If we were to walk backward in time at the rate of one century per step,
the first step would return us to 1872, the year that Yellowstone
National Park was established. But to return to the oldest recorded
event in its geologic history, we would have to walk (at 3 feet per
step) some 15,000 miles, or three-fifths of the way around the world!
Occurring far back in the antiquity of the Precambrian Era—approximately
2.7 billion years ago according to radiometric dating (fig. 6)—the
oldest event resulted in rocks so crumpled and changed by heat and
pressure that their original character is obscure. These rocks, having
been transformed from still older ones, are called _metamorphic rocks_.
Considered to form part of the very foundation of the continent itself,
they are also commonly referred to as _basement rocks_.
[Illustration: THE ROCKS of Yellowstone National Park, separated
into individual units or formations and arranged according to their
geologic ages (see fig. 6). A formation is a body of rock that
contains certain identifying features (such as composition, color,
and fossils) which set it apart from all other rock units. The
identifying features of each formation provide valuable clues
bearing on its origin. Most formations are given formal names, and
usually each formation is thick and widespread enough to be
recognized over broad areas. Some, however, change character from
place to place, and different names may be used in different areas
even though the rocks represent the same geologic time interval.
(Fig. 5)]