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When a fault cuts through other rocks, or when magma intrudes and crystallizes, we can
assume that the fault or intrusion is younger than the rocks affected. This is the principle of
cross-cutting relationships.
Sometimes inclusions can aid the relative dating process. Inclusions are pieces of one
rock unit that are contained within another. The basic principle is logical and straightforward.
The rock mass adjacent to the one containing the inclusions must have been there first in order to
provide the rock fragments. Therefore, the rock mass containing inclusions is the younger of the
two.
Text 2
Relative Dating—Key Principles
When we observe layers of rock that have been deposited essentially without interruption,
we call them conformable. Particular sites exhibit conformable beds representing certain spans
of geologic time. However, no place on Earth has a complete set of conformable strata.
Throughout Earth history, the deposition of sediment has been interrupted again and again. All
such breaks in the rock record are termed unconformities. An unconformity represents a long
period during which deposition ceased, erosion removed previously formed rocks, and then
deposition resumed. In each case, uplift and erosion are followed by subsidence and renewed
sedimentation. Unconformities are important features because they represent significant geologic
events in Earth history. Moreover, their recognition helps us identify what intervals of time are
not represented by strata and thus are missing from the geologic record.
ANGULAR UNCONFORMITY. Perhaps the most easily recognized unconformity is
an angular unconformity. It consists of tilted or folded sedimentary rocks that are overlain by
younger, more flat-lying strata. An angular unconformity indicates that during the pause in
deposition, a period of deformation (folding or tilting) and erosion occurred.
DISCONFORMITY. When contrasted with angular unconformities, disconformities are
more common but usually far less conspicuous because the strata on either side are essentially
parallel. Many disconformities are difficult to identify because the rocks above and below are
similar and there is little evidence of erosion. Such a break often resembles an ordinary bedding
plane. Other disconformities are easier to identify because the ancient erosion surface is cut
deeply into the older rocks below.
NONCONFORMITY. The third basic type of unconformity is a nonconformity. Here
the break separates older metamorphic or intrusive igneous rocks from younger sedimentary
strata. Just as angular unconformities and disconformities simply crustal movements, so too do
nonconformities. Intrusive igneous masses and metamorphic rocks originate far below the
surface. Thus, for a nonconformity to develop, there must be a period of uplift and the erosion of
overlying rocks. Once exposed at the surface, the igneous or metamorphic rocks are subjected to
weathering and erosion prior to subsidence and the renewal of sedimentation.

Task 3. Answer the following questions, using the vocabulary from Task 1.
1. What does relative dating mean?
2. What is Nicolaus Steno credited with?
3. What does the law of superposition state?
4. What does the principle of original horizontality state?
5. What is the principle of cross-cutting relationships about?
6. In what way can inclusions aid the relative dating process?
7. Distinguish between numerical dates and relative dates.
8. What is the significance of an unconformity? What does an unconformity represent?
9. Why are unconformities important features?
10. Distinguish among angular unconformity, disconformity and nonconformity.

Task 4. Look at Figure 4.1. It represents cross-cutting relationships as an important
principle used in relative dating. Look at the figure and say: