More than Sediment
By Trista L. Pollard
Geologists have learned that sediment composition depends on its source
and that it changes as it is transported. Clastic sediments have very distinct
physical characteristics. These characteristics come from the way the
sediment was carried to its deposition area. As you have learned, sediment is
transported by agents such as water, ice, and wind. It is also moved by the
effects of gravity. Depending on the erosion rate or speed, the size of the
sediment particles that can be carried and the distance they are carried will
vary. Scientists have found that sediment characteristics are determined by
the distance it travels and the agent that moves the sediment.
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Sorting and angularity are two factors that affect the characteristics of
sediment. As sediment is transported by currents of air or water, the
sediment is separated according to size. This is called sorting. Sediment that
is very well sorted has grains that are all roughly the same size. Moderately
sorted sediments have grains that are roughly two different sizes. Sediment
that is poorly sorted contains grains of many different sizes. Geologists
have also categorized sediment as well sorted and very poorly sorted. It's
the change in the speed of the agent (wind, ice, water, etc) that determines
the way sediment will be sorted. Imagine what happens when fast-moving
streams enter lakes. As the water from the stream combines with the water in
the lake, its speed slows down sharply. Once this occurs, the larger grains of
sediment become too heavy for the current to move. These are the sediment
grains that are deposited out of the water first. As you may have guessed, the
finer grains of sediment stay in the current for longer periods than larger
grains. As a result, finer grains of sediment are usually found farther from
shorelines. These finer grains also have a tendency to be deposited on
coarser or heavier sediment grains.
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Angularity describes the texture of sediment after it has traveled from its
source. As sediment grains travel, they collide with other sediment particles
and other objects. This action will change the shape and size of the particles.
Before sediment particles are transported, when they have first broken off
their source rock, their shapes tend to be angular and uneven. Sediment
particles that have traveled farther from their source tend to be smoother,
more rounded, and finer. Now that we have looked at sediment
characteristics, let's look at the characteristics of sedimentary rocks.
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Geologists refer to the final destination of sediment as the depositional
environment. These environments are rivers, beaches, deltas, and oceans.
Scientists have found that each depositional environment has different
characteristics. Due to these differences in characteristics, these
environments produce sedimentary rocks with their own special features. In
fact, there are seven special features of sedimentary rocks. Stratification is
the layering of sedimentary rock that happens when there is a change in
sediment type or the depositional environment changes. Beds are the actual
stratified layers that geologists observe in sedimentary rocks. Depending on
how long it takes for the sediment to deposit and the amount of sediment
deposited, that will determine the thickness of the beds. If large amounts of
sediment are deposited at one time, massive beds are created. These beds
have no internal structures and may also develop when sediment with similar
qualities is deposited for long periods of time. Cross-beds are usually found
in sand dunes or river beds. They are the slanting layers that form within the
beds of sedimentary rocks. Graded beddings have sediment materials that
are various sizes and of different types that settle in one layer. The sediment
in graded bedding tends to have different sizes and shapes that exist in
different levels. In this feature, the largest sediment grains form on the
bottom and move towards the smallest grains on the top of the level. When
mudflows occur, this order may be reversed. A special type of grade bedding
called reverse grading has the smaller grain levels on the bottom and moves
towards larger grains on the top of the level.
When wind or water meets up with sand, ripple marks occur. It's the
movement of these two agents on sand that causes the ripple marks. This
feature may be preserved as sand becomes sandstone. Ripple marks also tell
geologists and other scientists that the sediment was part of a river bed or
beach in its past life. Mud cracks occur after muddy deposits in a river's
flood plain or dry lake bed have dried and shrunk. Scientists may find
preserved mud cracks in hardened solid rock in areas that have been flooded
again.
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Earlier you read about organic sedimentary rocks which were formed
from the remains of plants and animals. Ancient plants and animals may also
leave their mark on sedimentary rocks in the form of fossils. Sediments that
pile up and accumulate over time may have the remains of these organisms
buried within their layers. As the rock hardens, the hard parts of the remains
may also be preserved. Scientists have also found fossils where the hard
sections have dissolved and the impressions of the organisms are left. The
last special feature of sedimentary rocks, concretions, are lumps of rock
with compositions different from the main body of the rock. As minerals
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precipitate from water and other fluids, these minerals may build up around
a central area. For example, the groundwater that carries the minerals may
deposit these minerals inside pockets or cavities of sedimentary rocks. These
minerals crystallize, forming lumps within the sedimentary rocks. This is
also how geodes are formed.
Sedimentary rocks have a tremendous amount of information in their
layers, including seven special features. These features help geologists learn
more about the Earth's surface and that there is more than sediment in those
layers.
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Name
_____________________________
Date
___________________
More than Sediment
1. Cross-beds occur when ______.
Slanting layers form within
rock beds
Horizontal layers form
within rock beds
Vertical layers form within
rock beds
None of the above
2. Based on what you have read
about the sorting categories, how
would describe the well sorted
and very poorly sorted
categories?
3. Explain the concept of angularity. 4. Why is it important for scientists
to understand the characteristics
of a depositional environment?
5. Compare fossils with concretions. 6. Ripple marks are found most
often in what type of depositional
environment?
Name
_____________________________
Date
___________________
More than Sediment
7. The agents that transport
sediment are wind, air, ice, and
gravity.
False
True
8. ______ form on rivers' flood
plains or in the dry beds of lakes.
Ripple marks
Concretions
Fossils
Mud cracks