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OVERVIEW

Geology is the study of Earth and the processes that shape it. Physical geology is a branch of geology concerned with
the materials of Earth, processes that bring about changes in the materials and structures they make up, and in the
physical features of Earth formed as a result. Historical geology, on the other hand, is a branch of geology concerned
with the development of Earth and the organisms on it over time. Physical geology and historical geology together
provide a basis for understanding much about Earth and how it has developed.
One reason to study geology is to satisfy an intellectual curiosity about how Earth works. Piecing together the
history of how a mountain range formed or inferring the history of an individual rock can be exciting as well as
satisfying. As a result, you appreciate the beauty of our Earth from a different perspective. Distinctive features such
as the granite domes of Yosemite, the geysers of Yellowstone, and the rocks exposed in a roadcut take on a whole
new meaning. Often, part of the new meaning is a story that tells the history of that distinctive feature and how it
came to be.
There are many different and fascinating stories that can be read from a given landscape. The structures in
the landscape, such as hills and valleys, tell a story about folding, faulting, and other building-up mechanisms, or
processes, that were described in chapter 19. There is also a story about the present stage of weathering, erosion, and
sculpturing, the processes that were described in chapter 20. Thus, the landscape has a story about the building and
sculpturing of surface features and what this must mean about the history of the region (Figure 21.1).
The story reaching back the furthest in time is told by individual rocks. Each rock was formed by processes that
were described in chapter 17. Each rock has its own combination of minerals that began to change the moment the
rock was created.
Altogether, the story of the individual rock and the landscape features describes the history of the region and how
it came to be what it is today. The resulting knowledge of geologic processes and events can also have a practical
aspect. Certain earth materials are used for energy or in the manufacture of technological devices. Knowing how,
where, and when such resources are formed can be very useful information to modern society.

21.1 FOSSILS EARLY IDEAS ABOUT FOSSILS
A fossil is any evidence of former life, so the term means more The story about finding a fossil fish on a mountain is not as
than fossilized remains, such as those pictured in Figure 21.2. far-fetched as it might seem, and in fact, one of the first recorded
Evidence can include actual or altered remains of plants and evidences of understanding the meaning of fossils took place in
animals. It could also be just simple evidence of former life, such a similar setting. The ancient Greek historian Herodotus was
as the imprint of a leaf, the footprint of a dinosaur, or droppings among the first to realize that fossil shells found in rocks far
from bats in a cave. from any ocean were remnants of organisms left by a bygone
What would you think if you were on the top of a moun- sea. Other Greek philosophers were not convinced that this
tain, broke open a rock, and discovered the fossil fish pictured in conclusion was as obvious as it might seem today. Aristotle, for
Figure 21.2? How could you explain what you found? There are example, could see no connection between the shells of organ-
several ways that a fish could end up on a mountaintop as a fossil. isms of his time and the fossils, which he also believed to have
For example, perhaps the ocean was once much deeper and cov- formed inside the rocks. Note that he also believed that living
ered the mountaintop. On the other hand, maybe the mountain- organisms could arise by spontaneous generation from mud. A
top was once below sea level and pushed its way up to its present belief that the fossils must have “grown” in place in rocks would
high altitude. Another explanation might be that someone left the seem to be consistent with a belief in spontaneous generation.
fossil on the mountain as a practical joke. What would you look It was a long time before it was generally recognized
for to help you figure out what actually happened? In every rock that fossils had anything to do with living things. Even when
and fossil, there are fascinating clues that help you read what hap- people started to recognize some similarities between living
pened in the past, including clues that tell you if an ocean had organisms and certain fossils, they did not make a connection.
covered the area or if a mountain pushed its way up from lower Fossils were considered to be the same as quartz crystals, or
levels. There are even clues that tell you if a rock has been brought any other mineral crystals, meaning they were either formed
in from another place. This chapter is about some of the clues with Earth or grew there later (depending on the philosophi-
found in fossils and rocks and what the clues mean. cal view of the interpreter). Fossils of marine organisms that

522 CHAPTER 21 Geologic Time 21-2