The Grand Canyon

A water created landform

The WOW Factor

Why is the Canyon Grand?

• The River rises at 5,000m and so has great potential energy

• The Colorado Plateau has risen 3,000 m in the last 5 million years lifting the river above its base level

• The sedimentary rocks are easily eroded• The discharge is variable and at times very high• The local climate is very arid reducing vegetation

and available moisture

The v shaped valley• Vertical erosion depends on

– The height above base level– The resistance of the rock– The energy of the stream

• Slope erosion depends on– The moisture available– The resistance of the rock– Permeability of the rock

The oldest, most resistant rocks produce the steepest valley sides

Less resistant rocks like shale give gentle slopes

Taken from Landmark AS Geography Prosser, Raw and Bishop, Published by Collins

Limestone – permeable, resistant,

Shale – impermeable, easily weathered,

Coconino sandstone – very resistant,

Mixed sandstones and shales- some more resistant than others

The Side Valleys

• Faults such as Bright Angel are easily exploited by water

• The gradient of side valleys is very steep

• Occasional rainstorms cause sheet-wash and free sediment

Bright Angel Fault is a point of weakness

Redwall Limestone

Shale

Study the Cross-section through the canyon

a.     Describe the shape of the canyonb.     Look at the exposures of limestone on the valley sides. Do they produce steep cliffs or gentle slopes?c.     What rock type consistently produces relatively gentle

slopes on the valley sides?d.     Most of the beds of rock are horizontal. Do you think this has affected the profile in any way?e.     Why do you think the valley at the base of the Canyon is

narrower than the canyon higher up?f.      How has the climate of the area encouraged the formation of the canyon?