Chalk, like limestone, is a form of calcium carbonate (CaCO3), which is a compound. A compound is formed whenever two or more elements are bonded chemically.
Chalk is formed at the bottom of the seafloor from limestone mud during certain conditions. Layers of lime build on top of each other until the sedimentary rock chalk is formed. Most of the chalk on earth was formed 60 to 100 million years ago during the Cretaceous period. Chalk gets its white color from Coccolithophores, tiny skeletons of plankton that fall down to the seafloor and form lime mud. Most of the Earth's chalk is found off the southern and eastern coasts of England, between Devon and Yorkshire. Chalk is effective in preserving fossils and is a common vessel for records of Cretaceous life on the sea floor.