Geologic Time

Find Your Park illustration of person near canyon, text "travel through geologic time"
While a human life spans decades, the Earth's history spans 4.6 billion years! Geologists have created a time scale to organize Earth's history into eons, eras, periods, and epochs.

The geologic time scale began to take shape in the 1700s. Geologists used fundamental concepts to understand the chronological order of rocks around the world. It wasn't until the advent of radiometric dating techniques in the middle 1900s that reliable dates could be assigned to the previously named geologic time divisions.

The National Park System contains a magnificent record of geologic time because rocks from each period of the geologic time scale are preserved in park landscapes.

Fundamental Geologic Concepts

Geologists use these concepts to place sequences of rock in chronological order:
  • Uniformitarianism: Geologic processes operating on the Earth's crust have acted in the same manner and relative intensity throughout geologic time.
  • Superposition: In an undisturbed sequence of sediments or rocks, the older layers occur at the bottom with successively younger layers on top.
  • Original Horizontality: Layers of sediment are originally deposited horiztonal to the Earth's surface.
  • Lateral Continuity: Layers of sediment start out as continuous beds spreading out in all directions.
  • 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.
  • Faunal Succession: Fossils often exhibit identifiable patterns or characteristics which progressively change over time. A rock containing a certain fossil must have been deposited during the time that creature existed on earth.
Digital Geologic Map

Geologic Time Views Module

Views of the National Parks (Views) multimedia modules are part of the natural resource Science in Action series. The Geologic Time module focuses on the fundamental concepts needed to build knowledge of geologic time. Students will develop a better understanding of how geologic time is measured, learn about the events of the past, and explore interactive geologic time case studies at some of America's greatest National Parks.
photo illustration oldest rocks!

Oldest Rocks in the Parks

Learn about the oldest rocks found in the parks that range in age from 3 billion to 600 million years old.

Earth from Space

Video: Big Ideas in Geoscience

From the American Geosciences Institute comes Big Idea 2: Earth is 4.6 Billion Years Old. Watch Earth form, and learn about Earth's history and the events of deep time. See what processes shaped the Earth we know today.
photo illustration with text discover fossils

Fossils Through Geologic Time

Fossils are found in the rocks, museum collections, and cultural contexts of more than 260 National Park Service areas and span every period of geologic time from billion-year-old stromatolites to Ice Age mammals that lived a few thousand years ago. Visit the parks that preserve fossils from each major time period.


Educational Resources


Learning Activity: It's About Time

Have you ever wondered how geologic time works? This interactive classroom learning activity helps build the basic understanding of geologic time for grades 4-9.

Geologic Time Poster

Geologic Time Classroom Poster

Every park contains a slice of geologic time. In this classroom resource we highlight a few parks associated with each geologic time period.

Ranger Led Program

Geology, Relatives, and Time

Using a simple three or four generation family tree, students will construct a relatives time tree that mimics the major divisions of the geologic time scale (Precambrian, Paleozoic, Mesozoic, and Cenozoic). For Grades 9-12.

Ranger Led Program

The Age of the Earth

Read about the age of the Universe and Earth while learning how scientists determine geologic age through Radiometic Dating.


Last updated: June 13, 2018


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