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Geology
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Geologic Map
Shaded Relief
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Generalized Geologic Map of the Conterminous United States
North America Shaded Relief
 

Article

  The North American Tapestry of Time and Terrain

Introduction
The Two maps
Zoom In
Features
Legend and Rock Ages
Rock Types
Political Boundaries
Credits

  Legend and Rock Ages
 
Ages in Millions of Years
 
Typically, the geologic time scale is displayed as a vertical bar, with the youngest age units at the top, and the oldest at bottom. This structure parallels the tendency of sediments to accumulate one on top of another, with the older layers of rock being covered over by new sediments.

(click on legend item to view map)

Time scale legend to quaternary map to paleogene map  to neogene map to cretaceous map  to jurassic map to triassic map to cenozoic map to mesozoic map  to permian map to pennsylvanian map to mississippian map  to devonian map  to silurian map  to ordovician map  to cambrian map to paleozoic map  to proterozoic map to archean map to precambrian map to late proterozoic map to middle proterozoic map  to early proterozoic map  to early archean map to middle archean map to late archean map Here, we've divided the time scale into its four major components: the Cenozoic era, the Mesozoic era, the Paleozoic era and the Precambrian. Since Cenozoic is the most recent of these it is at the top-left. Precambrian, being the oldest, is at the bottom.

The time scale is further divided. For example, the Mesozoic, the age of the dinosaurs, is made up of the Cretaceous, Jurassic, and Triassic periods.

The numbers on the scale indicate commonly accepted age boundaries of these eras. The Precambrian stretches from the origin of the earth, more than 4 billion years ago, to 540 million years ago (or mega anna, MA). The Paleozoic from 540 MA to 248 MA. Mesozoic covers the interval between 248 and 66 MA. Cenozoic covers the span from 66 MA to the present.

As you can see, the time scale is not drawn linearly, which is why the Precambrian, which lasts over 3.5 billion years, appears shorter than the Paleozoic, which lasted a little more than 290 million years.

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