Shropshire Routes to Roots

3. Geological forces

Continental drift

All the continents of the Earth rest on what are known as "Tectonic Plates". These are simply huge masses of compressed rock, floating on the underlying molten rock at the centre of the Earth, known as "Magma". They do not float around as we think of floating; rather they creep along, over millions of years. The speed has been estimated at 1 kilometre in 50,000 years, or about 2 centimetres in a year!

Sometimes they bump into other continents. In the case of the Indian sub-continent for example, which is still moving north into Asia, the power is so great that it raised, and is still raising, the Himalayan Mountains. Of course we have nothing as severe as that in Shropshire; but that same, slow, irresistable force has warped and twisted the rock beneath us. In places, such as the Long Mynd, it has tilted the original level bedrocks into vertical beds which can be seen in the rocky outcrops of valleys such as Ashes Hollow and Carding Mill.

Activity:

• Can you find somewhere near where you live, where the rocks are showing, and draw a picture of how the beds have been tilted?

Volcanic activity

Although Shropshire has its fair share of volcanic rocks such as the Wrekin, Earls Hill and Corndon, these hill were never volcanoes, even though they may look as if they were.

They are part of a great lava flow that once covered the area, which has been lifted up and warped by the tectonic forces described above. Their shape has more to do with the fact the rock is very hard and resists weathering far better than the surrounding softer beds.

Volcanic Rocks of the Wrekin Larger image, in a new window [37kb] [Reproduced with kind permission of Johnnie Atherton]

Activity:

• Find out how lava is formed.
• How many types of lava are there?
• How might different types of lava create a different landscape?

Faulting and earthquakes

There may not have been volcanoes in Shropshire, but there has always been some form of earthquake activity. Where two layers of rock are moving in different directions to each other, there is a very great build up of stress, and this is known as a "Fault Line". When this pressure reaches a critical level, the two sides suddenly jerk past each other. This is what we feel as an earthquake. It is a sign that the Earth is constantly on the move.

Folding and fault in America Larger image, in a new window [70kb] [Reproduced with kind permission of Johnnie Atherton]

Fault lines are to be found all over Shropshire,with a major one running up through Church Stretton. A fault can be seen most easily where there has been some quarrying. You can recognise it by the sudden change in the continuity of the rock.

At 2.40pm on 2nd April 1990, Shrewsbury experienced an earthquake, measuring 5.4 on the Richter Scale (Measure of force). Although this dislodged a few chimney pots, made the radiators rattle and set the dogs barking, its force was very minor compared to those experienced in more active areas.

Activity:

• Find out about the Richter Scale.
• Why is an earthquake measuring 7.3 nearly 100 times more powerful than one of 5.4?

Continue

Return to top of page