Objectives:
Students will be able to

• Draw the discussed valley shapes
• List the erosional processes that form the valleys
• Match the valley shape with the erosion process that created it
• Identify the landform in photos or nature

Time:
Preparation: 30 min.
Demonstration: 1 hr.
Discussion: 20 min.

Materials Needed:

• Sand
• Water
• Large flat plastic container or box
• Blocks or bricks
• Round wooden block or soup can (3 inch diameter)
• Jar or bucket

Vocabulary:

alpine glacier
continental glacier
deposition
erosion
floodplain
glacier
hanging valley
meandering
river plain valley
U-shaped valley
V-shaped valley

Included National Parks & other sites:

Grand Canyon N.P.
Black Canyon of the Gunnison NP
Yosemite NP
Glacier info page
Research page

Background:

There are three main types of valleys:

• U-shaped valleys,
• V-shaped valleys
• River plain valleys.

Each type of valley is formed by erosion. Valleys are landforms with two valley walls and a bottom of varying width. Many have river channels along the valley floor. V-shaped valleys and river plain valleys are formed by flowing water. U-shaped valleys are formed by flowing ice, or glaciers.

V-shaped valleys have steep valley walls with narrow valley floors. In cross section it has a "V" shape. They are created by fast flowing rivers with steep gradients. As rivers flow downhill quickly, they remove more sediment from the bottom of the stream channel than from the sides. This process is called downcutting. It can be compared to a knife cutting through a cake. The resulting landform is a narrow canyon with a fast flowing river and little bank, or floodplain, on the river sides. An example of such a valley is found in Grand Canyon National Park and Black Canyon of the Gunnison National Park. For more information about stream channels and erosion go to Changing Landforms in Mid-stream.

River plain valleys have broad flat valley floors with sides of varying angles. The valley bottom is formed by a meandering river. Plain valleys are created by slow moving rivers flowing at moderate to low angle slopes. A river at this gradient tends to erode more along the sides of the channel than along the bottom. Thus, the river slowly sweeps across the valley floor, forming a broad river plain.

U-shaped valleys are formed by slow flowing ice called glaciers. Glacially carved valleys have the name U-shaped because in cross section they resemble the letter 'U'. They have a curved floor with steep sides. Floor gradient does not affect the valley's shape, it is the glacier's size that does. Continuously flowing glaciers and large sized glaciers carve wide, deep incised valleys.

Glaciers flow downhill slowly, over long periods of time due to gravity. As glaciers flow they pick up rock from the valley floor and carry it along. Glacier transported sediment scrapes against the valley floor, digging deeper and carrying more sediment, carving out the shape of the valley. There are two types of glaciers: alpine glaciers (that form U valleys) found in mountainous regions, and continental glaciers (that destroy topography) which are huge ice sheets covering large portions of continents.

More information about glaciers can be found by checking out the links on the side column. The following three activities allow students to create the valleys discussed.

Instructional procedures:

1. V-shaped valley

   1. Pile sand into a elongate container and create two large mountains with a small, sloping valley between them.
   2. Raise the mountain end of the pan up on blocks to a 25 or 30 degree angle.
   3. Pour water between the mountains.
   4. Vary the rate you pour the water to see if different valleys will form.
   5. Experiment with both wet and dry sand.

    

2. Plain valley

   1. Pile sand into two hills with a slightly broad valley between them.
   2. Elevate the hill end of the pan to a 10 to 20 degree angle.
   3. Pour water slowly in the valley.
   4. Try to pour enough water to create a meandering stream in the valley. You may need to carve a curvy "river channel" into the sand before pouring water.
   5. Experiment with both wet and dry sand.

    

3. U-shaped valley

   1. Pile sand into two connecting mountains with a small valley.
   2. Using a rounded block of wood or soup can (representing ice) slide it downhill in the valley.
   3. Be sure to explain to students that glaciers carve out valleys by scraping rocks they have picked up along the valley floor and by slowly pushing sediment down hill. Note: the resulting bulge at the base of the glacier is called an end moraine.

Discussion:

After students create the landforms, have them draw a sketch of what was formed. On the sketch, label the direction the water or ice was flowing. Label which erosional process created the landform. Why do some rivers create V-shaped valleys and some create plain valleys? Are there any river valleys in your area? If so, what kind of valleys do you find? Are there any glacial valleys in your area? If so, do you know when the glaciers that created them were there?

Variations:

Instead of using water, use your hand as the river cutting through the sand. It works great and is less messy. Make sure you do not wiggle your hand for the V-shaped valley. For the meandering streams make sure you wiggle your hand back and forth, acting like a meandering stream. For the U-shaped valley you can substitute your fist.

Extension:

Ask students to perform the same experiments with different hillslope angles, with different rates of water flow (slow or fast). What landforms resulted from the variations?