THE ORIGINS OF THE GREENSAND RIDGE
Ancient Bedfordshire is relatively young in geological terms. The oldest rocks, deep beneath the surface, are crumpled deposits of mudstones and siltstones from only about 500 Ma (million years ago). According to recent geological maps, which estimate the position of Europe at this time, this area of what was to become Bedfordshire lay about 28o south of the Equator, near where Namibia is today. Massive movements of the earth’s crust broke up ‘Pangaea’, the original continent, and sent the smaller European continental plate northwards at the start of a long journey to its present position in the mid-northerly latitudes.
Heavy tropical and equatorial rains fed into rivers that discharged millions of tons of mud and silt into the shallow waters of the continental shelf. There, the coarser sediments accumulated at shallow depths close to shore and the finer material was deposited further out to sea. Sedimentation rates have been estimated at 1cm. every thousand years. Over a million years or so, many tens of metres would have been dumped. Overlying sediments would have compressed the original strata by two thirds, so ten metres would be squashed into three. Throughout these few hundred million years, this area was still beneath the sea and moving northwards at a rate calculated to have been up to 1o latitude every seven million years!
The main period when Bedfordshire’s surface rocks were deposited, started when much of Southern England was deep beneath the sea, lying just above the tropics at about 27o North, where Morocco is today. This was the beginning of the Jurassic period about 208 Ma. With the progressive rise in sea levels and migration north across the arid belt, it led to a reduction of eroded material from the continent. This allowed the build up of calcareous (limestone) deposits on the sea bed. Calcium carbonate was precipitated in the deep oceans to produce billions and billions of microscopic crystals, which, over many millions of years, rained down onto the seabed. They build up a limestone deposit over the ancient mud and silt.
Towards the end of the Jurassic period, continuing earth movements resulted in the sea bed sediments being uplifted to expose two great islands. They were joined together by two land bridges, the Charnwood and Nuneaton axes. Running parallel to each other, they stretched south-eastwards from the Midlands as a string of islands. The northerly island has been termed by geologists as the ‘Pennine High’ and the southerly as the ‘London-Brabant Platform’ or ‘East Anglian Massif.’ Bedfordshire lies on the latter’s north-western coast.
Analysis of bubbles of air trapped in amber shows that oxygen levels were as high as 35% at this time. Enormous dinosaurs dominated life on these ancient islands and their surrounding waters. To the south and west was an extension of the Tethys Sea, a huge lake covering southern England as far west as Dorset, and east across Belgium and northern France. Into it drained the rivers of Central and Eastern Europe that brought in huge deposits of silt and mud. To the north was a huge arm of what was to become the North Sea. It was on the floor of these expanses of water that the Lower Cretaceous strata were formed, strata which today can be seen as the Greensand Ridge with the ‘Potton Nodule Bed” or “coprolite” bed at its base.
How did the coprolites get there? Fine grained silicate particles, mudstones and maybe volcanic ash, washed out from the volcanic landmasses of Wales, Scotland and Scandinavia, were carried out to sea by the rivers. They rained down to settle over the limestone strata and built up to produce deep beds of what is known as Oxford Clay. The upper layer of the Jurassic clays is known as the Kimmeridge Clay, a bed that stretches in a belt from Dorset across this area to Norfolk. Abundant marine fossils have been found throughout these Jurassic clays with many classes of animals represented. Belemnites, nautiloids and ammonites - scavengers of the seabed - were in profusion. These resembled the squid and octopus of today but with a coiled shell. In the shallow, muddy seas around the two islands swam gigantic marine reptiles like the plesiosaurs, ichthyosaurs and pliosaurs, as well as sharks, which fed on the abundant marine life. There were numerous species of fish, ammonites, bivalves, gastropods, brachiopods and crustacea. The currents in the shallow waters washed the remains of these creatures, their shells, bones, teeth, claws and scales around on the sea bed. Many of their surface features were worn away and the remains covered by the continuous inflow of land sediments.
At the end of the Jurassic period, about 145 Ma, the region was about 35o north of the equator. This was close to where southern Spain is today. Tectonic movements folded the sediments upwards and this area was uplifted. Once above sea level, the soft upper clay beds, including the Kimmeridge Clays, were heavily eroded. Exposure to the elements - sun, wind, rain and river erosion - further lowered the surface until, in the area of eastern Bedfordshire, the Kimmeridge Clay was completely removed. This explains the nonconformity - the missing strata - in the geological succession. The Jurassic fossils were washed out by tides and rivers and built up as a fossil bed in the shallow, warm coastal waters. It was these that formed the lower part of the coprolite beds.
The waters provided the nutrients for a thriving swamp ecosystem that was dominated by the last dinosaurs of the Jurassic period. The geological papers on the finds unearthed by the 19th century coprolite diggings document megalosaurus, iguanadon, stegosaurus, craterosaurus, dinotosaurus, scelidosaurus and dakosaurus (a type of crocodile) as well as the gigantic marine reptiles. 20th century research has found two others in the greensand, the Criorhynchus (“battering ram snout”) a flying reptile, and the Hypsilophodon (“high crested tooth”), a plant-eating ornithopod..
By the earliest Cretaceous period (119 - 113 Ma), the London Platform was about 38o north of the Equator. Deposition of the Lower Greensand started. It originated from marine inundation and inland river flooding washing out coastal sands to build up deep beds above the coprolites that overlay the Kimmeridge Clays. Tectonic activity along the plate margin to the west produced slow sea floor spreading. The North Atlantic was created as the European plate moved ever further away from North America. Scotland broke off from Northern Ireland and Canada moved away westwards.
During the Albian stage, around 113 - 110 Ma, there was a break in sedimentation. The area remained above sea level and vegetation took over the exposed sea bed. Once again the dinosaurs and other life returned the graveyard of their ancestors. In time they were to become a second “upper” coprolite bed.
When the seas flooded the area again in the Late Albian (100 - 97.5 Ma) it was the result of extensive deep subsidence in the European continental plate. There was a complete change in the usual sedimentation allowing huge deposits of Gault clay. This formed during the period when the London Platform was flooded up to a depth of about 200 metres. Rivers draining the heavily weathered volcanic masses of Scandinavia to the northeast, Scotland and Wales to the north and west and Devon and Cornwall to the southwest, fed into this ancient sea. A blue ooze started to be deposited, composed of silicate minerals from the chemically weathered granites and basalts.
Underwater volcanic activity along the Mid-Atlantic Ridge, accompanied by sagging of the crust, caused sea levels to rise again. During the late Aptian or Cenomanian period (98.9 to 93.5 Ma.), Bedfordshire was on about the same latitude as Central Spain and was dominated by lakes and lagoons. As the African plate began to collide with Southern Europe the sediments in this area were pushed downwards and the rising waters eventually flooded the London Platform. The Mediterranean limestone sediments were uplifted to produce the Alps and Pyrenees..
It is thought that one or more tidal waves drowned the coastal areas of the London – Brabant Platform and wiped out East Anglia’s dinosaurs. The rising sea levels eventually flooded the Charnwood and Nuneaton land bridges. How fast the flooding was is unknown. Some scientists suggest it was rapid as the surface features of the fossils indicate a rapid burial. Some argue that the shift occurred during a period of intense asteroid activity between 96 and 94 Ma. Professor Trevor Palmer of Nottingham-Trent University reckons that if a 10-km wide asteroid hit the Earth at about 30 km a second it would vaporise instantly. A crater 180 km wide would be formed in seconds. It would be as “if it hit Milton Keynes then the crater would stretch from Nottingham in the north to London in the south and include Birmingham, Oxford and Cambridge. The crater would be lined with molten rock and an intense fireball would rise into the atmosphere, producing a violent, scorching wind.” Dr Emilio Spedicato of the Department of Mathematics and Statistics at the University of Bergamo in Italy reports that the atmospheric disturbance of such an impact “would be colossal and extend over hemispheric areas. For instance it can be estimated, if ten per cent of the initial energy goes into the blast wave, that at 2,000 km from the impact point the wind energy would be 2,4000 km per hour with a duration of 0.4 hours and the air temperature increase 480 degrees… At 10,000 km these numbers would be respectively 100 kms per hour, 14 hours and 30 degrees. (Palmer, T, ‘The Fall and Rise of Catastrophism’, Professorial lecture given at Nottingham-Trent University , 25th April 1995, p.11; Spedicato, E, Apollo Objects, Istituto Universitarion di Bergamo, 1990, p.17)
The location of impact zones, even in Siberia, would have affected this part of the European plate. Almost all the animal population would have been wiped out. Weak points in the oceanic crust allowed molten magma in the form of flood basalts to extrude upwards to produce underwater volcanoes. It is thought that these contributed to cause a rise in sea level. They also released volcanic carbon dioxide and other poisonous gases into the atmosphere to cause respiratory problems for the larger land creatures. The marine life that had to come up to the surface for air similarly suffocated and died.
Some of the nodules have flat bottoms that suggest they had been dropped onto the sand. Could they have been the droppings of these prehistoric creatures? A number have been found in the sand pit on Sandy Heath, washed out by the rains. The sandy coastal strip along the Bedfordshire Strait may well have been a dinosaur mating ground and nursery. Like crocodiles, some of the dinosaurs buried their eggs in the sand to protect their young. Like cats and dogs, some buried their droppings in the sand and could have become desiccated in the heat, leaving sun-dried “coprolites”. Those exposed to the atmosphere would have been consumed by insects or decomposed by bacteria. When the sea covered the coastal region again, new sediments compressed the buried ones. Without oxygen, they too would have become fossilised and, like the other remains, would have been washed out by repeated flooding and added to the fossil bed.
Even if their demise was not catastrophic, the advancing floodwaters changed the ecosystems of the coastal interior. Local dinosaurs and other land animals lost their habitat. They couldn't adapt quickly enough and died out. Even crocodiles couldn’t survive without land. The giant marine saurians, like the plesiosaurs, pliosaurs, ichthyosaurs, would have had a feast on the fish that came to scavenge - as would the sharks and other crocodiles. The pterosaurs and very early birds would have pecked and torn at their bloated bodies. Fish, crabs, ammonites and worms would also have scavenged and, over time, their carcasses would have sunk to the sea bed. Their bodies accumulated as debris in tidal embayments where their bones, teeth, scales and claws were rolled around on the eroded surface of marine clays. Many of the upper fossils would have had their surface features worn away to leave the generally amorphous lumps found today. Gradually they absorbed the phosphoric acid from overlying deposits of decaying organisms. Tides and underwater currents washed around their remains to form a massive underwater graveyard. The shallow, warm, phosphate-rich seawater soaked into the deposit and built up a layer of phosphate around them, rather like plaque build-up on your teeth.
Molluscs and other marine organisms lived and died amongst these remains and, once covered by further sandy sediments, they themselves become part of a huge phosphatised fossil bed. Plants like tempskya, tree ferns, have also been found in these beds and fragments of older rocks and minerals, among them vein quartz, chert, quartzite, lydian stone and fragments of Palaeozoic grit and slate (555 – 235 Ma). These must have been washed in from outside the area or dropped from the soils caught in the roots of floating trees.
Over many millions of years, marine sands accumulated over the clay and animal deposits, termed by geologists, the Lower Greensand formation. In the east of Bedfordshire they are known as the Potton Sands and further west, towards Buckinghamshire, as the Woburn Sands. In places these sands are up to 65 metres thick, composed of mainly coarse yellow or silver sands, often with large scale cross bedding, relating to the south east trend of the ocean currents. Above the Greensand in the region of Potton and Sandy lie about 10 metres of carstone, a bed of sands and iron rich sandstone.
Towards the end of the Gault deposition there was a local upward movement of the sea floor, which exposed the top zone of the Lower Greensand within range of marine and aerial erosion. Much of the newly formed fossil beds were removed and some of their denser constituents, including the fossils, were rolled around and redeposited in hollows on the sea bed. In some places they accumulated to a depth of several metres but the bed was generally uneven, averaging only about 30 cms. In places where the sea bed rose, the deposit was non-existent. Sedimentation was slow under such conditions and fossils of slightly different ages became mixed. A sand and pebble conglomerate was produced, made up of sand and chalky marl with phosphatised nodules at its base. This upper “coprolite” bed contains the fossils of pterodactyl, dinosaurs and crocodiles, marine plesiosaurs and ichthyosaurs, teeth of shark, crabs and lobsters, ammonites in abundance and brachiopods, echinoderms, sponges and corals. It has been argued that large numbers of infant oysters lying on the top of the fossil bed shows them to have been rapidly covered by new sediments.
This Upper Greensand, as it is termed, represents sandier deposits, formed in rather shallow water lying in a broad belt off the western coastline. It was caused by another rise in sea level, which flooded the region to the east. Further sandy sediments commenced south of the Bedfordshire Straits during the Upper Cretaceous period, 97.5 - 91 Ma. and when the seas deepened, the overlying layers of Gault Clay and Chalk sequences were deposited.
During the Cretaceous period it has been suggested that huge quantities of carbon were stripped out of the coal measures by erosion. What was not oxidised to produce large quantities of carbon dioxide became trapped in the organic matter on the sea bed. The chalky marl, a mixture of chalk and clay found above the Upper Greensand, was produced by gradual but long term sinking of the sea floor. This affected much of what is now Bedfordshire. With higher temperatures, desert conditions dominated the adjacent land mass, which led to less river discharge, otherwise the waters would have been contaminated with sands and muds. The deepening sea left this area so distant from land that no land-based sediments reached here, only the slow raining down of dead and decaying microscopic marine organisms. The vertical rate of accumulation during the Cretaceous period was about 80 metres every million years. These built up and were eventually compressed to produce the chalk layers of Southeast England - the Chilterns, Royston Downs and East Anglian Heights but further south the North and South Downs and even the chalk deposits in France and Belgium. In parts of Bedfordshire the lower chalk reaches a thickness of about 650 metres, the middle chalk about 950 metres and the upper chalk about 1,150 metres.
Deposition of the Lower Chalk began about 97 Ma and continued for about 14 Ma. The lower part has high concentrations of silts, sands and clay. Changing sea levels produced different deposits and over time and with further sinking a purer chalk was formed. This purer marine environment had marine sponges or sea cucumbers that became entrapped within the ooze. As they decomposed, the globules of silicate derived from their skeletons, migrated upwards through the sediments. Over time this gel that couldn’t escape to the surface, solidified and crystallised into amorphous nodules of flint.
These chalk deposits were gently folded and eroded before the Tertiary Period began 65 Ma. Widespread volcanic activity in NW Europe, and elsewhere in the world, increased the levels of carbon dioxide and other poisonous gases in the atmosphere. Analysis of trapped air in amber samples of this age show that oxygen levels had dropped to 11% at this time. This helps to explain the extinction of the larger dinosaurs. Their respiratory system wasn’t able to adapt as successfully as the smaller ones. With the meteorite smashing into the Yucatan peninsula in Mexico great clouds of pulverised rock and dust were thrust into the atmosphere. Sunlight was blocked out changing global weather patterns. No sun meant no photosynthesis, so there was no food. The high stress environment contributed to the demise of many land and sea organisms. The easily exhausted hot-blooded animals had to “dash and dine”. Some were probably too tired for sex! Research suggests that dinosaurs and great sea reptiles had been dwindling in numbers and size, adapting to the changes in atmosphere and climatic conditions before the impact.
Continued tectonic movements caused considerable uplift of these Tertiary sediments. The London Platform became a low basin. The Chilterns were folded upwards in the north and the Weald uplifted to the south. After the end of the Upper Chalk period there was a further period of uplift during which wave action and tidal currents eroded much of the newly exposed upper chalk deposits. The sun, wind and rivers eroded the higher inland chalk. It was around this time that the fossils of new species appeared in beds in Suffolk and Cambridgeshire. The mastodon, hippopotamus, horse, tapir, bear, hyena and ox have been found alongside whales and turtles. This fossil bone bed was reported to have been washed out of the London Basin and these mammal fossils were “discovered“ when the diggers cut through much more recent sediments to reach the far more ancient Greensand sediments for their phosphate deposits.
The folds of southern England are the outer ripples of the Alpine storm created when the continent of Africa smashed into Europe. All this area of Bedfordshire was uplifted with a slight 5o tilting to the southeast. The resultant dramatic increase in erosion removed much of the Upper Chalk. This exposed the underlying sandstones, which in turn were heavily eroded. In this area the Upper Greensand was completely removed and the Lower Greensand exposed to the atmosphere. Over time this left a northeast – southwest escarpment which rose, probably well over 100 metres above the flat Jurassic clays of the northern half of Bedfordshire. This is now the Greensand Ridge, about 51o N of the Equator.
About two million years ago Britain experienced the start of a dramatically cooler climate. At least sixteen ice episodes have been recorded but the greatest erosive event took place during the Anglian Glacial Period, about 400,000 years ago. Glacial ice from Scandinavia, Scotland, the Lake District and the mountains of Wales expanded to cover most of Southern Britain as far south as the Thames valley. Bedfordshire was under a few hundred metres of slowly moving ice. The Greensand, being made up of harder material, was not eroded as much as the softer clay and, when the ice sheet eventually retreated, the Greensand Ridge was left as a prominent feature on the landscape. Its highest point is 68 metres, just northeast of Sandy. The harder chalk of the Chilterns was also left as a remnant of a former upland. Being near the ice margin, this area was exposed to permafrost that shattered and weakened the surface deposits. The rising temperatures resulted in vast quantities of glacial meltwater removing and reworking much of these loosened surface sediments. The soft clays to the north and south of the Greensand ridge were stripped, leaving the fairly flat valleys of the Great Ouse and River Ivel. A sheet of glacial till was dumped in the wake of the melting ice. On the lower ground, boulder clay and extensive glacial sands and gravels were left, lake clays and silts were formed and, over the subsequent hundreds of thousands of years, later glacial action, soil creep and landslipping further eroded the area. River erosion and flooding continued their erosion and deposition to shape the landscape that is now Bedfordshire. Although humans have reworked the surface and dug small chalk, clay, sand, gravel and coprolite pits, human action has been nothing compared to the major agents of erosion and deposition.