English: Chalk from the Cretaceous of Britain.

Sedimentary rocks form by the solidification of loose sediments. Loose sediments become hard rocks by the processes of deposition, burial, compaction, dewatering, and cementation.

There are three categories of sedimentary rocks: 1) Siliciclastic sedimentary rocks form by the solidification of sediments produced by weathering & erosion of any previously existing rocks. 2) Biogenic sedimentary rocks form by the solidification of sediments that were once-living organisms (plants, animals, micro-organisms). 3) Chemical sedimentary rocks form by the solidification of sediments formed by inorganic chemical reactions. Most sedimentary rocks have a clastic texture, but some are crystalline.

Limestone is a common biogenic sedimentary rock composed of the mineral calcite (CaCO3), which bubbles in acid. Many geologically young limestones are composed of aragonite (also CaCO3). Numerous varieties of limestone exist (e.g., fine-grained limestone/micritic limestone/lime mudstone, coquina, chalk, wackestone, packstone, grainstone, rudstone, rubblestone, coralstone, calcarenite, calcisiltite, calcilutite, calcirudite, floatstone, boundstone, framestone, oolitic limestone, oncolitic limestone, etc.). Most limestones represent deposition in ancient warm, shallow ocean environments.

Chalk is distinctive variety of limestone that is soft, whitish, and powdery. Chalk is composed of calcite (CaCO3), and will bubble in acid. The most spectacular chalk locality on Earth is the White Cliffs of Dover (farm1.static.flickr.com/119/290719612_5a27cbaf61.jpg), along the southern shores of Britain. The rocks there are Cretaceous in age (“creta” means “chalk”).

Chalk is a biogenic sedimentary rock, but it is not obvious how this white powdery material represents the remains of once-living organisms. When examined under a scanning electron microscope, chalk powder is seen to be composed of immense numbers of exceedingly small microfossils, principally coccoliths (www.soes.soton.ac.uk/staff/tt/eh/pics/lith2.gif). Coccoliths are calcitic plates that once covered a living cell (upload.wikimedia.org/wikipedia/commons/d/d9/Emiliania_hux...). The cell was an entire organism called a coccolithophorid (Kingdom Protista, Phylum Chrysophyta, Class Coccolithophorida). Coccolithophorids are unicellular, photosynthetic organisms. They are often called “algae”, but they’re better called photosynthetic protists. When they die, the cell degrades, and the numerous hard calcitic plates covering the cell fall to the seafloor.

Chalk generally forms in moderately deep marine environments (but not in the deepest ocean depths), where high numbers of coccolith plates can accumulate as sediments, without calcite dissolution, and undiluted by muddy or sandy sediments washed in from the continents.