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The Cretaceous Chalk in
Southern England
By Brieanna Graham
Topics
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What is chalk?
Conditions in which chalk forms
Significance of flint and chert
Chalk formations along the Jurassic Coast
The Landscape
Economics uses of chalk
Chalk
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Chalk is made of
coccoliths
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Very small, single
celled autotrophs
Live in warm water, or
near the surface
Made of calcium
carbonate
http://earthobservatory.nasa.gov/Library/Coccolithophores/
Formation of Chalk
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Accumulation of coccoliths forms chalk
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Start as a calcareous ooze
Cemented into rock over a long period of time
Oozes accumulate at a rate of only 1 to 5 cm
per 1,000 years
Has the greatest outcrop area of any
formation in England
Environment in which the chalk
formed
http://www.scotese.com/cretaceo.htm
Environment
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Entire planet was warmer
Deposited on the outer edge of a
continental shelf
Water was 200 to 300 meters deep but
warm
Stratigraphy
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Ripples, but no erosion
Cyclic deposition
No continental material
Gradational change between layers
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Lower, Middle, and Upper Chalk
Evidence of bioturbation
Flint and Chert
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Both are cryptocrystalline quartz
Flint is a chemical sedimentary rock
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Filled in burrow holes
Nodules formed and then connected together
to form large beds of flint
Chert is a chemical or biogenic
sedimentary rock
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Chemical precipitate or accumulation of
microorganisms
Flint
Fossils in the Chalk
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Ammonites
Bivalves
Brachiopods
Fish teeth
Remains of sharks
Sponges enclosed in flint
Old Harry Rocks
Old Harry Rocks
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Chalk cliffs
Upper Chalk
Constantly changing
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Few thousand years ago
Old Harry connected to Isle
of Wight
1770 could climb out to Old
Harry
1896, Wife of Old Harry
collapsed
Brunsden
2003
Old Harry Rocks
Chalk along Lulworth Cove
http://www.soton.ac.uk/~imw/jpg/WB3-geology-map-new3.jpg
Lulworth Cove
Lulworth Cove
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Made of Lower and
Middle Chalk
One of the most
visited sites along the
Jurassic Coast
Brunsden 2003
Lulworth Cove
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Chalk has been faulted due to the uplift of the Alps
http://www.soton.ac.uk/~imw/jpg-Lulworth/5LC-fault-in-cliffs.jpg
Unconformity
Brunsden 2003
Unconformity
Landscape
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Chalk created the Isle
of Purbeck
Chalk is so hard that
it can only be crossed
in two places so the
area south is as
isolated as an island
Salmon Watercolour Post Card
Vegetation
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Chalk breaks down to
form poor soil
Only short turf and
very few trees grow
on top
Economic uses for the Chalk
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Upper Chalk
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Middle Chalk
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Writes well
Local building stone
Roads
Lower Chalk
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No economic uses
The end of the Cretaceous
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Ended 65 million years ago
Asteroid hit earth and resulted in the
second greatest extinction on earth
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Dinosaurs, marine reptiles, and ammonites
extinct
Chalk forming environment gone
New environment dominated by
mammals, flowering plants, and grasses