Chalk
From Free net encyclopedia
Image:Dscn0646-needles 600x800.jpg
- For other uses, see Chalk (disambiguation).
Chalk is a soft, white, porous form of limestone composed of the mineral calcite. It is also a sedimentary rock. It is relatively resistant to erosion and slumping compared to the clays with which it is usually associated, so forms tall steep cliffs where chalk ridges meet the sea. Chalk hills, known as chalk downland, usually form where bands of chalk reach the surface at an angle, so forming a scarp slope.
Chalk is formed in shallow waters by the gradual accumulation of the calcite mineral remains of micro-organisms, over millions of years. Embedded flint nodules are commonly found in chalk beds.
Because chalk is porous, chalk downland usually holds a large body of ground water, providing a natural reservoir that releases water slowly through dry seasons. The River Somme is an example of such water release from chalk.
Chalk has been quarried from prehistory, providing building material and marl for fields. In southeast England, deneholes are a notable example of ancient chalk pits.
The Chalk Formation is a European stratigraphic unit in the upper Cretaceous period. This includes the famous White cliffs of Dover of Kent in England, which are formed entirely of chalk deposits.
Image:DSC 6623-MR-Craies.jpg Blackboard chalk is a substance used for drawing on rough surfaces, as it readily crumbles leaving particles that stick loosely to these surfaces. Blackboard chalk, often supplied in sticks about 5 cm long, is currently made from the mineral gypsum (calcium sulfate) rather than calcium carbonate. Similarly, the "chalk" used by tailors is usually made from talc (magnesium silicate).
The Champagne region of France is mostly on chalk formations.
See also
- Chalk Formation
- Southern England Chalk Formation
- Limestone
- Sidewalk chalk
- Agricultural lime
- Chalk stream
External links
et:Kriit es:Tiza fr:craie he:קירטון ja:チョーク (岩石) nl:Krijt (grafisch materiaal) pl:Kreda (skała) simple:chalk sv:Kalk zh:白垩