The Cretaceous Chalk in Southern England By Brieanna Graham

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The Cretaceous Chalk in Southern England By
Brieanna Graham
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Topics

• 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

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Chalk

• Chalk is made of coccoliths
• Very small, single celled autotrophs
• Live in warm water, or near the surface
• Made of calcium carbonate

http//earthobservatory.nasa.gov/Library/Coccolith
ophores/
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Formation of Chalk

• Accumulation of coccoliths forms chalk
• 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

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Environment in which the chalk formed
http//www.scotese.com/cretaceo.htm
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Environment

• Entire planet was warmer
• Deposited on the outer edge of a continental
  shelf
• Water was 200 to 300 meters deep but warm

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Stratigraphy

• Ripples, but no erosion
• Cyclic deposition
• No continental material
• Gradational change between layers
• Lower, Middle, and Upper Chalk
• Evidence of bioturbation

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Flint and Chert

• Both are cryptocrystalline quartz
• Flint is a chemical sedimentary rock
• Filled in burrow holes
• Nodules formed and then connected together to
  form large beds of flint
• Chert is a chemical or biogenic sedimentary rock
• Chemical precipitate or accumulation of
  microorganisms

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Flint
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Fossils in the Chalk

• Ammonites
• Bivalves
• Brachiopods
• Fish teeth
• Remains of sharks
• Sponges enclosed in flint

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Old Harry Rocks
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Old Harry Rocks

• Chalk cliffs
• Upper Chalk
• Constantly changing
• 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
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Old Harry Rocks
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Chalk along Lulworth Cove
http//www.soton.ac.uk/imw/jpg/WB3-geology-map-ne
w3.jpg
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Lulworth Cove
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Lulworth Cove

• Made of Lower and Middle Chalk
• One of the most visited sites along the Jurassic
  Coast

Brunsden 2003
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Lulworth Cove

• Chalk has been faulted due to the uplift of the
  Alps

http//www.soton.ac.uk/imw/jpg-Lulworth/5LC-fault
-in-cliffs.jpg
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Unconformity
Brunsden 2003
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Unconformity
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Landscape

• 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
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Vegetation

• Chalk breaks down to form poor soil
• Only short turf and very few trees grow on top

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Economic uses for the Chalk

• Upper Chalk
• Writes well
• Local building stone
• Middle Chalk
• Roads
• Lower Chalk
• No economic uses

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The end of the Cretaceous

• Ended 65 million years ago
• Asteroid hit earth and resulted in the second
  greatest extinction on earth
• Dinosaurs, marine reptiles, and ammonites extinct
• Chalk forming environment gone
• New environment dominated by mammals, flowering
  plants, and grasses