Cretaceous

Up onto the Downs

The road north out of London follows an old Roman road known as Watling Street. Climbing up the side of the Thames Valley gives a view across the whole of London. In Smith's day, much of the city would have looked like a smouldering ruin covered by a pall of smoke, as thousands of household fires wafted strands of wood smoke into the atmosphere. Several decades later at the height of the Industrial Revolution, the city would have barely been visible at all as coal-burning fires belched thick acrid fumes into the atmosphere, blackening all the buildings around with the sooty pollution. Only in recent decades has this industrial grime eventually been cleaned off the more prestigious of London's buildings, but there are still plenty that retain the disfiguring layer of soot.

In places the grey, silvery glint of the river is just discernible as it wends its weary way down through the port of London to the sea. Smith would have seen a busy port with a forest of wooden masts belonging to sailing vessels that plied their trade from all over the globe to one of the world's busiest centres of commerce. Only in the twentieth century with the growth of much bigger steam-powered vessels did the shallow waters of the Thames make the port more or less redundant. Downstream at Greenwich there was the famous Observatory, from which the Greenwich meridian of longitude was established in 1884, and the fine Regency buildings of the Royal Naval College, with direct access to the Thames estuary and eventually the sea.

The landscapes surrounding the estuary were low-lying muddy marshes made famous or rather notorious by Charles Dickens' novel Great Expectations. The criminal Magwitch, one of the main characters, escaped from one of the prison hulks moored offshore these bleak and damp mistbound seascapes.

Right across the Thames Valley to the south another line of low hills can be seen rising in the distance, forming the southern rim of the geological basin in which London lies. The lines of hills that rim both sides of the basin are established on the same kind of rock strata - the distinctive white and soft limestones known in England as Chalk Downs. Historically, chalk has been all too familiar to countless generations of schoolchildren as the material by which the wisdom and information of the ages has been transmitted from teacher via the blackboard to pupil, although it has been largely superceded by clay-based chalk- and whiteboards.

'Downs' is a typically topsy-turvy English name for hills. It is very ancient and is derived from the old English and Celtic word 'dun', meaning 'hill'. It was famously used by Shakespeare in The Tempest (iv, i, 81), 'my boskie acres and my unshrubd downe'. This accurately describes the characteristic features of downland for 1000 years or more since the original tree cover of beech and elm was cut down and land turned over to grassland pasture. As Smith noted, 'great flocks of sheep are kept upon these Downs and supplied with early spring feed from water meadows in the vallies'.

The thin soils can easily be broken through to reveal white patches of limestone that underlie the landscapes, but rarely are there any natural rock outcrops because the limestone is so easily weathered and eroded. Seldom is chalk hard enough to be of any use as a building stone, although there are places in Norfolk where it is used because some layers are harder than usual. Since most of the trees have been cut down the hard chalk is the only vaguely durable material available locally, apart from flint and bricks manufactured from clay.

Shallow pits and quarries have been excavated over the centuries as the chalk has been dug out for various purposes. Smith remarks that 'much chalk ... is used on the land either in a crude state or burned to lime____ and for the recovery of its hard siliceous nodules called flint'. Lime was extensively used as a cheap form of cement and the ubiquitous 'whitewash' form of paint. But chalk has also played an important role in the history and development of fine art in Europe (see box on Painting with chalk).

Painting with chalk

Chalk has played an important role in the painting of fine art over the ages. A particularly important historical painting, known as the Thornham Parva retable, is the oldest oil painting in Britain and one of the oldest in Europe. Its three panels originally formed a backdrop to an altar and inevitably it was a religious work painted on wooden panels. The magnificent depiction of Christ flanked by his disciples has recently been restored. In order to carry out the work in a way that used original materials, the technical details of the painting's construction have been scientifically analysed. Dendrochronological study of the wood's growth rings revealed that the panels were cut from tough, slow-growing Baltic oak, which was felled in 1336. The age has been further constrained by Dr Sophie Stos-Gale of Oxford University. She analysed the lead pigments in the paint and showed that the original lead mineral used to make the pigment has a characteristic isotopic 'fingerprint'. This identifies it with lead ore from a specific lead mine in Derbyshire. Historical records show that the mine had closed by the end of the fourteenth century.

A microscopic sample of the white 'ground' or base paint layer from the Thornham Parva has been examined to check its provenance. With a scanning electron microscope it became evident that the paint was clearly made from micron-sized fossils of unicellular organisms called coccoliths, which comprise the bulk of chalk rock. By identifying the fossils, Professor Katharina von Salis of the Swiss Federal Institute of Technology in Zurich has shown that the artists obtained their chalk from a particular layer of rock, which she identified as coccolith biozone CC 17 of the Late Santonian, which was deposited some 84 million years ago, near the end of the Cretaceous Period of Earth Time. A soft limestone that is common throughout large regions of northwest Europe, it was commonly ground up for use as a ground for paintings over hundreds of years and has only been replaced by other base paints since the late nineteenth century.

Since prehistoric times peoples have searched for the best type of rock to make stone tools and weapons. Flint, along with a volcanic glass called obsidian, has been recognised as one of the best materials. The mineral is so tough that its strange, irregular-shaped cobbles or nodules persist when the surrounding chalk enclosing them has been worn away. Consequently, there are places on the land surface, such as beaches and riverbeds, where flints accumulate in great numbers.

The mineral has many of the properties of glass. It is both hard and brittle and can be broken into flakes and shards with exceedingly sharp edges. Blades and points can be fashioned from it, along with blunter axes or hammer stones. In places it is so abundant that prehistoric peoples used the sites as centres of manufacture. Smith thought it worth noting that in Norfolk Chalk landscapes, 'The plougher land between Swaffam and Castle Acre [is] strewn with Flints.' But flints that have been exposed at the surface for a long time become weathered and discoloured and are weaker than ones freshly dug out of the chalk.

Quarries and even mines, such as Grimes Graves in Norfolk, were excavated as long ago as Neolithic times to obtain the best flints and the industry persisted right through into the eighteenth century. As its use for stone tools became redundant, flint was still knapped (meaning shaped) to make small, brick-shaped blocks for building and for use in old-fashioned flint-lock percussion weapons. Again, Smith notes that 'Gunflints formerly manufactured at Salisbury', in Wiltshire, from flint nodules occur in the Chalk there. There was a particular demand for flints in the region because it was and still is a military barracks town. The striking of a hammer flint against an iron metal sheet generated sparks that ignited the gunpowder, which exploded and propelled the ball-shaped bullet down the weapon's barrel. The actual generation of flint within the chalk rock is a complicated process.

The chalk landscape consists of rolling downs dissected by narrow tree-filled 'hangars' or valleys. The valley bottoms are often filled with weathered clay and flints and, as Smith observed, 'copious springs of clear water flow from the foot of the chalk hills'. Rarely do the streams seem nearly big enough to have carved their valleys and in many places the chalk valleys are completely dry, without any sign of running water even in the wettest winters. Rainwater falling on the soft and porous chalk limestone generally soaks straight into the ground and percolates down until it reaches the local water table or an impervious layer of clay. Here the strata are soaked in water and wherever the water table reaches the ground surface springs occur. In chalk landscapes they are often found along the bottom of the valley sides, so that historically farms and even hamlets have been sited where there are naturally occurring and persistent springs that provide beautifully clean hard, lime-rich water.

The formation of the dry valleys found higher on the downs was seen as a puzzle for many centuries. Again, Smith remarked that 'the scarcity of water and other reasons for the paucity of habitations on the hills accounts for the numerous sites of population in the vallies'. Since the mid-nineteenth-century discovery that even the landscapes of the British Isles had suffered extensive glaciation in the not too distant geological past, it has also been realised that beyond the southernmost extent of the ice sheets, the ground would have been permanently frozen just as much of northern Alaska and Siberia is today.

If the chalk was frozen, it would have become impermeable during the brief periods of summer thaw. Any rainwater would not have soaked into the ground but would have run across the surface and eroded normal river valleys. When the glacial climates modulated and the permafrost melted, the chalk would have resumed its normal porosity and rainwater sank more or less straight down into the water table, leaving the distinctive upland dry valleys and grasslands that formed ideal pastures for domesticated sheep and cattle.

William Smith described the Chalk as forming 'Extensive sheep pastures on the Downs' and 'Water Meadows in the Vallies' and noted that 'fossil oyster shells and echini' (sea urchins) were to be found within it. Indeed, he went on to collect, describe and illustrate the common clams, ammonites and other seashells found in this ancient seabed deposit. What Smith was not able to observe are the fossils that make up the bulk of the chalk rock, because they are far too small for the microscopes available in Smith's day to resolve.

We now know that chalk is largely composed of the remarkable microscopic fossil skeletal remains of myriads of tiny unicellular marine algae known as coccoliths. A single sugar-cube-sized piece of chalk contains hundreds of thousands of these calcareous skeletons, which accumulated on the seabed when

the parent organism died. Today similar organisms live in subtropical waters and are known to have bursts of reproduction, known as 'algal blooms' or 'whitings', when trillions of organisms are suddenly generated and discolour the sea locally. So many are produced that they soon use up whatever nutrients are available and then die off, with their minute calcareous skeletons sinking onto the seabed, where they accumulate as a white calcareous mud, which eventually is compacted and hardened off to form chalk.

The shellfish of the chalk seas were largely creatures that are still familiar today, such as clams, snails, sea urchins, crabs, lobsters and shrimps, along with quite modern-looking bony fish and cartilagenous sharks, some of which were significantly bigger than today's Great Whites {Carcharadon). Two less familiar groups of fossil shellfish are the extinct belemnites and ammonites of the chalk seas. Both were abundant, squid-like, swimming cephalopods, some of which lived in vast shoals.

All that normally remains of belemnites are peculiar bullet-shaped fossils made of a limy mineral (calcite), which originally projected from the posterior end of the animal and acted as ballast for buoyancy control over the depth at which they swam, gave the back end a pointed shape and stiffened the body. It did not protect them from being consumed by the dozen by the voracious marine reptile predators of these ancient seas. The discovery of hundreds of small belemnites clustered on ancient seabed surfaces has been interpreted as ichthyosaur vomit. The idea is that the predators swallowed the belemnite animals whole and, rather than pass the indigestible mineral 'bullet' through their digestive system, were able to 'cough' them up, in a similar way to many other predators such as owls and dogs.

Of the coiled ammonites, altogether there were many different kinds that lived at different water depths and ranged in size from a centimetre or two up to metre-sized giants with heavy shells that could not move from the seabed. By the middle of the nineteenth century German scientists who studied ammonites in detail realised that they were very useful for helping make fine distinctions between successive layers of strata and their relative dating (see p. 144).

Much more spectacular were the giant creatures of the Chalk seas, some of which reached astonishing dimensions and were the Cretaceous equivalent of

Napoleonic forces capture fossil

In 1780, giant fossil jaws over a metre long were recovered from underground chalk workings in Maastricht. Armed with an impressive array of teeth, the jaws were soon the subject of discussion among scholars all over Europe and were described in 1786 as the jaws of a fossil toother whale. Napoleonic forces besieging Maastricht in 1795 were on the lookout for any interesting and valuable war booty. The jaws had been hidden by their owner Canon Godin, but the French General Pichegru offered a reward of 600 bottles of wine for the recovery of the jaws. It did not take long before they appeared and were shipped off to Paris, where French scholars were only too delighted to have such a famous fossil to themselves. In 1799 Faujas de Saint-Fond described the jaws as belong to a giant crocodile, but his younger colleague Georges Cuvier later changed the diagnosis to that of a giant extinct and predatory monitor lizard called Mosasaurus that lived in the Chalk seas. The fossil jaws are still in the Natural History Museum in Paris and the Dutch have to make do with a plaster cast.

Today we know that the mosasaurs included voracious predators that were among the top predators of the Cretaceous seas. Analysis of their teeth shows that they had some of the most advanced cutting edges of any teeth seen in marine reptiles. The teeth evolved along similar lines as those seen in predatory dinosaurs and modern sharks. Each tooth had numerous cutting or breaking facets that were capable of both crushing bone and slicing through flesh. The mosasaurs had no serious competitors except other mosasaurs. The discovery of mosasaur jaws with repaired fractures shows that they probably did fight among themselves and may even have been cannibalistic. Some grew to over 17 m in length and their fossil remains have been found from the Netherlands to North America and New Zealand; in other words they had a truly global distribution. They ruled the oceans of the world for some 27 million years in the latter part of Cretaceous times, but apparently died out as suddenly as they had appeared.

Barthelemy Faujas de Saint-Fond, 1742-1819, a successful lawyer but, influenced by Buffon, took up geology and became professor of geology in the National Museum of Natural History in Paris and authored the Natural History of the Mount St. Pierre, Maastricht in 1799.

today's mammalian whales. The discovery of one particular chalk seamonster was part of an important development in the overall understanding of the history of life. This particular fossil monster was found in the chalk limestones of Maastricht just across the North Sea near the Dutch/Belgian/German border.

The recovery of the 'Grand Bête de Maastricht' in 1780 (see box Napoleonic forces capture fossil) from underground chalk workings and subsequent discussion of its exact nature and biological affinities became a pivotal point in an ongoing argument about whether any of God's creatures could have become extinct. After all, why would a benevolent God allow any of his creations to die out?

Dutch anatomist and naturalist Pieter Camper saw that while the metre-long fossil jaws might superficially look like those of a giant crocodile, they also had features that were, to his mind, like those of a toothed whale. When French scholars got their hands on the fossil they decided it was in fact a crocodile, only to have their argument countered by Adriaan Camper (Pieter's son), who had inherited his father's collections. Adriaan made a more detailed study of the fossil's anatomy and concluded that it was neither whale nor crocodile but a new kind of reptile previously unknown to science - a giant marine lizard.

Adriaan Camper wrote to the most famous anatomist of the day, Georges Cuvier in Paris, telling him of his reasons for his new diagnosis, but Cuvier dismissed the suggestion and sided with the previous French diagnosis as a crocodile. But young Camper must have sown some doubts in Cuvier's mind. For once in his life Cuvier changed his mind and in 1808 named the beast Mosasaurus, meaning 'lizard from the River Meuse', and suggested that it was a monitor lizard of kind intermediate between the iguanas and varanids; not that he gave Adriaan Camper much credit for the diagnosis. But most importantly, Cuvier acknowledged that the 'beast' from the Chalk strata was also extinct. Once the great Cuvier had spoken, the notion of extinction became more acceptable.

Greensands, Gault Clays and some big reptile bones

The chalk downs of Norfolk, Cambridgeshire and Wiltshire look northwest out across extensive low-lying and wet vales and floodplains (the Bedfordshire

Levels, the Vales of Aylesbury, the White Horse and so on). The underlying strata are not immediately obvious, except in a few places along the foot of the chalk escarpment and where low-lying sandstone ridges occur.

Mostly the strata are too soft to form significant topographic prominences. But historically the clays have been widely exploited for making bricks and phosphate minerals in some of the clays have been quarried for fertiliser. In a few places iron has been smelted from the so-called greensands, especially in mediaeval times. As Smith notes, 'the ancient ironworks were chiefly on opposite sides of the "Forest Ridge" where Marl occurs with Ironstone and thin beds of Limestone'. The iron was smelted using charcoal made from the local trees in a tradition that stretched back to Iron Age times some two thousand five hundred years ago.

Smith also ruefully observed that 'at Bexhill the extremity of the Forest Ridge against the sea was the late very expensive and useless search for Coal'. As we shall see (p. 163), it was the search for coal that directly and indirectly provided Smith with much of his income and helped fuel the early development of the Industrial Revolution in England.

Most of the Wealden deposits were laid down in shallow seas, with the greensands accumulating in very shallow offshore and coastal waters. Although typically the sandstones are various shades of brown, orange and yellow, they are called 'greensand' because when freshly excavated, they can be seen to contain grains of a distinctive green clay mineral called glauconite, with a complex iron-magnesium silicate composition. And glauconite is so named because it only forms in marine environments ('glaucus' meaning sea).

Excavation of the clays and sands has occasionally revealed spectacular skeletons of extinct marine reptiles as well as a host of other important fossils over the ages. Historically, the most important and interesting were the bones that were turned up in strata of similar age that occur to the south of the Thames Valley in a region of the south of England known as the Weald.

Between London, nestling in the Thames Valley Basin and the south coast of England, there is a large geological 'window' that opens on these pre-Chalk Age strata in the Weald. The Chalk Downs that form the southern rim of the Thames Valley Basin also form a huge elongate semi-oval escarpment running westwards from the Kent coast in the east, through Surrey to Wiltshire and then swinging sharply south and around in a semicircle before returning eastwards through Hampshire and Sussex to the sea again. Along the southern 'limb' the Chalk escarpment faces north and thus encloses the Wealden region within a rim of Chalk hills.

The eastern half of this semi-oval structure is closed offshore and in the Artois region of France on the other side of the Straits of Dover. William Smith was well aware of its geometrical form, which geologists call an anticline or upfold. The strata have been arched upwards in a huge dome by pressures coming from far away in the south. Originally the Chalk would have arched right over the Weald, but its soft limestones were easily and rapidly eroded and worn away to reveal the older greensands and clays underneath. Topographically the original dome now forms a wide vale, although the central part is elevated with significant outcrops of sandstones, some of which have been extensively quarried in the past for building stone.

The reason for taking the southward diversion here is that historically these sandstones of the central Weald have been of enormous importance in the development of our understanding of the life of Cretaceous times and the whole era, as we shall see.

Giant saurians make their first appearance

In the first decade of the nineteenth century, a young apprentice physician by the name of Gideon Mantell had the luck to be introduced to Dr James Parkinson, the famous medic and radical reformer who advocated universal suffrage in order to avert bloody social revolution. In 1811, the final volume of his five-volume work Organic Remains of a Former World was published, in which he described the then known distribution of fossils within their original strata. Not surprisingly at this date, he still concluded that the Mosaic account 'is confirmed in every respect, except as to the age of the world, and the distance of time between the completion of different parts of creation' and that overall the creation of the Earth 'must have been the work of a vast length of time'. It was Parkinson

Gideon Algernon Mantell, 1790-1852, English physician who was the son of a shoemaker and subsequently devoted his energies to geology. He struggled to be recognised but described Iguanodon, one of the first dinosaurs to be discovered, and was elected a Fellow of the Royal Society in 1825 and published Wonders of Geology in 1836.

who encouraged the young and ambitious Mantell to pursue his interests in geology.

Following a London medical apprenticeship, Mantell qualified as a physician and returned to Lewes in Sussex where he had grown up. By 1819 he had become a well-established, successful medical practitioner and was married with a child, but he was also actively pursuing his geological interests in the region. From a network of local contacts, gifts of stones and fossils from the rock strata of the Weald flooded into the Mantell house to be piled everywhere, so that it became as much a museum as a home. News of his collection brought well-connected visitors who in turn spread news among the scientific élite about Mantell, the talented young country doctor.

In June 1820, Mantell was sent some fossils that had been unearthed in the Wealden quarries around the nearby town of Cuckfield. They included bits of

James Parkinson, 1755-1824, English physician who first described the 'shaking palsy' (in 1817) now known as 'Parkinson's Disease', also a radical reformer, naturalist and founder member of the Geological Society of London.

backbone, a fragment of a very large leg bone and some teeth. Greatly excited by the fossils, Mantell visited the quarries and had the luck to find more pieces of large bones and the metre-long fragment of a fossil tree trunk covered with diamond-shaped scars like tropical palms. His initial thoughts were that the fragmentary remains belonged to one of the seamonsters newly discovered in the older strata of Dorset and North Yorkshire (see below), but he soon changed his mind and was more inclined to think that they had something to do with crocodiles. At the same time, Mantell recognised that the fossil tree trunk indicated that land could not have been far away when the deposits were originally laid down.

In the early 1820s, another piece of fossil tooth of unusual appearance was found, whether by Mantell or his wife is unclear - the popular story has it that it was his wife. With a distinct wear surface on the crown, which gave it a blunt end, the tooth was clearly not that of a crocodile but more like that of a mammalian plant eater. The problem was that at the time Mantell knew of no mammal fossils being found in such ancient rock strata. Nor was he aware of any reptile that masticated plant food to produce such a wear surface.

The strangely shaped fossil teeth found in Sussex in 1820 puzzled Gideon Mantell and even Georges Cuvier because they knew of no living creature with similar teeth.
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