Relative Time and Geologic Principles

The methods that geologists use to establish relative time scales are based on the Geologic Principles. Before we discuss the different geologic principles, it would be worthwhile to remind ourselves of the different rock types. Sedimentary rocks, like sandstone, are made from broken pieces of other rock that are eroded in higher elevations, transported by wind, water, ice, and gravity to lower areas, and deposited. The cooling and crystallization of molten rock forms igneous rocks. Lastly, the application of heat and pressure to rocks changes them into metamorphic rocks. This distinction is important because these three rock types are formed differently and therefore require different interpretations with respect to dating.

Uniformitarianism

The overarching principle of Uniformitarianism states that the Earth processes we observe today also occurred in the past and over long periods of time. This principle is often summarized as “the present is key to the past”. This principle was originally based on the work of James Hutton and made popular by Charles Lyell in the 19th Century. Uniformitarianism was originally applied only to common, everyday geologic events, and excluded major catastrophic events like an asteroid impact. Many modern geologists will often instead use the term actualism, which examines how the universal physical laws govern both common and catastrophic events (Table 14.1).

**Table 14.1:** Comparison between catastrophism, uniformitarianism, and actualism. (CC-BY 4.0, Chloe Branciforte)
	Catastrophism	Uniformitarianism	Actualism
Type of change	Big, sudden events	Continuous, uniform events	Both
Rate of change	Rapid, long events	Constant	Rate vary; natural laws do not
Description of change	Begins, usually violently, then changes to something different	Same events occur over time at the same rate; "The present is key to the past"	Universal physical laws are constant; however, Earth may become different if conditions change

Original Horizontality

Originally named the “Law of Original Horizontality” by Nicolas Steno in the 17^th Century, the Principle of Original Horizontality states that undeformed sedimentary rocks are first deposited horizontally. The deposition of sediment is controlled by gravity and will pull it downward. If you have muddy water on a slope, the water will flow down the slope and pool flat at the base rather than depositing on the slope itself. The implication is that, if a sedimentary rock layer is tilted or folded, it was first deposited flat then folded or tilted after lithification. In other words, sediments are deposited in horizontal layers; only after deposition does deformation occur (Figure 14.2).

Left, cross-sectional view of flat-lying, horizontal layers. Right, cross-sectional view of illustrating the layers have been tilted or inclined from the horizontal. — **Figure 14.2:** The principle of original horizontality states that undeformed sedimentary rocks are deposited horizontality (left). The rocks on the right have been tilted. This tilting event would have to occur after deposition of the horizontal layers. (CC-BY 4.0, Chloe Branciforte, own work)

Superposition

Originally named the “Law of Superposition” by Nicolas Steno, the Principle of Superposition states that, in an undeformed sequence of sedimentary rocks, the oldest rocks will be at the bottom of the sequence while the youngest will be on top. Imagine a river carrying sand into an ocean: the sand will spill out onto the ocean floor and come to rest on top of the seafloor. This sand was deposited after the sand of the seafloor was already deposited, and is therefore above it. We can envision a relative time scale of rock layers from the oldest rocks at the bottom (#1) to the youngest at the top of an outcrop (#7) (Figure 14.3).

Cross-sectional view of flat lying, sedimentary rock layers. — **Figure 14.3:** The principle of superposition states that in an undisturbed sequence of sedimentary rock, the oldest rocks will be found on the bottom, while the youngest will be located at the top. (CC-BY 4.0, Chloe Branciforte, own work)

Cross-Cutting Relationships

Originally named the “Law of Cross-cutting” by Nicolas Steno, the Principle of Cross-Cutting Relationships states that when two geologic features intersect, the one that cuts across the other is younger, or happened more recently. A feature must be present before something can affect it. For example, if a fault fractures through a series of sedimentary rocks, those rocks had to have existed before the earthquake that formed the fault (Figure 14.4).

Left, a cross-sectional view of flat-lying, horizontal layers. Right, layers that have been bisected by a fault (top right) and igneous intrusion (bottom right). — **Figure 14.4:** Block diagram showing the principle of cross-cutting relationships. The geological features (fault or Igneous intrusion) that cuts across the sedimentary layers must be younger. (CC-BY 4.0, Chloe Branciforte, own work)

Lateral Continuity

Originally named the “Law of Lateral Continuity” by Nicolas Steno, the Principle of Lateral Continuity states that layers of sediment, which lithify to form rock, form laterally extensive horizontal sheets. This means that the same type of sediment will be deposited across a landscape until the environment in which those sediments are deposited ends (e.g. beach sand will be deposited only in a beach environment) or until a physical barrier stops their deposition (e.g. river sediments are structurally confined to river channels). This is useful when correlating layers of rock across a landscape where erosion may have disrupted their continuity (Figure 14.5).

A cross-sectional view of flat-lying, horizontal layers which have been bisected by a river valley. — **Figure 14.5:** The principle of lateral continuity states that layers of sediment, form laterally extensive horizontal sheets. The river which has carved the valley, disrupted the continuity of the layers, however we can match the rock on either side of the valley using this principle. (CC-BY 4.0, Chloe Branciforte, own work)

Faunal Succession

The Principle of Fossil (Faunal) Succession acts as an independent check to superposition and states that fossils succeed one another in known order. Fossils are the preserved remains of ancient organisms that are normally found within sedimentary rocks. Organisms appear at varying times in geologic history and go extinct at different times. These organisms also change in appearance through time. This pattern of the appearance, change, and extinction of thousands of fossil organisms creates a recognizable pattern of organisms preserved through geologic time. Therefore, rocks of the same age likely contain similar fossils and we can use these fossils to date sedimentary rocks. This principle was discovered by William “Strata” Smith as he worked to survey where the canals could be placed in England.

Certain fossils are particularly useful in telling time; these are called index fossils. These fossils are organisms that were abundant when they were alive, were widespread geographically, have hard parts (shell or skeleton), and have a limited geologic range (the amount of time an organism is alive on Earth). Index fossils are often the quickest and easiest way to date sedimentary rocks precisely and accurately (Figure 14.6).

Simplified geologic time scale with associated fossils. — **Figure 14.6:** Keyed to the relative time scale are examples of index fossils, the forms of life which existed during limited periods of geologic time and thus are used as guides to the age of the rocks in which they are preserved. (Public Domain, USGS)

Inclusions

The Principle of Inclusions states that any rock fragments (inclusions) that are included in a rock must be older than the rock in which they are included. For example, a xenolith in an igneous rock, or a clast in sedimentary rock, must be older than the rock that includes it (Figure 14.7).

A cross-sectional view metamorphic, igneous and sedimentary rock. Inclusions of the schist are present in both the granite and sandstone. — **Figure 14.7:** Block diagram showing the principle of inclusions which states, inclusions are always older than the rock they are included within. (CC-BY 4.0; Chloe Branciforte, own work)

Baked Contacts

The Principle of Baked Contacts states that rock encountering molten rock, lava or magma, will become cooked or changed (metamorphosed). The presence of a baked contact indicates the igneous intrusion is younger than the rocks around it. If an intrusive igneous rock is exposed via erosion, then later buried by sediments, the surrounding rocks will not be baked, as the intrusion was already cold at the time of sediment deposition (Figure 14.8).

A cross-sectional view metamorphic, igneous and sedimentary rock. Inclusions of the schist are present in both the granite and sandstone. — **Figure 14.8:** Block diagram showing the principle of inclusions which states, inclusions are always older than the rock they are included within. (CC-BY 4.0; Chloe Branciforte, own work)