Calcium sulfate an unusual property: when mixed with water at normal (ambient) temperatures, it quickly reverts chemically to the preferred dihydrate form, while physically "setting" to form a rigid and relatively strong gypsum crystal lattice: This reaction is exothermic and is responsible for the ease with which gypsum can be cast into various shapes including sheets (for drywall), sticks (for blackboard chalk), and molds (to immobilize broken bones, or for metal casting). Mixed with polymers, it has been used as a bone repair cement. Small amounts of calcined gypsum are added to earth to create strong structures directly from cast earth, an
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