Chapter 18 Fossils and Geologic Time

434 CHAPTER 18: FOSSILS AND GEOLOGIC TIME
ogists make decisions about these issues. Correlation is an
important part of drawing geologic maps such as Figure 18-8.
Geologic maps can be used to establish the geologic history of
an area.
Correlation is also useful in locating natural resources
such as petroleum (crude oil), mineral ores, and groundwater.
These resources are often found by drilling to a particular
level in the rock sequence. Study of rock fragments from the
drill hole and knowledge of the local strata can help geologists
determine whether the drill is above or below the level
at which the resource would most likely be found.
The first step in correlating rock layers in different locations
is to find outcrops of bedrock. In most places, soil and
sediment cover the rocks. An outcrop is a place where the solid
lithosphere is exposed at Earth’s surface. In some places,
bedrock is exposed by erosion such as on the steep bank along
a stream or river. Fresh outcrops are also common at road cuts
where making the road straight or level required the crew to
drill and blast solid rock. Sometimes builders expose bedrock
when they level building lots to construct homes or other
structures.
Comparing Fossil Types
Correlation is usually accomplished by comparing rock types
or fossils in different locations. The fact that two places have
the same kind of rock does not mean they are the same age.According
to the principle of uniformitarianism, the processes
and events that make different kinds of rock have occurred
throughout geologic history. Furthermore, at any given time,
different kinds of rock are made in different environments all
over the world. However, if geologists observe the same sequence
of rock types in two nearby locations, there is a good
chance that they are observing rocks of the same age in both
locations. The probability that the layers correlate is even
greater if they can follow the rock units from one outcrop to the
next. Geologists try to do this when they create geologic maps.
Fossils are very useful in correlation, especially if a geologist
can use index fossils that are widespread in geographic