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:
“The women cry,
Come, my fox,
heal me.
I am chalk white
with middle age
so wear me threadbare....”
—Anne Sexton (1928–1974)
“What harm cause not those huge draughts or pictures which wanton youth with chalk or coals draw in each passage, wall or stairs of our great houses, whence a cruel contempt of our natural store is bred in them?”
—Michel de Montaigne (1533–1592)
“Go, birds of spring: let winter have his fee;
Let a bleak paleness chalk the door,
So all within be livelier than before.”
—George Herbert (1593–1633)