2) WHAT IS A BIOLOGICAL REEF?

Biological reefs are best known today as coral reefs, but even that is not a correct description. The attraction of coral reefs are the beautiful and complex display of coral types and colours, but there are many other components of great importance, perhaps greater importance, that construct a reef. For example, sponges are a major component in some reefs, and of huge significance are the microbial organisms which form much of the structure; however, microbial organisms are just not attractive to look at on modern reefs, just blobs and amorphous masses, and are easily overlooked. Nevertheless, some fossil reefs are constructed principally by microbial material, and form stunningly beautiful stone. A famous example is the Marbre Rouge from reefs in southern Belgium, a decorative stone often seen polished as facing stones on walls of houses that most of us can't afford. >>

One example is in Blenheim Palace in Oxfordshire, England; another less famous example is the standing section of the gentlemen's toilet at the Geological Society in Piccadilly, London, where it is possible (for gentlemen) to examine the structures at close quarters, if only for a short time without attracting attention. The pictures below show some features of modern biological reefs. Note that the term REEF is used by mariners to describe any rocky structure that could damage a ship, and in that sense reefs are made of any kind of solid rock, from granite, gneiss, sandstone, limestone etc., through to actual coral reefs. All the pictures in this section are modern coral reefs.

Figure 1: Photographs of modern coral reefs as seen on the sea surface, from. The pictures illustrate the danger to shipping represented by coral reefs the Great Barrier Reef off the coast of Queensland, Australia, and the Bahamas.

Figure 2: Photographs of modern coral reefs on the Great Barrier Reef underwater. The pictures show the dense coral buildups, and emphasise the topography of the structure.

Figure 3: Upper two photos: platy and branching corals from the Great Barrier Reef. Lower two photos: Close ups of a different component of coral reefs - lime-secreting algae (called coralline algae) which coat areas of loose sediment and bind it to form a solid structure. The surface of this encrusted mass is colonised by a variety of small attached shelly organisms, as minor encrusts that take advantage of the available surface. In this way, you can see how the reef becomes a complex and diverse assemblage of organisms. The same kind of complexity can be seen in fossil reefs.

Figure 4: a close-up view of a coralline-algal-encrusted sediment mass, with lots of encrusting tubes and red-coloured Homotrema, a kind of lower life form that inhabits the darker parts of reefs.

Figure 5: Views of two common corals on modern reefs: Fungia and Diploria (brain coral). You can see top, bottom and side views of these samples showing the flat form of Fungia and the domed form of Diploria.

Figure 6: Views of branching corals from modern reefs, showing the wide variety of form of branches. Because these are obviously physically weaker than the types shown in Figure 6, similar-shaped corals in ancient reefs are used to interpret lower energies of the areas of fossil reefs where they are found. It is this comparison between the growth forms of ancient and modern fossils that is used to develop environmental reconstructions of ancient coral reefs, as we will see later on this page.

Reefs are not always made from corals. Below is an example of a small reef encrusting bedrock in northern Sicily. It is made of snails !!!!! Curly snail tubes in huge masses, that also define the sea level very precisely.

Figure 7: A modern reef on the north coast of Sicily, Italy. The reef is made of coiled snails (vermetid gastropods), shown in a cut vertical section on the right, and in close-up in the bottom photo. Such snail reefs occur in several places on the north coast of Sicily, but curiously are not found at all on the east coast of Sicily, for reasons which are not clear.

3) HOW CAN YOU RECOGNISE A FOSSIL REEF?

You begin by realising that a reef is big (usually 10s of metres to kilometres in diameter, and up to several tens of metres thick), and that you are unlikely to be able to see all of it. Reef observing is also governed by the fact that reefs are seen in rocky outcrops, road cuts or quarries, so what you can see is to a large extent governed by the laws of natural erosion, and by the laws of road builders and quarry managers! Almost all fossil reefs are made of limestone, so are an important resource for building and chemical (e.g. cement) industries. Because the limestone of the reef core is normally purer calcium carbonate than the surrounding bedded rocks, the reef core is more useful; this has the unfortunate effect that the reef is often largely quarried away, and often you can see only the margins, in old quarries. Coastal sites where reefs form cliffs are also very good, but not too common, because they depend on the coincidence of modern coastlines with the occurrence of the reefs. A famous example is the island of Gotland in the Baltic Sea, where the northwest coast is high cliffs, and reefs intersect the coastline. In fact 'high cliff' in Swedish, gives a name to one of the best exposed reef outcrops in the world, the Högklint reefs, which can be viewed for tens of km along the coastline. There is one key feature that distinguishes the study of fossil reefs from modern reefs; modern reefs are normally viewed from above, or perhaps the side if enough topography is visible. If you have ever snorkelled or SCUBA-dived on modern reefs, you are amazed by corals, but what you mostly view is obtained by looking down from near the water surface. In contrast fossil reefs are mostly studied by looking at their vertical cross sections in coastlines or quarries. Therefore fossil reef histories can be studied, as you can see from the base to the top of the reef. For a modern reef you can usually only see the current growing surface. Exceptions are where researchers have cored through a reef to study its sequence, but because of the damage this causes to our fragile reef system, such work is limited. >>

Very rarely, fossil reef systems are exposed in three dimensions if the natural erosion has carved out a hill, for example, which contains the reef (reefs are often harder rock than the surrounding rocks, so it is possible, but not common, to get hills made of single reefs. An example of this is in Sichuan Province in central China, where it is possible to walk through and around much of the reef and appreciate its three dimensional structure. Another example is in Castleton, in Derbyshire, England, where stromatolite-rich reefs form on the edge of a rock platform, and can be studied in three dimensions. The best places to visit reefs in Britain are around Torbay, where old limestone quarries on the coastline have cut through reefs, leaving rather beautiful cut faces in which corals and calcified sponges can be seen, photos later. Unfortunately, however, these reefs have been subject to some serious tectonic disturbance during the Variscan folding event at the end of the Carboniferous Period; this means the reefs are chopped up by faults, so you can see only parts of the reefs. Nevertheless, they are pretty good to study. Some of this rock is also used in stone walls, where details of reefs can be seen. Below are some photograph compilations showing what fossil reefs, and their associated sedimentary rocks, look like.

Figure 8: These two pictures are of the northwest coast of the island of Gotland, Sweden, and show reefs in the cliffs, cut by modern erosion. They are in the Högklint formation of the Silurian period of geological time (about 400 million years ago) on the northwest coast of Gotland, Sweden, show the size and shape of these fossil reefs. This place is incredible: the development of erosion of this part of the Earth surface has led to the fortuitous exposure today of these reefs along the cliff line; Gotland is one of the best places in the world to study reefs, and there are more photos on this page.

Figure 9: MAIN PHOTO: cross section of a Silurian reef in Wenlock Edge, Shropshire, UK, showing the lens-shaped unbedded reef limestone surrounded by bedded limestones. LEFT INSET: a polished sample of vertically-cut reef frame showing the reef-building fossils in white, the frame being infilled by pale green fine-grained sediment. In this case the reef frame is not coral, it is calcified sponge (called stromatoporoid). RIGHT INSET: a close-up from another reef showing the reef margin, where the unbedded reef material is interlayered with the surrounding bedded sediment. Notice the sharp contact between reef and bedded limestone; this suggests that the reef had a coherence of structure so that you can put your finger on the boundary between reef and non-reef rock. These reefs were constructed not just from corals and sponges, but had also a significant component of microbial sediment, which bound together the structure into a solid mass. Microbial sediment is very common in some modern reefs where it can make up nearly half of the reef volume.

Figure 10: These pictures show details of bedded limestones around the reefs. These ancient ones (Silurian Period) are made of fragments of crinoids, which are related to the sea urchins and starfish, but lived a fixed life on the sea bed, attached by a stem made of circular hollow discs (called ossicles) which you can see here. The lower two pictures show some reef fossils that grew in these non-reef beds.

Figure 11: These pictures show a coral reef in China, from the lower Silurian Period. The small picture shows the reef has been exhumed by modern erosion from the surrounding bedded limestones, because the reef is made of harder limestone. The main picture from a different reef shows a reef frame made of corals. Post script: these pictures were taken in 2002. A return trip in 2011 revealed that although the exhumed reef is still there, the site where the main picture was taken is now the foundation rock for a high-level motorway pylon, and this picture can no longer be taken because the reef is buried in concrete ! That's progress.

Figure 12: These pictures show some details of what biological reefs can be made of. There is a mixture here. The TWO LEFT HAND photos are from pieces of Silurian reef rock from the Högklint reefs on Gotland, Sweden, given to me by Nigel Watts. TOP RIGHT: molluscs from a reef frame of Holocene age (last 10,000 years) in the Gulf of Corinth, Greece. BOTTOM RIGHT: a mini-frame made from a calcimicrobe (see Mass Extinctions section for description of calcimicrobes) from the Devonian reefs in western Australia. This structure is called Renalcis.

Figure 13: Reef fossils are often banded (you can see nice examples in the site at Long Quarry Point, in South Devon section of this website). The significance of banding has been long-debated, some people think it represents yearly growth in the fossils when they were alive (like tree rings), but really it is difficult to know exactly what they signify. TOP LEFT: coral with banding; note the bands match the irregular edges of the fossil, and may reflect changes in growth rate of the coral. BOTTOM LEFT: A microscope section (ca. 1cm across) showing a stromatoporoid (Petridiostroma convictum) with laminations that are bunched in some places suggesting changes in growth rate, which may or may not be annual. The vertical tubes are a coral that has grown inside the stromatoporoid in symbiosis; see other pictures later. RIGHT: a beautifully banded stromatoporoid (identified in microscope sections as Plectostroma scaniense), but here the banding almost certainly relates to chemical alteration of the calcium carbonate structure of the stromatoporoid (the white areas are silica). Whether or not these reflect a growth rate variation is under investigation.

Figure 14: Reef fossils can show a lot of complexity in detail. Although reefs are big structures, you must remember that each organism has to grow and be part of the reef. Therefore the growth history of each fossil has some meaning in the development of the reef. Although you can view a reef from the large scale, it is also valuable to understand the small scale features. These pictures show some nice examples of small-scale changes in reef fossils. TOP LEFT: a complex of stromatoporoids and corals that have grown one on the other, and form in themselves a mini-reef mass. Width of this photo is about 15 cm TOP RIGHT: a coral (the vertical lines) grew on the dead surface of a stromatoporoid, forming the basis of reef growth. BOTTOM: this complex of stromatoporoids and corals show how storms have rolled the sample around, and each time it develops new growth in a different direction (we assume they always grew upwards). Diagram on the RIGHT shows the sequence of change, with numbers.

Figure 15: Reef fossils (and also non-reef fossils) may show situations where two or more organisms grew together, called a symbiotic relationship. They produce stunningly beautiful images. In these pictures taken down a microscope, stromatoporoids have corals inside them. TOP LEFT: vertical section of a stromatoporoid with two types of coral inside it (large coral branches, and the smaller tubes which are here cut in oblique section in most tubes). BOTTOM RIGHT: a horizontal section of the same specimen, showing the two coral types in horizontal cut view. TOP RIGHT: a stromatoporoid with a single coral tube inside it.

In the photos, the large coral tubes are about 5 mm across. The exact nature of this symbiosis is not clear; the corals do not seem to have adversely affected the stromatoporoid, so they are not being parasitic. Whether or not there was a mutualistic relationship, so each partner benefits from the association (like some marriages in humans) is open to debate. Nevertheless, these would look nice as a pattern on your bathroom floor, but might make you feel a bit dizzy after a shower.

Figure 16: Reefs are classically considered to be rounded structures, with lens-shaped outline when seen in vertical section. However, some reefs are flat layered structures, and the one shown here is part of a reef complex that covers several tens of sq km in eastern Gotland, Sweden. It is packed full of stromatoporoid sponges, and is one of the densest accumulations of reef fossils anywhere in the world. It is absolutely amazing.

Figure 17: These pictures from Western Australia show the bedding arrangements of a famous reef complex. The reef (arrowed) is unbedded, but it has steeply dipping fore-reef beds to the left, where reef debris and other sediment rolls down the slope in front (seaward) of the reef. Flat-lying back-reef beds to the right formed behind the reef. It is one of the best views of a cross section through a reef complex in the world, on a bend of then Windjana Gorge inland from Derby in northern Western Australia. The inset photo shows a stromatoporoid from the backreef.

4) FOSSIL REEFS IN BRITAIN

Where can you see reefs in Britain? The map in Figure 18 shows the locations of some well-known fossil reef formations in Britain; some of these are difficult to recognise or to get access to, so in the next section of this page is an illustration of reefs in one of the easiest accessible areas, in south Devon, where you can see reef rocks in many places along the coast.

Figure 18: Locations of some fossil reefs in Britain; the one highlighted in blue is detailed in the next section.

5) FOSSIL REEFS OF SOUTH DEVON

Below are some descriptions and photos of reefs that you can visit in the Torbay area of southwest England. These are in publically-accessible locations, where it is easy to appreciate the composition of the reefs, and the beauty of the fossils they contain. The Torbay area is a nationally important area for fossil reef studies and is a Geopark; there are information boards that tell you some things about the geology, so do look at them. Note, however, that sampling of these fossils is strictly forbidden; they are a resource for everybody and you must take care of the environment. In any case the fossils are in solid rock and impossible to extract; so you would end up with a piece of broken fossil that is not very attractive. Leave them where they are and just look. You also need to take care of yourself when studying them. Below is some health and safety advice; it is not intended to cover all possibilities, but will help you understand the dangers. >>

People with mobility problems will have difficulties accessing these sites, but there are places where you can see details of the reef fossils. For example, Meadfoot Beach in Torquay has a car park, the wall of which is made from blocks of limestone containing reef fossils. This is readily accessible for wheelchair users, for example; photos on this website will give you a good idea of the outcrops in case you can't access them. Note that the authors of this website are not responsible for any fossil reef visit you make! Below is information on health and safety in the field; this is not exhaustive, and you must take careful notice of instructions on display boards and by official personnel. Of course, use your common sense at all times in the field.

Figure 19: Display board of the English Riviera Geopark, showing the kind of information you can easily see, about the ancient reefs in this area.

HEALTH AND SAFETY IN THE FIELD

Observing rocks in the field is very rewarding, but requires some basic common sense and awareness of potential dangers, as well as attention to the environment and to other people. Here are some key points to note:

1. Some sites are in nature reserves so it is important not to try to collect any material; just take your camera. If you have a close-up lens, you can get some very nice photos of fossil details.

2. Some places are on or near roads, and you must take great care of traffic dangers.

3. Britain can be a wet and cold place, and the weather can change. Geologists do something strange that most people do not do: they stand around outside in the cold and rain !!!!! You may need to wear warm and waterproof clothing, carry food, water, first aid kit; you will need good shoes that do not slip on rock and wet grass.

4. If in a tidal place, watch the tide movements, and get the tide tables.

5. Do not ever do anything that could place you in personal danger: for example, walking on narrow ledges, walking under cliffs, or near the edge of the top of a cliff. Do not drink alcohol if you are going to study fossils in the field. Also for the benefit and health of other people, it is considerate not to smoke; it is not generally realised that smoke can be carried considerable distances by air currents.

6. When walking along paths, stay on the paths. Remember that paths take you somewhere that other people have been to and therefore are safer than wandering off to potentially dangerous places.

7. If you go on your own, tell somebody else where you are and when you will be back; carry a mobile phone but be aware you may not be always in range of a signal.

8. Carry a torch, whistle, compass.

9. If you see somebody else doing something dangerous, tell them (I have occasionally told families who sit under cliffs on beaches that they may not all be going home today; but generally they just look at me and do not reply; they are on holiday!).

10. Finally, remember that going out to study fossils is a really pleasurable thing to do. It beats lying in bed. In fact, lying in bed is (statistically) the most dangerous thing you can do; more people die in bed than anywhere else. So go and look at some fossil reefs, safely.

Figure 20: Photo of the coast at Torquay showing overviews of the two sites described in the next sections on this page, Hope's Nose and Long Quarry Point.

5.1. Hope's Nose, Torquay

This lovely promontory on the north side of Torbay is the easternmost point of the bay, with great views across the cliffs and sea. The reef site is near sea level and is accessible from Marine Drive, but because it is a private road, it is best to park at Meadfoot Beach car park, and walk (I should say ”slog“) up the hill to the style that lets you onto Hope's Nose, then walk down to the left as in the photo. This is an old coast quarry; ships tied up at the cliff and limestone loaded onto the ships. The quarry is through a partly-formed reef and visible in the limestone are nice examples of corals and calcified sponges called stromatoporoids. Stromatoporoids were the main reef builders during this time, and dominate the rocks. Between the corals and stromatoporoids, you can see fragments of fossil shelly organisms. The most abundant are crinoids, and here the stems are preserved, as short lengths made of small piles of circular objects about 0.5 cm across, with holes in them. If you took a packet of mint sweets with holes (Polo mints) and stack them up on a table, then this resembles a crinoid stem in broad appearance. >>

Also you can see some branching corals and stromatoporoids, in one part of the rock platform. Be very careful at the edge of the rock platform by the sea; the rock here can be slippery if raining, and it is a vertical drop into the sea. This is NOT a place to go swimming. By the way, at the end of the point of Hope's Nose is one of the most spectacularly exposed thrust faults anywhere in UK; you can walk on the fault and appreciate how the earth moves. Back at Meadfoot beach, the car park wall, which is also the ramp down to the sea, is made of blocks of reef limestone, which are easily studied in detail, and where superb examples of stromatoporoids and corals are seen, very photogenic.

Figure 21: View of Hope's Nose at Torquay, showing an overview of the site (top right). The other three photos show some flat-shaped stromatoporoids with coarse-grained debris of crinoids. These are vertical sections through the reef structure; the flat stromatoporoids indicated that the rate of sediment accumulation must have been low, otherwise they would have been smothered by sediment. Actually that may be how they eventually died.

Figure 22: A close-up view of a stromatoporoid and crinoidal debris. Notice the banding in the stromatoporoid, which might be annual. If it was annual, can you estimate how old this specimen was when it died?

Figure 23: Close-up views showing some variations of the reef fossils, in coarse-grained crinoidal sediment. You can see corals and stromatoporoids here.

Figure 24: A nice example of coral banding in this reef.

Figure 25: Views of branching corals and stromatoporoids, present as broken fragments in the reef. They may have grown in quieter water in the back reef, and then transported here during a storm.

5.2. Long Quarry Point, Torquay

This equally lovely promontory is accessible from the top of Wall's Hill, but its access is down a steep cliff path. Picnic-ers and fishermen/women go down here, so it is readily accessible, but this is not for people who have fear of heights, or with mobility issues. Therefore you must exercise great care to access this site. Also, down on the rock platform there is a steep cliff drop to the sea, so not a place for swimming, and definitely a place to be careful. >>

However, you can see the fossil reef easily and safely if you take care, and a warm dry day is best. This site has what is probably the largest fossil reef in Britain, and if you are able to get down to it, the experience is excellent.

Figure 26: Long Quarry Point. Here the reef is exposed around the edges of the grass, and shows some spectacular cut sections of stromatoporoids.

Figure 27: An enlargement of the top right photo in the last picture.

Figure 28: Close-up view of a stromatoporoid that was encrusted by a coral, and then another stromatoporoid grew on top of the coral.

Figure 29: Views of the car park wall at Meadfoot Beach, Torquay, showing the blocks of reef limestone used to make the wall. This is an easily accessible site, and is advantageous for anybody with mobility problems who cannot access the field sites at Hope's Nose and Long Quarry Point.

6) FOSSIL REEFS IN CAPTIVITY

Fossil reef fabrics can be found inside some buildings. As mentioned in the introduction to this section, the famous red-brown coloured Marbre Rouge in some stately homes such as Blenheim Palace is from Devonian reefs in Belgium. Actually these are not considered as strict reefs by many geologists because they are composed mostly of sediment, with rather little of the reef-building corals and stromatoporoids. In London, the most readily accessible reef limestone as decorative stone is in the entrance hall of the Natural History Museum, Exhibition Road entrance, but this rock (Ashburton Limestone) can be found in other places.

Figure 30 shows a photo of a doorway to a building in Ludgate Hill, near to the top of the hill close to St. Paul's Cathedral. You can see the large rounded stromatoporoids and branching corals & stromatoporoids, in this beautiful polished stone. In this case the location of the environment is in the back reef of the reef complex, where the water is quieter and large rounded stromatoporoids, and branching corals and stromatoporoids, could survive without damage. The sediment here is also very fine-grained (the dark material between the corals and stromatoporoids). Fine-grained sediment indicates a low energy of deposition, since fine-grained particles will be held in the water if the energy is high, and are not deposited.

Figure 30: Polished slab of Devonian-age Ashburton Limestone from south Devon, in the doorway of a building near the Cathedral of St Paul, London. You can see bulbous stromatoporoids and branching stromatoporoids and corals. The diagram on the left is a simple drawing of the cross section through a Devonian reef to show the position of this material in the reef system; it is in the backreef area, because the branching corals and stromatoporoids are not very strong and the dark sediment is fine-grained.

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