Frequently Asked Questions

This section answers questions about the content of the site. For an overview of the site, take the site tour. For navigation, technical and trouble-shooting assistance go to the help page.

What is paleontology?

Paleontology is the study of fossils. A fossil is defined as any preserved remains or traces of a past life form. Paleontologists study fossils to reconstruct the history of the Earth and the life on it.

What does the Exploring Time and Space map portray?

The map used here was created by the US Geological Survey in Menlo Park. Through computer processing and enhancement, they wove together the topography and the geology of the United States into a single image. The new map,, portrays a 3-D perspective of the land surface with the addition of the fourth dimension of geologic time, shown in color. The colors correspond to those used in the stratigraphic column representing the eras and periods of the Geological Time Scale.

Why do some of these maps look different than other geologic maps of the same area and time period?

For purposes of the web, the USGS map has been simplified from a more detailed geologic map. It was meant to indicate the major exposures of rocks of a particular time interval.

What does the colored column portray?

The colored column represents intervals of geologic time. The colors correspond to those used in the map and are more or less consistent with those used as the Sedimentary Age Colors of the U.S. Geological Survey:

What is the geologic time scale?

The geologic time scale is a sequential arrangement of time units defined by geologists. It was based originally on two fairly simple observations. The first was that, given a stack of sedimentary rock layers, each layer of rock must be younger than those below it and older than those above it. The second observation was that the presence or absence of various life forms also occurred in a predictable order. Taken together, these observations enabled scientists anywhere in the world to tell the relative age of rocks containing fossils.
More about relative age:i

Geologists subdivide time hierarchically into eras (e.g. Paleozoic, Mesozoic, and Cenozoic), which are further divided into periods (e.g. Cambrian, Permian, Cretaceous), which are further divided into epochs (e.g. Miocene, Pleistocene). Scientists also have a method for determining the actual number of years ago that various rocks formed. This method is called radiometric age dating, and it has allowed scientists to continue refining the accuracy of the geologic time scale.
More about radiometric age dating:
More about the geologic time scale:
More about the time intervals:

What are undifferentiated rocks?

These are rocks for which finer age divisions are not specified on the map. In some cases this is because small areas of rocks of different ages are too close together to be shown at the scale of the map. In other cases, the exact age relationships of the rocks in a given area may not yet have been determined.

What are fossil assemblages?

This term can have several meanings, depending on how it is being used. The simplest meaning is "a collection of fossils that occur together in the same layer or layers of rock."

You may be interested in additional information about:

Using radiometric dating:
The names of the time intervals:
Understanding about geologic time (for students):
Plate tectonics:
How fossils are formed:
What fossils can tell us (for students):
The taxa in the Fossil Gallery:
More general FAQs about paleontology:

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