5  Landslips

Landslips

Task Site 5 is a good place to see the third element that influences the landscape here – the presence of major landslips. The most distinctive of these occur below the Darens, where the landslips disrupt the smooth valley sides. You can see this on a clear day from Cat’s Back.

 

Landslip into Olchon Valley

The picture illustrates the slide of material into the Olchon Valley, leaving behind a large scar in the face of the Black Darren.

 

Features of the landslip

At Black Darren, the landslip exhibits a distinct headscarp (steep cliffs), immediately below which there is a depositional sequence of scree, ridges, and hummocky ground.

 

Boulder field

Extensive area of boulders which have fallen from the cliff face above. The boulders would have broken away due to freeze-thaw action, whereby water penetrates the rock, expands on turning to ice and forces the rock apart. Often this process repeats many times until eventually the rock is split apart. Freeze-thaw action would have occured a lot under periglacial conditions, but is still at work during winters today. The boulder field in this picture is below the cliff face at Red Daren.

 

Ridge formed by fallen blocks

The biggest of the ridges has been enhanced by the accumulation of falling rocks at the foot of a former semi-permanent snowbank.

 

Snow bank in winter

Even nowadays, in the winter, rocks continue to fall as the back wall / head scarp freezes up, as it stays in the shade all day when we have a low winter sun.

 

 

Hummocky ground

Further downslope, the landslip terrain becomes more chaotic and consists largely of mounds.

 

Broad apron of landslip

Finally, below the mountain road, the foot of the landslip is visible as a broad apron, indicative of long-travel processes such as soil and rock flowage.

 

Summary of features

The Black Darren landslips, together with the smaller one at Red Daren, as well as similar features in the Black Mountains, are all primarily a consequence of the geological set-up, where strong rocks (the Senni Formation) overly weaker strata (the Freshwater West Formation, and the slopes have been over-steepened by former glacial action.

 

Task Site 5 Question

Which of the following are key features in creating the landscape of the Black Mountains?

a) rocks of differing erosion resistance
b) glaciation
c) landslips
d) all of these