Blackboard ‘chalk’ are produced especially for use with blackboards in white and also in various colours. White chalk sticks are made mainly from calcium carbonate derived from mineral chalk rock or limestone, while colored or pastel chalks are made from calcium sulfate in its dihydrate form, CaSO·2HO, derived from gypsum. Chalk sticks containing calcium carbonate typically contain 40–60% of CaCO (calcite). As compared to whiteboards, blackboards still have a variety of advantages: On the other hand, chalk produces dust, the amount depending on the quality of chalk used. Some people find this uncomfortable or may be allergic to it, and according
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