Chapter 18 Fossils and Geologic Time

432 CHAPTER 18: FOSSILS AND GEOLOGIC TIME
• In the column labeled “Time Distribution of Fossils,”
thinner vertical lines indicate when certain fossil
organisms lived. Notice the creatures labeled A-Z on at
the top of the page. These letters appear on the vertical
lines to show when these organisms were alive. For
example, trilobites A, B, and C lived at different times
during the Paleozoic Era.
• The maps at the right side of the page show plate
tectonic motions of the continents. Specific positions of
North America are indicated. The maps also show how
North America was positioned with respect to other
continents at five specific times in the past.
New York Bedrock
Figure 18-8 is a geologic map from the Earth Science Reference
Tables. New York’s oldest rocks are Precambrian. These
rocks are exposed in the Adirondacks of northern New York
State and the Hudson Highlands between New Jersey and
Connecticut. Paleozoic sedimentary rocks extend westward
from these regions to lakes Erie and Ontario. Long Island has
the youngest “bedrock.” It is composed of geologically recent
sediments of glacial origin. Note that the absolute age of
these bedrock regions can be determined by using this map
along with the geologic history chart in Figure 18-7, also
found at the center of the Earth Science Reference Tables.
HOW DO GEOLOGISTS CORRELATE
ROCK LAYERS?
Geologists often compare rock layers in two or more locations
to determine whether they are the same layers, or their
relative age. Matching bedrock layers by rock type or by age
is called correlation. The principle of superposition (explained
in Chapter 17) and the use of index fossils help geol-