Landslide!

Before the break we travelled across the slopes of Mam Tor, Derbyshire, to see what we could learn about the structure of a landslide from the new position of the wrecked road.

Here the rock layers get lubricated when there is a good dose of rain and everything is steadily sliding down the hillside, often in bursts, though not particularly dramatically, at an average rate of ¼ metre per year.

At the bottom of Mam Tor everything comes to rest in a jumbled mess and the pile of debris helps to slow down the rate of movement on the rest of the slope.

But what happens when the debris is being removed by erosion.

The village of Overstrand, Norfolk, is located on the top of 35-metre-high cliffs.

These cliffs are formed from loose sands and clays, which are easily broken and washed away, and at the bottom of the cliffs, doing just that, is the sea.

As the sea washes material away from the base of the cliffs the steep slope becomes unstable, resulting in rotational landslides and mudflows. Between May 1990 and January 1994, three landslides caused the cliff edge to move 90 metres inland, taking out the coast road and threatening properties.

Click here to see a map of the area. The coast road now curves into Clifton Way. Can you guess where it used to go?

The removal of debris from the base of the cliff, by the sea, is allowing more rapid landscape change than at Mam Tor, where the toe of the landslide is helping to hold up the slope.

At Mam Tor there’s only farmland sliding down the hillside, but at Overstrand there are around 1000 people living on top of the cliffs. Not surprisingly the council was under pressure to “do something”.

We’ll take a closer look next week.