Chalk

Chalk is a soft, white, porous form of limestone composed of the mineral calcium carbonate. It is relatively resistant to erosion and slumping compared to the clays that it is usually associated with, and 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.

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Because chalk is porous, chalk downland usually holds a large water table, providing a natural reservoir that releases water slowly through dry seasons.

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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.

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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 formed entirely of chalk deposits.

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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 not actually made from the mineral chalk but from gypsum (calcium sulfate). Similarly, the "chalk" used by tailors is usually made from talc (magnesium silicate).

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Teachers and many universities use it to teach students, as it is easy to see.

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Limestone: Limestone (CaCO3) is a sedimentary rock composed of the mineral calcite (calcium carbonate). The primary source of this calcite is most commonly marine organisms. These organisms secrete shells that settle out of the water column and are deposited on ocean floors as pelagic ooze (see lysocline for i...

Mineral: This article is about minerals in the geologic sense; for nutrient minerals see dietary mineral; for the band see Mineral (band)....

Erosion: Erosion is the displacement of solids (soil, mud, rock, and so forth) by the agents of wind, water, ice, movement in response to gravity, or living organisms (in the case of bioerosion). Although the processes may be simultaneous, erosion is to be distinguished from weathering, which is the decompos...

Chalk related Images and Photos (experimental)

Chalk Garden 1964	P.J. Harvey: White Chalk CD	P.J. Harvey: White Chalk CD	El Chavo Chalk and Hopscotch Game
Billiard Balls Chalk Cue and Rack on Table Felt	Crayola Outdoor Sidewalk Chalk Tools Set	Disney Mickey Mouse Chalk Board Peel and Stick Wall Applique	Poppies on Gold Chalk
Wine Cellar Old Chalk Quarry Champagne Ruinart Reims Marne Ardennes France	East Sussex Beachy Head Is a Chalk Headland on South Coast of England England	US Operation Desert Storm F-16 Fighter Crewman Adding 5th Exclamation Point to Chalk-Drawn Message