Chalk It is formed in the sea by sub-microscopic plankton, which fall to the sea floor and are then consolidated and compressed during diagenesis into chalk rock. Most people first encounter the word "chalk" in school where it refers to blackboard chalk, which was originally made of mineral chalk, since it readily crumbles and leaves particles that stick loosely to rough surfaces, allowing it to make writing that can be readily erased. Blackboard chalk manufacture now may utilize mineral chalk, other mineral sources of calcium carbonate, or the mineral gypsum (calcium sulfate). While gypsum-based blackboard chalk is the lowest cost to
Chalk stream which would normally be too large for the term "stream". The Somme River in Northern France is a chalk stream on a larger scale. Chalk streams of the Southern England Chalk Formation in Berkshire, Hampshire, Wiltshire, and Dorset: Chalk streams of the Southern England Chalk Formation in the Chiltern Hills and Hertfordshire (tributaries of the River Thames, River Lea and River Colne): Chalk streams of the Yorkshire Wolds: Chalk streams of Lincolnshire: There are several chalk streams in the Lincolnshire Wolds including Chalk streams of Kent: Chalk streams of Surrey: Chalk streams of Norfolk: Chalk stream Chalk streams are streams