| The rocks highlighted here are some of the youngest
in the United States. They formed in the late Tertiary Period, between
38 and 1.6 million years ago. The Tertiary Period is subdivided
into epochs. The youngest Tertiary epochs, the Miocene, Pliocene,
and Oligocene are shown here. During this time, sediments eroded
from the uplifting Rocky Mountains were being deposited in huge
alluvial fans that formed the Great
Plains. In the eastern U.S., sediments were also being deposited,
forming the Coastal Plain.
In the west, subduction led to the formation of a chain of volcanic
mountains called the Cascade
Range.
Upper Tertiary features of the United States:
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