Geological Time and Stratigraphy Principles Explained

What Is an Index Fossil?

An index fossil is a fossil known to have lived in a particular geologic age that can be used to date the rock layer in which it is found.

Age of the Earth

The Earth is approximately 4.54 billion years old.

Difference Between a Mold and a Cast

A cast is taken of a footprint, while a mold is made of a bone or other part. One is the reverse of the other; in a mold, you are creating a negative impression to eventually produce a copy of the original object.

What Is an Unconformity?

An unconformity is a buried erosion surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous.

What Is a Disconformity?

A disconformity is the surface of a division between parallel rock strata, indicating an interruption of sedimentation.

The Three Geological Eras

• Paleozoic
• Mesozoic
• Cenozoic

Current Geological Periods

• Paleozoic Era: Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian.
• Mesozoic Era: Triassic, Jurassic, and Cretaceous.
• Cenozoic Era: Tertiary and Quaternary.

Law of Superposition

The law of superposition (or the principle of superposition) is a key axiom based on observations of natural history that serves as a foundational principle of sedimentary stratigraphy and other geology-dependent natural sciences:

Sedimentary layers are deposited in a time sequence, with the oldest on the bottom and the youngest on the top.

Law of Original Horizontality

Sedimentary rocks are always deposited as horizontal, or nearly horizontal, strata, although these may be disturbed by later earth movements. The law was first proposed in the mid-17th century by Nicolaus Steno.

Law of Cross-Cutting Relationships

This is the principle that a fault or body of rock is younger than any other body of rock that it cuts through.

Half-Life and Dating Methods

• Relative Age: Relative dating determines if something is younger or older than something else. It does not provide a specific age, only the sequence of events. For example: the sandstone in this area is older than the limestone.
• Absolute Age: Absolute age allows us to precisely assign a numerical value (in years) to the amount of time that has passed. For example: the sandstone is 300 million years old.