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 Group includes what were formerly designated the Middle Chalk and Upper Chalk Formations, together with the Plenus Marls (topmost part of the former Lower Chalk Formation). In the southern province it is divided in the following way (youngest/uppermost at top): In the northern province the sequence is divided thus: In the southern province, the former Middle Chalk, now the Holywell Nodular Chalk Formation and overlying New Pit Formation, averages about in thickness. The sparse fossils found in this sequence include the brachiopod "Terebratulina" and the echinoid "Conulus". The former Upper Chalk by comparison is softer than the underlying sequence and the