on this walk is visible at outcrop just behind the Reading Blue Coat School
Boathouse, beyond the iron fence, and in the undergrowth high in the “cliff”
is within the private grounds of Reading Blue Coat School.
a limestone, virtually pure calcium carbonate, comprising the ultra-small
fragments of calcareous plants (algae) called coccoliths, which lived
in the near surface waters of a warm Cretaceous seaway that stretched
across nearly all of Europe.
|Scanning electron microscope image of the Chalk from
Sonning showing coccolith ringlets. Scale bar line is 5 micro-metres
scanning electron microscope you can see that the rock is nearly entirely
composed of tiny ringlets of calcite.
fossils fell, within crustacean droppings (copepods swim in near surface
seawater and eat coccoliths), onto the seafloor, in waters that were many
tens of metres deep. Sea level at this time ( 70 million years ago) was
perhaps 500 metres higher than now.
climate was more like that of the Canaries. Clams and sea urchins lived
on the sea floor, along with sponges and crustaceans.
fish and large reptiles swam in the sea and pterosaurs flew in the skies
above. These organisms, as fossils, are to be found in the Chalk. On the
nearest landmasses (Central France, Scandinavia, Greenland) dinosaurs
lived. The sponges, made of silica, provided the chemical source of the
silica which comprises flints.
|Reconstruction of the floor of the chalk sea around
70 million years ago with shrimps, sea urchins (echinoderms) and clams
©The Cretaceous World by P. Skelton (Editor) 200.3.
black bands in the chalk were generated during burial as sponge skeletons
dissolved, and silica diffused towards localised irregularities within
the sediment (shrimp burrows) and then re-precipitated.
would be sought out by our ancestors for tools and, much later, as building
materials. A large time-break (an unconformity) separates the Chalk from
the Reading Beds. During this break, which marks the beginning of the
growth of the Alps, the dinosaurs (and many other animal groups) became
extinct, probably because a large meteorite struck Mexico triggering poisoning
of the atmosphere.
Beds consist of sands and clays that formed 55 million years ago. The
land had been uplifted above sea level, but a new seaway flooded in, this
time from the east and the base of the Reading Beds (which can be seen
in a number of places around Reading, such as Pincent’s Kiln in West Reading,
and at Bradfield, is a fossilised seafloor with shrimp borings and burrows
and encrusting oysters.
Beds here are around 20 metres thick and are overlain by London Clay,
which occurs towards the top of the hill. The London Clay (3 m thick here)
beyond the iron fence, from which bricks were made locally until 30 or
so years ago (e.g. at Arborfield) formed in a warm tropical sea around
50 million years ago, when Reading had the climate of Malaysia, with deep
tropical weathering and marine muds (now the London Clay) formed in a
shallow near-shore environment, the land being over towards Newbury.
TO PROFESSOR BRUCE SELLWOOD OF READING UNIVERSITY FOR ALL OF HIS HELP
WITH THIS WALK