Guided Notes
Geologic History
Relative Age
Sequence of Events
Correlation Techniques
Volcanic Ash Markers
Index Fossils
Geologic Time Scale
Evolution
Radioactive Dating
6) Why do scientists correlate events
that occurred in different locations?
▪ What does correlate mean? To match
▪ Correlation: matching rocks and geologic
events in one location with rocks and geologic
events in other locations.
7) How do scientists correlate rock
outcrops?
1) Walking the outcrop: matching nearby
rock layers that appear to be the same type
and thickness.
▪ Outcrop: surface of exposed bedrock
7) How do scientists correlate rock
outcrops?
• The Grand Canyon provides excellent visuals to
show how layers of rock can be correlated by
walking the outcrop. You can see that the layers
match up across the canyon.
Grand Canyon Rock Layers
✓ Checkpoint – Walking the Outcrop:
1. Correlate the rock layers by drawing an arrow from
Outcrop A to the matching rock layer in Outcrop B.
2. The sandstone layer is present in Outcrop A but
not in Outcrop B. What may be a cause for these
missing layers?
7) How do scientists correlate rock
outcrops?
2) Comparing Properties: matching the
color, texture, and composition of the rock
strata
7) How do scientists correlate rock
outcrops?
• Using an arrow, match the layers from Location X to
Location Y. Then, match Location Y to Location Z.
Finally, match Location X to Location Z.
7) How do scientists correlate rock outcrops?
▪ To determine which layers are missing due to an
unconformity, match the layers in the outcrop in
sequence.
o Which layer is represented by the unconformity in
Location Z? __________________________________
7) How do scientists correlate rock outcrops?
• You can determine the oldest layer of all of the
columns. It will be the bottom layer that does not
correlate with another rock outcrop.
o Which rock layer is oldest? ____________________________
7) How do scientists correlate rock outcrops?
• You can determine the youngest layer of all of the
columns. It will be the top layer that does not
correlate with another rock outcrop.
o Which rock layer is youngest? _____________________
7) How do scientists correlate rock
outcrops?
• Volcanic Markers: volcanic eruptions
leave a layer of ash.
• Why would this be so effective in correlation
of rock strata?
Represents an event that took place rapidly and
covered a large area. We can use this to
pinpoint the age of the ash layer.
7) How do scientists correlate rock
outcrops?
7) How do scientists correlate rock
outcrops?
4) Index Fossils: fossils of organisms that
lived over a wide geographic area for a
relatively short period of time.
7) How do scientists correlate rock
outcrops?
(1) Which fossil appears to be the best index
fossil in the diagram below?
The shell in the 2nd layer from the top. It is only in
this layer and it is found in every column
7) How do scientists correlate rock
outcrops?
(2) Which letter best represents an index fossil
C
on the diagram below? ____
✓ Checkpoint – Correlation Techniques
(1) The cross sections below represent three widely separated
bedrock outcrops labeled A, B, and C. Letters W, X, Y, and Z
represent fossils found in the rock layers.
Which fossil could best be used as an index fossil?
a. W
b. X
c. Y
d. Z
✓ Checkpoint – Correlation Techniques
(2) The map below shows the distribution of ash across the United
States as a result of the May 18, 1980 volcanic eruption of Mount St.
Helens.
Volcanic ash deposits such as these are usually excellent geologic time
markers because they
a. occur at regular time intervals
b. spread over a large area in a short amount of time
c. represent a time gap in the rock record
d. contain index fossils from different time periods