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Back to gritstone and shale

Landscape from gritstone and shale

Edges

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gently sloping areas dotted with huge blocks of gritstone jumbled mass of broken rocks below the vertical face nearly vertical cliffs eroded along cracks into huge blocks

Gritstone is strongly bedded with horizontal and vertical cracks gradually enlarged over time through erosion. The edges look like they have been made up of huge building blocks – some of which balance precariously on top of others.

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Tors

The 'cakes of bread' tor above the Upper Derwent. Can you guess why?

Tors are isolated masses of bare rock varying in size.

It is not certain why tors form but they are thought to be the result of one of the following processes:

• deep chemical weathering within the rock with further weathering into weird shapes
• water freezing and thawing during periods of Ice Age, isolating tors from the bedrock beneath
• blocks of gritstone isolated by a landslip

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Landslips

The combination of gritstone, shale, rain water, gravity and dipping bedding planes can result in landslips (also known as landslides).

The alternating gritstone and shale layers of Mam Tor. The bedding planes slope down towards the old A625 road below

The most dramatic Peak District landslip is at Mam Tor, near Castleton, where the scar of the landslip towers 65 metres above the remains of the A625 road to Buxton. The A625 was the main all weather east-west route across the Peak District until 1979 when a large landslip took out the road below. It was decided that the repairs would be too costly. Nature had won the day.

In the hummocky ground, below Mam Tor and the 'broken road' lies the debris from the landslip. It is easily identified by the darker vegetation found on the more acidic gritstone/shale as compared to the bright green grasses found in the shale/limestone valley bottom.

Sheep make their way over the broken tarmac of the once busy A625, with Mam Tor above

The reason for the landslip is uncertain but it is primarily due to the geology of Mam Tor and the Losehill ridge.

Gritstone and shale have very different properties, and the alternating layers stacked one on top of the other make the steep hillside unstable. Rain water flows along the bedding planes between the gritstone and shale, eroding the shale until the mass above becomes unstable. At a critical point, the shale bed slips (a bit like greasy butter between two slices of bread).

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High Moorland Plateau

High moorland plateau to the north of the Peak District

Around the edges of the plateau are steep, rocky-sided valleys or cloughs with rivers that rush down to lower ground. Few trees now live on the plateau tops but the cloughs are often thickly wooded.

The westerly, upland location of the Peak District moorland means that rainfall is high. Although porous, much of the gritstone and shale is impermeable. This combination of high rainfall and impermeable rock leads to boggy, peat conditions that support an incredible diversity of wildlife. Follow this link for more information about the wildlife found on heather moorland and in blanket bogs.

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Wide Shale Valleys

Edale Valley is a wide open shale valley that falls away from the high gritstone plateau above

Bands of softer shale are more easily eroded and thicker beds tend to form wide open valleys. The two main shale river valleys of the Peak District are the Derwent and the Dove.

These two major rivers of the Peak District flow mainly along these shale valleys that separate the White Peak and Dark Peak. These valleys are also major transport routes.

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