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Principle of inclusion
One of the basic principles of stratigraphy is the "principle of inclusion." The principle states that if fragments of a rock unit are found included in a second rock unit, the second unit is younger than the first. In this picture, fragments of the Mesoproterozoic Pikes Peak granite (like the clast on which the pencil is resting) are included in a conglomerate layer of the Pennsylvanian Fountain Formation, CO. We infer that the granite must have existed as a solid rock body that was broken and eroded before and during the deposition of the conglomerate layer. Therefore, this principle helps establishing a relative sequence of events. The principle was already understood by Nicolas Steno, the father of stratigraphy. In his "Prodromus," published in 1669, he writes: "these same bodies had already become hard at the time when the matter of the earth and rock containing them was still fluid. And not only did the earth and rock not produce the bodies contained in them, but they did not even exist as such when those bodies were produced in them” (pp. 15-16).
Clinostratification
The principle of original horizontality is one of the basic principles of stratigraphy. It states that sediments are deposited in essentially horizontal layers, and it was enunciated already by Steno in his Prodromus (1669, p. 30): "...adeoque strata omnia praeter infimum duobus planis horizontis parallelis contineri" (...all strata, therefore, except the lowest, were bounded by two planes parallel to the horizon). There are, however, exceptions to this principle. Strata can be deposited at an angle when sediment is being transported over a sloping substrate. This occurs, for example on the stoss face of sand dunes, on fluvial bars, or, at an even larger scale, on the front of submerged deltas. This type of stratification is known as "clinostratification" and the inclined layers or laminae are called "foresets." This picture shows nice clinoforms (foresets dipping to the left) overlain by horizontal beds, in the Pennsylvanian Minturn Formation (CO, USA). Backpack for scale, about 50 cm in size.