Laboratory Manual for Introductory Geology 4e

Exercise 6.3 will help you practice identifying rock samples by using these steps.
Remember that you can look to either the table or the chart for help. Both present
the same information, so use whichever style you are more comfortable with.
EXERCISE 6.3
Identifying Sedimentary Rock Samples
Name:
Course:
Section:
Date:
Examine the rock samples in your set that you classified as sedimentary rocks. Read Section 6.4 closely to remind yourself
of the steps. Fill in the study sheets at the end of the chapter to identify each sample. Keep these samples and your study
sheets until you have finished the chapter. At that point, you will be able to add an interpretation of the histories of the rocks.
6.5 Interpreting Clastic Sedimentary Textures
Identifying a sedimentary rock is the first step in learning what it can tell us about the
processes and conditions of its origin. The texture of the rock is a rich source of information
about the geologic history that eventually led to the deposition of the sediment
from which the rock formed. Specific aspects of texture—grain size, sorting, and grain
shape—provide clues to the agent that transported the sediment (streams, glaciers,
or wind?) and the amount of weathering that the sediment experienced between the
time it eroded from its source to the time it became buried at the site of deposition.
6.5.1 Grain Size and Sorting
Clasts in sedimentary rocks range from the size of a house to specks so small they
can’t be seen without an electron microscope. As noted in Table 6.1, geologists use
familiar words such as sand and pebble to define clast size. The size of grains in a rock
reflects the kinetic energy of the agent that transported them—an agent with a lot
of kinetic energy can move bigger clasts than one with less.
The kinetic energy of a transporting agent depends on both its mass and its velocity.
Air has much less mass than running water, so at the same velocity, a stream can
move larger particles than air. Similarly, a small stream flowing at the same velocity
as a large river has less kinetic energy (less mass) and therefore cannot move clasts
as large as those that the river can.
Glaciers are the only solid transporting agent. Even though rivers flow much faster
than glaciers, glaciers transport blocks of material far larger than those that even the
largest river can move. There are two reasons for this: first, the mass of a glacier is
enormous; second, its solid nature provides a permanent strength absent in running
water. Thus, even though glacial ice moves slowly, it can carry debris weighing tons.
The sorting of a clastic sedimentary rock is a measure of how uniform its grain size
is (FIG. 6.6). Sorting shows the degree to which clasts have been separated by flowing
currents, and it can also help identify the transporting agent. For example, aeolian
(wind-deposited) sediment is very well sorted because wind can pick up only clasts
of a narrow size range. A fast-moving, turbulent stream can carry clasts ranging from
mud- to boulder-sized. But as a stream slows gradually, particles of different sizes settle
out progressively. The coarsest grains (boulders and cobbles) drop out first, then
the medium-sized grains (pebbles and sand), and only when the water is moving very
slowly can silt and mud settle out. Thus, streams do sort clasts, but not as well as wind.
Glacial ice is solid, so it can carry clasts of all sizes. As a result, glacially transported sediments
are very poorly sorted or not sorted at all. Exercises 6.4 and 6.5 show how much
of a sedimentary rock’s history can be interpreted from the size and sorting of its clasts.
6.5 INTERPRETING CLASTIC SEDIMENTARY TEXTURES
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