Geology Reference
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Figure 17.9
Formation of an Angular Unconformity
Deposition
Angular
unconformity
Erosion
Uplift and tilting
b
Angular unconformity at Siccar Point, Scotland. James Hutton
fi rst realized the signifi cance of unconformities at this site in 1788.
Deposition
a
Formation of an angular unconformity.
history, we will say that it intruded after the deposition of
bed Q (Figure 17.12g, h).
Following the intrusion of dike R, lava S flowed over
bed Q, followed by the deposition of bed T (Figure 17.12i, j).
Although the lava flow (S) is not a sedimentary unit, the
principle of superposition still applies because it fl owed onto
the surface, just as sediments are deposited on Earth's
surface.
We have established a relative chronology for the rocks
and events of this area by using the principles of relative dat-
ing. Remember, however, that we have no way of knowing
how many years ago these events occurred unless we can ob-
tain radiometric dates for the igneous rocks. With these dates,
we can establish the range of absolute ages between which the
different sedimentary units were deposited and also deter-
mine how much time is represented by the unconformities.
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