Chapter 17 Unraveling Geologic History

388 CHAPTER 17: UNRAVELING GEOLOGIC HISTORY
Inclusions
zontal. Diagram 2 shows a sequence of layers that are horizontal
in spite of the curvature of the basin. Diagram 3 represents
an outcrop of rock layers from the depositional basin. In
diagram 4, the layers have been pushed up on one side (or
down on the other) to create tilted layers in the outcrop. The
point is that when you see tilted layers like those in diagram
4, you can infer that they started as flat, horizontal layers of
sediment that later were tilted.
Geologists sometimes find that bedrock contains and surrounds
pieces of a different kind of rock. An intrusion is a
body of rock that was injected into the surrounding rock as
hot, molten magma. Therefore, all intrusions are igneous
rocks. An inclusion is a fragment of one kind of rock that
is enclosed within another rock. This can happen in several
ways. An intrusion of basaltic magma following a zone of
weakness through sedimentary rocks can pick up fragments
of the sedimentary rock. When the magma solidifies it contains
and surrounds pieces of the rock it invaded. Alternatively,
consider a region of granite in which the granite
weathers to boulders of granite sitting on granite bedrock.
If the region then collects layers of sediment, the sediments
will surround the granite boulders. The result could
be sedimentary rocks that contain granite boulders. Figure
17-3 shows an inclusion of sandstone rock surrounded by
granite.
Whenever a body of rock contains inclusions of another
rock, the inclusions are older than the surrounding rock.
Some of the oldest masses of rocks found on Earth contain inclusions.
This tells us that another rock unit existed before
these very old rocks. Unfortunately, it may not be possible to
determine how much older the inclusions are than the main
body of rock, but the sequence can be inferred. Particles of
sediment can be thought of as inclusions in sedimentary
rock. Most sedimentary particles are the weathered remains
of an older rock. Therefore, grains of sand in sandstone or