Chalk
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. The
Champagne region of
France is mostly on chalk formations, with the famous caves being carved out beneath the hills.
|
French chalks used at school |
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.
*In Lawn
Tennis, when grass courts are used, chalk is used to mark the boundaries. This gives the advantage that, if the ball hits the line, a cloud of chalk dust can be seen.
*In
gymnastics,
rock-climbing,
bouldering and
weight-lifting, chalk - usually in the form of
magnesium carbonate - is applied to the hands to prevent slippage.
Tailor's chalk traditionally is hard chalk used to make temporary markings on cloth, for instance by tailors. Nowadays it is usually made from
talc (
magnesium silicate).
*
Chalk cliffs in the North of Ireland*
Chalk: Sedimentary rocks
