The nature of the rocks reveals their origins
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?