Ancient
Bedfordshire is relatively young in geological terms. The oldest rocks, deep
beneath the surface, are crumpled deposits of mudstones and siltstones from
about 500 million years ago (Ma). At this time southern Britain was part of an
enormous continental land mass floating on semi-molten magma lying about 28o
south of the Equator, near where Namibia is now. Massive movements of the
earth’s crust sent the continental plate northwards to where it still floats
today in the mid-northerly latitudes.
Heavy
tropical rains fed into rivers that discharged 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. Throughout these few hundred million years this area of what was to
become Bedfordshire was still beneath the sea and moving northwards at an
approximate rate of 1o latitude every seven million years! By this
time it was about 18o North of the equator, where the Sahara is
today.
The main
period for the deposition of Bedfordshire’s surface rocks started at the
beginning of the Jurassic period about 208 Ma. This was when much of
Southern England was deep beneath the sea lying just above the tropics at about
27o North, where Morocco is today. With the progressive rise in sea
levels there had been a reduction of eroded material from the continent that
led to the build up of calcareous (chalk) deposits northwards towards Europe.
Carbonate precipitated in the deep oceans to produce microscopic crystals,
which, over the millennia, rained down onto the seabed.
Towards
the end of the Jurassic period continuing tectonic pressure resulted in a
gradual uplift of the sea bed sediments 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.’
Evidence
from amber samples shows that oxygen
levels were as high as 35% at this time. Life on these islands and their
surrounding waters was dominated by the enormous dinosaurs. To the south and
west was an extensive 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 which brought in huge deposits of silt and mud. To the north
was a huge arm of what 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, weathered from
some of the nearby volcanic landmasses, were carried out to sea by the rivers.
They rained down to settle over the limestone strata, accumulating as deep beds
of clay. The upper layer of the Jurassic clays is known as the Kimmeridge clay
that stretches in a belt from Dorset across this area to Norfolk. Abundant
marine fossils have been found throughout the Jurassic clays with many classes
of animals represented. The cephalopod - ammonites - scavengers of the seabed
were in profusion. These resembled the
squid and octopus of today. In the
shallow muddy seas around the two islands swam gigantic marine reptiles like
plesiosaurus, ichthyosaurus and pliosaurus, 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 produced a high-energy environment which
resulted in considerable sediment transport and deposition.
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. The
sediments were folded upwards and this area was lifted above sea level. The
soft upper clay beds, including the Kimmeridge clays, once exposed above sea
level were eroded by the waves. Exposure to the elements - sun, wind and rain -
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 fossils were washed out by
tides and rivers accumulated in the shallow, warm coastal waters. These waters
provided nutrients for a thriving swamp ecosystem that was dominated by the last
dinosaurs of the Jurassic period. These included the megalosaurus, iguanadon,
stegosaurus, craterosaurus, dinotosaurus, dakosaurus and the gigantic marine
reptiles of ichthyosaurus, plesiosaurus and pliosaurus.
By
the earliest Cretaceous period, (119 - 113 Ma) the island known as the London
Platform or the East Anglian Massif was about 38o north of the
Equator. Deposition of the Lower Greensand took place as far south as Potton
and Sandy, a sandy tract on its western coast. Tectonic activity along the plate
boundary to the west of Britain produced sea floor spreading. The plates
separated and moved away from each other. The North Atlantic was created as
Britain moved ever further away from North America. Scotland broke off from
Northern Ireland and Canada moved away westwards. Underwater volcanic activity
along the mid-Atlantic ridge caused sea levels to rise again. In about 116 Ma,
during the late Aptian or Cenomanian period, it eventually flooded the London
Platform. By this time this area was on about the same latitude as Central
Spain.
As
the continents continued to shift, sea floor spreading caused localised
volcanic activity. Weak points in the oceanic crust allowed molten magma to
extrude upwards to produce underwater volcanoes. Some reached the sea surface
to produce the volcanic island chains in the major oceans. Occupying a
considerable proportion of oceanic space they caused global sea levels to rise.
They also released Carbon Dioxide and other gases into the atmosphere to cause
respiratory problems for the bigger dinosaurs. The rising sea levels eventually
flooded the land bridge. How fast the flooding was is unknown. Some scientists
suggest it was rapid as the surface features of the fossils indicate a rapid
burial. As the floodwaters advanced the ecosystems of the coastal interior
changed. Local dinosaurs and other land animals lost their habitat, couldn't adapt quick enough and died out.
Even crocodiles couldn’t survive without land. Bodies would have been washed
around in tidal embayments. The giant marine saurians like the plesiosaurs,
ichthyosaurs and pliosaurs would have had a feast on the fish that came to
scavenge - as would the sharks and crocodiles. The reptile Pterodactyl 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 sank to the sea
bed. Their bones, teeth, claws and scales would have been washed around by
tidal and underwater currents to form an extensive underwater graveyard. Rolled around on the eroded surface of
marine clays many of them would have had their surface features worn away to
leave the generally amorphous nodules found today. The shallow, warm, phosphate-rich
seas caused these fossils to absorb the phosphate and build up a layer of
phosphate around them.
Molluscs
and other marine organisms lived and died amongst these remnants and, once
covered by further sandy sediments, they themselves become part of a huge
phosphatised fossil bed. Plants like tempskya 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. These must
have been washed in from outside the area or dropped from the roots of floating
trees.
Some
of the nodules have flat bottoms which 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
some would have been desiccated in the heat, leaving sun-dried “coprolites”.
Those exposed to oxygen in the atmosphere would have been consumed by insects
or decomposed by bacteria but the buried ones, when the sea covered the coastal
region again, were buried under a new layer of sediments. Without oxygen they
too would have become fossilised and, like the other bones, teeth, scales and
claws, over successive floods would have been washed out and added to the
fossil bed.
Over
the millennia marine sands accumulated over the clay and animal deposits. They
are termed 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
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 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 Scotland and Wales to the north and west and Devon and Cornwall to
the Southwest fed into this sea. A blue
ooze was deposited, composed of silicate minerals from the chemically weathered
granites and basalts.
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
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, saurians and crocodiles, marine polyptychodons and
ichthyosaurus, teeth of shark and bones of edaphon, 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, 91 - 97.5 Ma. and
when the seas deepened the overlying layers of Gault 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. This became entrapped 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. 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.
This
period had a lot hotter climate with little surface drainage otherwise the
waters would have been contaminated with sands and muds. 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 migrated upwards through
the sediments. Over time this gel solidified as nodules of flints.
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. It has been suggested that the numerous volcanic mounts beneath the
sea forced sea levels to rise as well as increasing surface temperatures.
Analysis of trapped air in amber samples show that oxygen levels dropped to 11%
at this time which helps 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.
Continued
tectonic movements caused considerable uplift of these Tertiary sediments. The
low and high regions of Southeast England were compressed. The London Platform became a low basin, the Chilterns
were folded upwards in the north and the Weald to the south. After the end of
the Upper Chalk there was a further period of uplift during which wave action
and tidal currents eroded much of the newly exposed upper chalk deposits. It
was around this time that the fossils of new species appeared in beds in
Suffolk and Cambridgeshire. The mastadon, hippopotamus, horse, tapir and ox
have been found alongside whales and turtles. This fossil bone bed was washed
out of the London Clay.
Most
of what is now the British Isles was land but this part of SE England was again
submerged by earth movements with new chalk deposits being laid down. In the
Middle Tertiary the strata were further bent, tilted and broken. This was
between 15 and 20 Ma, when the continent of Africa smashed into Europe. The
marine deposits in between were uplifted to produce the Alps, Apennines and
Atlas Mountains. The tectonic movements caused by the break up of the Atlantic
plate produced extensive volcanic activity resulting in layers of ash, some of
which have been found in the London Clay.
The
folds of southern England are the outer ripples of this Alpine storm. All this
area of Bedfordshire was uplifted with a slight 5o tilting to the
south east. 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 removed and the Lower Greensand exposed to the atmosphere. Over
time this left a Northeast trending escarpment which rises up to 66 metres from
the flat Jurassic clays of the northern half of Bedfordshire. This is now the
Greensand Ridge.
About
two million years ago Britain experienced the start of a dramatically cooler
climate. At least eighteen 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. This area was
near the ice margin and exposed to permafrost. When the temperatures rose again
the glacial meltwaters removed and reworked much of the sediments in
Bedfordshire. Boulder clay, sands and gravels were deposited. They stripped the
soft clays to the north and south of the Greensand ridge, leaving flat vales.
The Greensand, being made up of harder sands, was not eroded as much as the
softer clay and when the ice retreated the Greensand ridge was left as a
prominent feature on the landscape. The harder chalk of the Chilterns was also
left as a remnant of a former upland. A sheet of glacial till was dumped in the
wake of the melting ice. On the lower
ground extensive glacial sands and gravels were left, lake clays and silts were
formed and over the subsequent hundreds of thousands of years this till was
eroded by later glacial action, soil creep and landslipping. River erosion and
flooding have also contributed their erosive force and deposition to shape the
landscape that is now Bedfordshire. Man has reworked the surface and dug small
chalk, clay, sand, gravel and coprolite pits but human action has been nothing
compared to the major agents of erosion and deposition.
Hart,
M.B. ‘Foraminiferal Evidence for the Age of the Cambridge Greensand’, Proc.
Geol. Ass.Vol.84, pt.I (1973) p.71)
Markland,
Graeme ‘Greensand Ridge’, ms in Bedford Museum
Markland,
Graeme ‘Geological History of Bedfordshire’, ms in Bedford Museum
‘Regional
Geologies - East Anglia’, HMSO
Wooldridge
and Goldring, ‘The Weald’, Collins, 1972 pp.5-37
Conversation
with Dr Keith Rigby, palaeontologist, University of Notre Dame, USA