Chalk ( /ˈtʃɔːk/) is a soft, white, porous sedimentary rock, a form of limestone composed of the mineral calcite. Calcite is calcium carbonate or CaCO3. It forms under reasonably deep marine conditions from the gradual accumulation of minute calcite plates (coccoliths) shed from micro-organisms called coccolithophores. It is common to find chert or flint nodules embedded in chalk. Chalk can also refer to other compounds including magnesium silicate and calcium sulfate.
Chalk has greater resistance to weathering and slumping than the clays with which it is usually associated, thus forming 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. Because chalk is porous it can hold a large volume of ground water, providing a natural reservoir that releases water slowly through dry seasons.
Read more about Chalk: Deposits, Formation, Composition, Uses
Famous quotes containing the word chalk:
“No matter how much faculty of idle seeing a man has, the step from knowing to doing is rarely taken. ‘Tis a step out of the chalk circle of imbecility into fruitfulness.”
—Ralph Waldo Emerson (1803–1882)
“The women cry,
Come, my fox,
heal me.
I am chalk white
with middle age
so wear me threadbare....”
—Anne Sexton (1928–1974)
“Through throats where many rivers meet, the curlews cry,
Under the conceiving moon, on the high chalk hill,
And there this night I walk in the white giant’s thigh
Where barrren as boulders women lie longing still
To labour and love though they lay down long ago.”
—Dylan Thomas (1914–1953)