Three rules of relative dating
From top to bottom: Rounded tan domes of the Navajo Sandstone, layered red Kayenta Formation, cliff-forming, vertically jointed, red Wingate Sandstone, slope-forming, purplish Chinle Formation, layered, lighter-red Moenkopi Formation, and white, layered Cutler Formation sandstone.Photo from Glen Canyon National Recreation Area, Utah.He put forth three propositions, the first being this: "If a solid body is enclosed on all sides by another solid body, of the two bodies that one first became hard which, in the mutual contact, expresses on its own surface the properties of the other surface." (This may be clearer if we change "expresses" to "impresses" and switch "own" with "other.") While the "official" Principles pertain to layers of rock and their shapes and orientations, Steno's own principles were strictly about "solids within solids." Which of two things came first? Thus he could confidently state that fossil shells existed before the rock that enclosed them.And we, for example, can see that the stones in a conglomerate are older than the matrix that encloses them.This principle allowed Steno to link identical rocks on opposite sides of a river valley and deduce the history of events (mostly erosion) that separated them.Today we apply this principle across the Grand Canyon—even across oceans to link continents that once were adjoined.Today we know that some strata start out tilted, but nevertheless this principle enables us to easily detect unnatural degrees of tilt and infer that they have been disturbed since their formation.And we know of many more causes, from tectonics to intrusions, that can tilt and fold rocks.
The regular order of the occurrence of fossils in rock layers was discovered around 1800 by William Smith.But in the case of minerals, he could assert that crystals accrete from outside rather than grow from inside.