Chapter 18 Fossils and Geologic Time
Chapter 18 Fossils and Geologic Time
Chapter 18 Fossils and Geologic Time
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Figure <strong>18</strong>-9 Correlation.<br />
HOW DO GEOLOGISTS CORRELATE ROCK LAYERS? 435<br />
range, but restricted in time by rapid evolution. Most rocks,<br />
even sedimentary rocks, do not contain abundant fossils. A<br />
geologist must look hard to find fossils that can be used to<br />
correlate rocks from one location to another.<br />
Figure <strong>18</strong>-9 shows how rock types <strong>and</strong> fossils can be used<br />
in correlation. If locations I <strong>and</strong> II are not too far apart, the<br />
following inferences can be made. Layers A, B, <strong>and</strong> C are<br />
probably the same age as layers 1, 2, <strong>and</strong> 3. The rock type <strong>and</strong><br />
the sequence are the same. Layers D <strong>and</strong> 4 not only are the<br />
same rock type <strong>and</strong> occur in the same sequence, but they contain<br />
the same fossil. <strong>Fossils</strong> indicate that layers G <strong>and</strong> H correlate<br />
with layers 5 <strong>and</strong> 6, although G <strong>and</strong> 5 are not the same<br />
kind of rock. (Fossil correlation is usually more dependable<br />
than correlation by rock types.) It appears that layers E <strong>and</strong><br />
F are missing at location II. E <strong>and</strong> F may not have been depositated<br />
at location II or they may have been eroded This<br />
would indicate an uncormity between layers 4 <strong>and</strong> 5 at location<br />
II. Furthermore, it is likely that the oldest rock layer in<br />
both locations is the schist at the bottom of location II.
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