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 Most people first encounter the word "chalk" in school where it refers to blackboard chalk, which was originally made of mineral chalk, since it readily crumbles and leaves particles that stick loosely to rough surfaces, allowing it to make writing that can be readily erased.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 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