Principles of Geology

On this page, we will discuss the Principles of Geology. These are general rules, or laws, that we use to determine how rocks were created and how they changed through time. We also use these laws to determine which rock formations are older or younger.

Different rock layers of different colors and angles, labeled A through L.

The Law of Superposition states that beds of rock on top are usually younger than those deposited below. This is logical, consider a layered cake or a stack of books, you can’t add another layer unless one already exists to begin with. By understanding the Law of Superposition we can make general statements about the ages of these rock units.Consider these top layers – Unit K (dark green) is younger than Unit J (burnt orange) because it lies atop it, this also directly relates to the relative age dating.

The Law of Original Horizontality suggests that all rock layers are originally laid down (deposited) horizontally and can later be deformed. This allows us to infer that something must have happened to the rocks to make them tilted. This includes mountain building events, earthquakes, and faulting.The rock layers on the bottom have been deformed and are now tilted. The rock layers on the top were deposited after the tilting event and are again laid down flat.

The Law of Lateral Continuity suggests that all rock layers are laterally continuous and may be broken up or displaced by later events. This can happen when a river or stream erodes a portion of the rock layers. This can also happen when faulting occurs. Faulting causes displacement in rock units. The figure here shows the offset between the layers signified by the black line cutting across the rocks. Trace the colors or letters across to find the layers that match. The rock layers on the top seem to form a valley but we can tell that Unit I (dark blue) on one side is the same as the Unit I (dark blue) on the other side. There is missing rock in between and a displacement caused by deformation.

Cross-cutting relationships also helps us to understand the timing of events. Younger features cut across older features. Going back to the fault on this image, we know that these rock layers were involved in the fault movement because they are all offset. We can also determine which beds of rock were tilted and that relationship to the rocks that are not tilted. The rock layers on the bottom have been deformed and are now tilted. The rock layers on the top were deposited after the tilting event and are again laid down flat.

Conglomerate rock containing many smaller pieces.
Conglomerate rock containing many smaller pieces.

The idea of Components is simple. If you find a rock that has other smaller pieces of rocks within it, the smaller rocks inside must have existed before the larger rock was created.

The Principle of Faunal Succession states that a species appears, exists for a time, and then goes extinct. Time periods are often recognized by the type of fossils you see in them. Each fossil has a ‘first appearance datum’ and a ‘last appearance datum’. This is simply the oldest recorded occurrence of a fossil and then the youngest recorded occurrence of a fossil.

Rocks that contain fossils occur in a very real and understandable order. Rocks of certain time periods can be recognized and separated by their fossil content (Boggs, 2012). This is a skill that geologists acquire as they do field work and explore the Earth! The fauna from the Mississippian is very different from the Ordovician and easily distinguishable! Groups of fossils, or fossil assemblages, can be used to correlate rock units across continents.

Ten vertical rock layers with different fossils. A legend is to the side organizing the fossils from oldest to youngest.
Relative age dating with fossils is very common and we can use this data to make interpretations about the environment.

Something else that fits into Geologic Principles and basic stratigraphy (study of rock layers) are unconformities. Unconformities are simply gaps (missing data) in the rock record, these gaps could indicate a variety of processes. Such as: erosion, deformation, or changes in sea-level. There are three main types of unconformities: (1) Angular unconformities; (2) Disconformities and; (3) Nonconformities.

Angular unconformity between the older orange package of rock layers and the younger green package of rock. The curvy line indicates the erosional surface. A disconformity between the older purple sedimentary layers and the younger orange sedimentary layers. Nonconformities occur between an igneous body and sedimentary rock layers.

Angular unconformities are represented by an older group of rock layers has been tilted, eroded, and another younger set of rock layers were deposited on top of this erosional surface. The tilting process is commonly by a mountain building event, it doesn’t necessarily have to be in the mountains but the effects of mountain building processes are long reaching.

Disconformities are an erosional surface between two sets of rock layers. Unlike with angular unconformities, there is no tilting of the older rock layers. This makes disconformities difficult to recognize because the erosional surface is often very difficult to find.

Nonconformities are unconformities that separate different rock types. This is commonly the separation between igneous and sedimentary or metamorphic and sedimentary rocks. These types of unconformities usually indicate that a long amount of time has been eroded away before the younger sedimentary rocks were deposited.