• Essex.

The remains of these river gravels which were deposited throughout southern England, for example, in the Thames, Avon and Severn valleys, are the basis for several Geological Conservation Review networks. The older the gravel deposits, the less likely they are to have been preserved, and many site 'gaps' exist in the lowland river valley Quaternary networks because the evidence has been destroyed, by either natural processes or gravel extraction.

The Thames Valley is an example of a river system that has evolved over nearly two million years. Three principal phases occurred in its development.

1. When the 'Thames' flowed between what is now Reading, Watford, St Albans and Hertford into East Anglia and then the North Sea. Early in this phase its headwaters may have extended into Wales, because igneous rocks from North Wales are contained in its river gravels. These may have been introduced into its headwaters by early Welsh upland glaciers.
2. When a large ice sheet during the Anglian glaciation, some 450,000 years ago, blocked the formerly north-east flowing course of the 'Thames' and it adopted its present path through what is now central London to the Thames Estuary.
3. The development of the Thames since the Anglian glaciation, when extensive river gravels were deposited along its present-day valley: for example, the Taplow Terrace which forms the widespread 'flat' areas around Slough and Heathrow Airport.

These three phases are represented by more than one Geological Conservation Review network, each of which exemplifies stages in the evolution of the Thames Valley. (Figure 54) shows a diagrammatic section of the Lower Thames Valley. Twelve distinct stages are represented here, each one corresponds to a major event in the history of the valley that is correlated with a global event as revealed in the oxygen isotope stratigraphy of deep-ocean sediments (Table 2). Oxygen isotope stages 1, 5e, 7, 9 and 11 correspond to interglacials, when global ice-volume was similar to the present. Stages 2, 3, 4, 5, 6, 8, 10 and 12 correspond to ice ages. The ages of the mid-points of the interglacials are shown. These are based on amino-acid dating, which is also the basis for correlating the Lower Thames deposits with those elsewhere in Britain.

The first column in (Table 2) shows the named geological deposits and down-cutting events of the river. All of the river gravels were deposited in a periglacial environment, when ice sheets occupied northern Britain. Their braided streams had very wide floodplains.

(Figure 54) shows the main geomorphological terrace features of the Lower Thames. Its complicated history may be explored further in (Table 2). This complex evolution occurred during five major ice ages and five interglacials (including the present one).

The degree of natural preservation of the Thames deposits varies considerably. The older ones are the most dissected, while those deposited after the Anglian glaciation are relatively better preserved. All of them, however, have been subject to destruction or modification through natural erosion, as well as by quarrying activities for sand and gravel aggregates. They are still an important and valuable economic resource.

The coastal geomorphology of Scotland Geological Conservation Review block

Much of the coastline of Scotland consists of relatively old rock types of variable resistance to erosion, ranging from the ancient, durable Lewisian rocks of the north-west to the younger, less resistant Devonian Old Red Sandstone sediments and Carboniferous lavas of the east. Younger Tertiary lavas, prone to landsliding where underlain by weaker sediments, form the coastline of parts of the Inner Hebrides. The variations in hardness and rock structure (jointing and bedding) have a fundamental control on the overall coastline form and on the shape of the landforms. In addition, rock structures, such as major faults, have played a part in coastline form, most notably in the pattern of sea lochs in western Scotland.

Apart from rock type and structure, factors such as climate, wind, tide and wave action, the effects of glaciation and sea-level change, vegetative 'protection' and the availability of sediment supply all play a role in shaping the coast, the stability of the present coastal environment and the effects of present-day geomorphological processes.

Broadly, the interrelationships between these factors make it possible to classify types of coastline into beach complexes, rock-coast features and saltmarshes. The beach complexes can be categorised further into distinctive types for the Highlands and Islands (beach-machair system, beach-dune-machair system, beach-bar system, prograding coastal foreland) and lowland Scotland (beach-dune system, prograding coastal foreland and shingle structures). Similarly, the rock coast and saltmarsh coastlines can be subdivided further. These categories form the basis of Geological Conservation Review networks.

Beach complexes of Scotland

Beach complexes are widely distributed throughout Scotland. Typically they comprise beaches, sand dunes, machair (dune pasture with lime-rich soils), links or some combination of these. In the north and west, and particularly in the Outer Hebrides and Shetland, they are associated with high-energy, exposed environments; in the south and east, lower levels of energy and exposure prevail. Modern active shingle structures are relatively rare, although there are extensive sites of raised shingle ridges in many areas. In a British context, the beach complexes of Scotland are nationally important for:

• the machairs of the Highlands and Islands which are exceptionally rare in western Europe (some examples occur in north-west Ireland)
• features associated with high-energy,
• exposed environments
• some of the largest British blown-sand features
• some of the most extensive areas of sand coast progradation
• features associated with glaciated coasts.

Beaches of the Highlands and Islands networks

Sandy beaches comprise less than 5% of the total length of the coastline of the Highlands and Islands. Most of the beaches occur on the islands on the open, highly exposed Atlantic coasts, and to a lesser extent in north and east Sutherland. This distribution is related to four principal controlling factors: exposure, relief of the coastal zone, rock type and glacial history. Many beaches in the region are associated on the landward side with blown-sand deposits in the form of sand dunes or machair, forming what are called beach complexes. Glaciation has been an important factor in beach-complex development: first, in providing a source of sediments, and second, in influencing changes in sea level over time and space, which have left a strong imprint.

The beach-complex network illustrated here is the beach-dune-machair system of the Highlands and Islands.

Beach-dune-machair system

The beach complexes associated with machair development are among the most distinctive soft coast systems of Britain. The Geological Conservation Review network which addresses this system includes all the associated major features (landforms and processes) and encompasses their history of development and their variations according to the principal controlling factors. This network is shown diagrammatically in (Figure 55).

Twelve sites were selected for this network to illustrate the features of the beach-dune-machair system, with preference being given to those sites that show outstanding examples of landforms or landform assemblages in the beach-dune machair system and sites that offer opportunities to elucidate the links between landforms and the processes which form them as well as the history of their development. Emphasis was placed on selecting sites with assemblages of features. The importance of each site is summarised in the table on page 67.

Beach and dune coasts of lowland Scotland networks

Sandy beaches form a relatively large part of the coastline of lowland Scotland, notably on the east coast. This reflects the less indented nature of the coastline in comparison with that of the Highlands and Islands, and the reworking of large volumes of sediment which had been deposited on the continental shelf during deglaciation. Larger beaches are typically long and curved with a single dune line, and extensive links are developed on older low raised beaches. The beach and dune coasts of lowland Scotland can be grouped into three models:

• beach-dune systems with parallel dune ridges, illustrating the development of different dune types and blown-sand deposits
• prograding foreland systems, showing the scale, complexity and diversity of the landforms and processes associated with coastal progradation, such as stabilised dune sands and dynamic spit and bar environments
• shingle structures showing the assemblage of active and raised shingle ridges and shingle spits.

Rock coast geomorphology networks

Rock coast features are characteristic of extensive parts of western Scotland from the Clyde Estuary to Shetland and including the north coast of the mainland. They include large glaciated sea lochs, low ice-scoured coasts, low cliffs with shore platforms of variable width and extent, and high cliffs, varying according to patterns of rock type, glacial erosion and isostatic uplift. To the east and south-west, rock coasts are more intermittent or buried in drift or sand overlying shore platforms. The rock coast features of Scotland are important for

• features associated with igneous and metamorphic rocks
• features associated with high-energy exposed environments
• some exceptionally high-cliff coast features
• features associated with glaciated coasts.

Saltmarsh geomorphology networks

Saltmarshes are limited in distribution in Scotland and are confined to a number of estuary, barrier-beach and loch-head settings. They are important for:

• features associated with crustal uplift
• relatively young features that allow comparisons to be made with more developed systems in England
• features associated with loch-head environments.

The Geological Conservation Review Site Series

The examples described above show something of how the network approach deals with the range of situations and circumstances that British geology and geomorphology encompass, and the flexibility of the approach in developing a comprehensive series of representative sites that illustrate the characteristics of each block.

The suite of Geological Conservation Review sites is derived from the selection of the international, exceptional and representative sites discussed above. Of course, the great majority of the internationally important and exceptional sites were also selected as representative sites because they were the best example for portraying an essential characteristic within a relevant block.

The Geological Conservation Review site series is summarised in (Figure 56). The figure lists the Geological Conservation Review blocks and indicates how these blocks will be grouped for publishing within the Geological Conservation Review volume series. The numbers and distribution of sites across Great Britain within these volumes are also shown.

The selection of individual sites to represent the essential characteristics of the blocks was a major element of the work of the Geological Conservation Review. The methods employed to do this are described in the next chapter.