In geological time, the last period of the Mesozoic Era, preceded by the Jurassic Period and followed by the Tertiary Period. The rocks formed during Cretaceous time constitute the Cretaceous System. Omalius d'Halloys first recognized the widespread chalks of Europe as a stratigraphic unit. W. O. Conybeare and W. Phillips (1822) formally established the period, noting that whereas chalks were remarkably widespread deposits at this time, the Cretaceous System includes rocks of all sorts and its ultimate basis for recognition must lie in its fossil remains.