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 Chalk stream Chalk streams are streams that flow through chalk hills towards the sea. They are typically wide and shallow, and due to the filtering effect of the chalk their waters are alkaline and clear. There are at least 210 chalk streams in the world, and 160 of those are in England. Chalk streams are popular with fly fishermen who fish for trout on these rivers. Chalk is a highly porous and permeable rock and rain falling onto chalk topography percolates directly into the ground, where the chalk acts as an aquifer. The groundwater flows through the chalk bedrock, re-emerging