Geologists believe that “the present is the key to the past.” After
observing lava erupting from a present-day volcano or limestone forming
in marine waters, we infer that similar types of ancient lavas or
ancient limestones formed in virtually the same ways. This kind of
reasoning is used to interpret the origins of all types of ancient
rocks, for all the known geological processes that form rocks seem to
have been operating since the earth’s beginning.
Figure 5 shows the many different rock units that have been recognized
in Yellowstone National Park. Arranged in a vertical column according to
the geologic time intervals in which they formed, these rocks represent
a large part of total earth history (fig. 6). A generalized geologic map
(plate 1) shows the distribution of the various units (or groups of
closely related units) exposed at the surface throughout the Park area.
This map and figure 5 summarize much of the information that is
necessary to interpret the Park’s geologic history—in essence, to
provide answers to these two important questions: What were the geologic
events that formed the rocks? When did these events occur?