Download Lesson 4: What is erosion?

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Lesson 4 Summary
Use with pp. 276–281
Lesson 4: What is erosion?
Erosion is the movement of materials
away from one place. It is a destructive
process. Deposition is the placing of the
materials in a new place. Deposition is a
constructive process. Erosion and deposition
together create valleys, deltas, and sand
dunes.
Gravity is the main force causing erosion.
Gravity pulls rocks and dirt downhill. This
is called a landslide. A landslide is one kind
of erosion.
Gravity also causes rivers to flow. Rivers
pick up and carry sediment as they flow
downhill. The sediment can erode riverbeds
and form deep canyons.
Rivers also cause deposition. A river flows
much slower when it meets an ocean. This
causes the river to deposit heavy sediment
like gravel and sand. The sediment create a
delta.
Glaciers are big bodies of ice that can
cause erosion. Gravity pulls glaciers down
along a valley. Rocks beneath the glaciers
are broken down into sediment. The
glacier moves the sediment downhill. This
can make the bottom of a valley more
U-shaped over time.
Waves are a source of erosion and
deposition. Waves from storms or tides
crash against rocks along the coastline. The
rocks break. Sand and gravel in the waves
acts like sandpaper. They wear down the
rocks even more. This is how some of the
sand on beaches is created.
Not all parts of a shoreline are eroded
at the same rate. Harbors and inlets form
when some areas erode more quickly than
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Chapter 9, Lesson 4 Summary
others. A harbor is an area that is protected
from ocean waves. Waves can make caves
when parts of a cliff erode more quickly
than other parts of the cliff.
Waves push sand when they hit beaches.
A spit is a narrow piece of sandy land. A
baymouth bar is like a spit. But it forms
across a bay. Barrier islands form along
coastlines. Erosion can move barrier
islands.
The wind can move dust, dirt, or sand
from one place to another. This dust and
sand can blow against a rock. Tiny bits of
the rock might break off and blow away.
This is erosion.
The wind deposits large, loose amounts
of sand to create sand dunes. Wind pushes
sand up one side. This sand will move over
the edge of the dune’s top. The sand will
pile up until gravity pulls it down. This
creates a steeper slope than the one that
faces the wind.
Winds can move a sand dune. The wind
will pick up sand from one side of the dune
and deposit it on the other side. This causes
the dune to slowly move in the direction of
the wind.
Wind erosion can cause problems on
farms. If bare, plowed fields become very
dry, the wind can blow the topsoil off
the fields. Topsoil is the best kind of soil
for growing crops. It cannot be quickly
replaced.
Farmers try to prevent wind erosion by
planting trees along the edges of fields to
block some of the wind.
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Erosion and Deposition
Quick Study
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Lesson 4 Checkpoint
Use with pp. 276–281
Lesson 4 Checkpoint
1. Suppose sand, gravel, and clay are being carried by a river. As
the water enters a lake and slows down, in what order will these
sediments settle out of the water? Explain why they settle out in
this order.
2. Describe how waves, currents, tides, and storms affect the geological
features of the ocean shore (beaches, barrier islands, inlets, and
harbors).
3. Define erosion, and tell how gravity works with water, ice, and wind
to cause erosion.
4. How does a delta form?
© Pearson Education, Inc. 5
5. How do sand dunes form? Why is one side of a dune different than
the other side?
Quick Study
Chapter 9, Lesson 4 Checkpoint 73