Geologic Principles—Superposition and Original Horizontality

Introduction

In 1669 Nicolaus Steno made the first clear statement that strata (layered rocks) show sequential changes, that is, that rocks have histories. From his work in the mountains of western Italy, Steno realized that the principle of superposition in stratified (layered) rocks was the key to linking time to rocks. In short, each layer of sedimentary rock (also called a “bed”) is older than the one above it and younger than the one below it.

Superposition

Steno’s seemingly simple rule of superposition has come to be the most basic principle of relative dating. Steno originally developed his reasoning from observations of sedimentary rocks, but the principle also applies to other surface-deposited materials such as lava flows and beds of ash from volcanic eruptions.

Principle of Superposition

The geologic axiom or general law that states that in a sequence of layered beds, the lowest bed is the oldest and the highest bed is the youngest with each bed being younger than the one underneath it and older than the one above it. This principle is based on the processes in which successively younger sedimentary layers are deposited on lower and older layers.

Strata in Canyonlands National Park illustrating the principle of superposition. The oldest rocks in this sequence are at the lowest elevation and the youngest at the highest.

Photo by Allyson Mathis, first published in Moab Happenings.

Original Horizontality

In addition, Steno realized the importance of another principle, original horizontality, namely that strata are always initially deposited in nearly horizontal positions. Thus, a rock layer that is folded or inclined at a steep angle must have been moved into that position by crustal disturbances (i.e., mountain building, faults, or plate tectonics) sometime after its deposition.

Principle of Original Horizontality

The geologic axiom or general law that states that sedimentary strata are deposited in horizontal or nearly horizontal layers, and parallel or nearly parallel to the Earth’s surface.

Strata in Canyonlands National Park and Capitol Reef National Park illustrating the principle of original horizontality were deposited in horizontal or nearly horizontal layers. In Capitol Reef, the strata have been tilted from their original horizontal position by the Waterpocket Fold.

Photo by Allyson Mathis, first published in Moab Happenings.

Related Links

• NPS—Geologic Concepts

• NPS—Rocks and Minerals

• NPS—Geology

Geologic Time