Series: Fundamental Geologic Principles

In relative age dating, geologists use these principles to place sequences of rock in chronological order:

• Article 1: Geologic Principles—Cross-cutting Relationships

  

  A rock layer that is cross-cut by a fault, rock intrusion, or an erosion surface is older than the cross-cutting feature. Read more

• Article 2: Geologic Principles—Faunal Succession

  

  Fossils often exhibit identifiable patterns or characteristics which progressively change over time. In almost every case, rock containing a certain fossil was deposited during the time that creature existed on earth. Read more

• Article 3: Geologic Principles—Organic Evolution

  

  The process by which new species of organisms develop from earlier life forms and slowly change over long periods of geologic time. Read more

• Article 4: Geologic Principles—Organic Extinction

  

  Species of plants and animals may go extinct for a variety of reasons. Mass extinction events with widespread and rapid termination of organisms, as well as rapid increases in diversity of lifeforms, are used as major divisions in the geologic time scale. The extinction of dinosaurs at the end of the Mesozoic Era is perhaps the best known extinction event. Read more

• Article 5: Geologic Principles—Superposition and Original Horizontality

  

  Layers of sediment are originally deposited horizontal to the Earth's surface. Folded or tilted layers tell a story of change that occurred after deposition. Undisturbed sedimentary layers or beds are ordered sequentially with the oldest at the bottom and the youngest at the top. Read more

• Article 6: Geologic Principles—Uniformitarianism

  

  Geologic processes operating on the Earth's crust today have acted in a similar manner and relative intensity throughout geologic time. Understanding the Present is the key to understanding the past. Read more

• Article 7: Mohs Hardness Scale

  

  Brief description of Mohs Scale of Mineral Hardness Read more