Nicolas Steno (Danish: Niels Stensen), a Danish scientist, anatomist and geological pioneer, was born on 11 January 1638 and died on 5 December 1686. He is widely regarded as the father of stratigraphy.
Steno observed that streams spread across their floodplains during high water events, covering them with a layer of sediment and burying any organisms living there. Renewed flooding produced further layers of sediment, thus building up a sequence superposed on the lower strata. He is credited as formulating three of the defining principles of stratigraphy: the law of superposition discussed above: '...at the time when any given stratum was being formed, all the matter resting upon it was fluid, and, therefore, at the time when the lower stratum was being formed, none of the upper strata existed'; the principle of original horizontality: 'Strata either perpendicular to the horizon or inclined to the horizon were at one time parallel to the horizon', i.e. sediment particles are deposited according to the law of gravity; the principle of lateral continuity: 'Material forming any stratum were continuous over the surface of the Earth unless some other solid bodies stood in the way'. Steno's theory that fossils represented a chronological record of different living creatures in different eras was pivotal to Darwin's theory of natural selection.
A fourth principle, known as Steno's law, or Steno's law of the constancy of interfacial angles, states that the angles of intersection of equivalent crystal faces remain the same for all specimens of the same mineral, a fundamental breakthrough which was the prerequisite for all subsequent investigations into crystal structure. He also postulated that mineral crystals are built up of identical building blocks.