Gorgeous Erosion

Source:

Adapted from The Den of Airlie Expedition Pack (produced by Wildwood Designs and SNH)

Time

1 hour or more

You will need

• A sandy river bank with an S bend in the river
• A gorge further upstream, with a viewpoint, ideally crossed by a bridge.

Did you know?

During the last ice age the ice covering many parts of Scotland was up to 1km thick. Scotland has been glaciated many time in the last million years. Each glacial episode changed the shape of the land. The landforms left by the last glacial period, which ended about 10,000 years ago, are the most obvious. Knockan Crag National Nature Reserve is renowned internationally as one of the most important sites for understanding how the landscape of Northern Britain was formed.

Before the activity

Show the class a map of how Scotland looked in the last ice age (see downloads below)

The activity

Find a suitable spot where the class can gather by the river bank and see an 'S' bend in the river. Watch the river for a time and see if they can work out whether the water travels faster on the inside or the outside of the bend. See if they can see how the water cuts into the far bank on the outside of the curve.  What is happening on the inside of the curve? Can they spot that the sediment load in the river is being dropped there, because the water is flowing more slowly (on the inside of the curve there may be a shallow beach rather than the steep cut away bank on the far side). Explain that the force of the water is causing Erosion on the far bank. The sand and gravel washed away by erosion further upstream is carried down and deposited where the water moves more slowly.

If the far bank was made of solid rock, would the river cause erosion then? Can the water cut through solid rock? See what suggestions the class comes up with (they might suggest that the water could wear the bank smooth). Ask them to hold a small rock in the river, how long would it take the river to wear it down? Then ask them to try rubbing two rocks together. Has either rock been affected. Then put some wet sand on one of the rocks and rub the second one on it again. Wash of the sand, has either of the rocks been affected? 

Travel further upstream and find a viewpoint or bridge over-looking a gorge. How was the gorge made? Remind the class that Scotland used to be covered in ice 100's of metres thick. Try to get them to imagine a layer of ice towering above their heads. Read them the following passage:

.....It has been freezing for centuries but it's getting warmer now. Not so long ago the ice was hundreds of metres thick here, pressing down on the land......Now the ice is melting. The rivers are growing, spreading across the land , carving new channels overnight as the flow increases. The water isn't clear - it's the colour of red clay, thick as soup with rock dust and gravel. Rocks are smashing against each other as they are rolled along the river beds adding their fragments to the flood. The loaded water bites deeper every day. The gouging slows as it hits bedrock - but not for long - then the rock starts to wear away....

Recap that the gorge was carved, not by the river you see today but when glacial melt-water loaded with sediment and rocks was channelled down weaknesses in the rock or over relatively soft rock. Does the class think that the sides of the gorge are made from hard rock or soft rock?

Suggested Follow up

Find out more about the many landscape features which  are the clues to Scotland's geological past. See the Activity Rock Detectives in this section and visit the classic sites section of the Scottish Geology website (see below)

Visit Knockan Crag National Nature Reserve - renowned internationally as one of the most important sites for understanding how the landscape of Northern Britain was formed. The far northwest of Scotland is one of the oldest landscapes in Europe, the rocks here tell of ancient oceans, vast deserts and ice sheets. The story of the landscape is told using poetry, sculpture, interactive demonstrations and cartoons. 

Visit other National Nature Reserves with interesting geological features including Staffa, the Isle of May and Beinn Eighe (see the 'Where to go' section for more details). 

Contact your local RIGS (Regionally Important Geological and Geomorphological Sites) group for information on visiting sites near you (see additional information below)

Downloads

• Map of the ice cover over Scotland during the last ice age
• Scottish Geology Website 
• Classic geological sites in Scotland
• Map of the ice cover over Scotland during the last ice age
• Knockan Crag National Nature Reserve website
• Fun geology-based activity sheets

Additional Information

• Advances series posters from SNH - Posters and teachers notes on landscapes and the natural heritage
• Landscapes fashioned by geology series - SNH Publications
• RIGS groups contact information

Curriculum Links

science

Age Range

2,3,4