Geology – Blog About Britain

Scilly Geology

If you had a go at Monday’s Discovering Where quiz, on islands and island groups, you will hopefully have discovered the location of the Scilly Isles. At just over 25 miles beyond Land’s End in Cornwall, they contain the most southerly point in the British Isles.

The Scilly Isles are largely made of granite rock. This is an igneous rock, which means that it formed when liquid rock cooled down and solidified. It wasn’t a volcano. The liquid rock stayed underneath the ground surface and cooled relatively slowly, allowing crystals to form.

This is one piece of rock, but can you see 3 different colours in it? There are at least 3 different types of crystals in there. The white and pink colours are quartz and feldspar, while the tiny black pieces are crystals of mica.

Although it is softer than the granite on mainland Britain, the granite on Scilly is still a relatively hard rock, so the wind and rain wear it away very slowly.

There are lumps of it sticking up all over the place.

The edges have been rounded by the wind and rain gradually wearing it away over a long period of time.

Weaknesses in the rock also wear more easily resulting in deep cracks and crazy shapes. This one is called the tooth.

If you are familiar with Dartmoor, with its hill tops littered with granite tors, then Scilly is much the same, except the granite tors are right by the sea.

Go and have a look at some granite rock and see if you can see the crystals for yourself. If there isn’t any nearby in the natural landscape, try the DIY store or garden centre – granite is often used for kitchen worktops or decorative paving.

Here’s Where It All Began

Each Thursday Blog About Britain will be re-running posts from a previous series. This was my earliest post, with enlarged images. Enjoy!

These are the Ochil Hills, Clackmannanshire, and they just happened to be the view from my camper on a very chilly November morning, when I decided to stop talking about Blogging and start collecting the information to put it into action.

Why do these hills suddenly rise out of the surrounding lowland?

The hills are made of volcanic rock and that same rock is hidden deep beneath the lowland, under layers of younger rock.

Due to pressures within the earth, a huge crack developed. This is called a fault. The land on the north side of the fault was pushed upwards. Any younger rocks that were on the north side have long since been worn away, leaving the Ochils as you see them today – low hills with an abrupt edge.

I’ll leave you with a picture from the hills looking back at the lowland.

I’ve plenty more to tell you about these valleys and the villages that they lead down to, so look out for the next Clackmannanshire post next Thursday.

A Canal through Limestone?

Limestone is a permeable rock, since it has cracks through which water can pass.

The Sapperton Tunnel, on the Thames and Severn Canal, tunnels through the limestone rock so that the canal can pass through the Cotswold Hills.

The 3½ km tunnel took 5½ years to build. Along the line of the route, 25 shafts were dug down from the ground surface and the tunnel was excavated from the bottom of these, until they were all connected together. Once the alignment had been checked, the tunnel was enlarged to the correct size.

Before water could be allowed in, the channel had to be made watertight. This was done by lining it with clay, an impermeable rock that doesn’t let water through. The channel was then finished with brick and the sections that were not through solid limestone were also given a brick roof.

Water could then be pumped in to this section of canal, enabling it to be used by boats.

The problem was that the rainwater soaking down into the permeable Cotswold Hills was leaking into the tunnel. This provided an additional water supply in winter, but in summer the springs dried up. Where the spring had pushed a hole through the clay canal lining, the canal water could then leak out through the same hole.

More water was needed to make up for leakage than to transport boats through the locks. The wind pump, by the Thames source, was replaced by a steam engine pump, which was used in the drier summer months, typically from June to October, and could deliver 3 million gallons a day into the canal.

There was a constant need for repairs to deal with the holes. Sections were eventually lined with concrete, but that was over 100 years after the canal first opened. Meanwhile loads of water had been taken from the ground near the source of the Thames. It had leaked back into the ground but not at the same place.

The Thames and Severn canal was closed in 1933, due to the need for repairs, and the Sapperton Tunnel is now impassable as the roof has collapsed in a number of places. You can find lots more information, including diagrams and pictures, on this page of the Cotswold Canals website.

Water shortages caused problems for boats both in the canal and in the river. Join me next Friday to find out more about travel on the Thames.

Springs in the Cotswolds

The River Thames once began as a spring, in this field, on the lower slopes of the Cotswold Hills.

The Cotswolds are made of limestone, the same type of rock that we encountered in Wharfedale, where the river Wharfe disappears into the ground. On the Cotswolds you won’t find much in the way of rivers for the same reason. Limestone is full of cracks, enabling rainwater to sink into the ground. It is a permeable rock.

The water flows down through the permeable limestone until it can’t go any further, either because the rock has changed and no longer has holes, or because the holes are already full. The water will then flow sideways, out of the hillside, forming a spring, the source of a surface stream.

So if there hasn’t been rain in a while, water stops flowing out and the spring dries up.
But where the Thames is concerned, that is only part of the story. Find out more next Friday.

So where to begin…?

Well I am going to start here.

Ochil Hills from Tullygarth

Why do these hills suddenly rise out of the surrounding lowland?

Ochil Hills at Alva

The hills are made of volcanic rock and that same rock is hidden deep beneath the lowland, under layers of younger rock.

fault diagram

I’ll leave you with a picture from the hills looking back at the lowland.

Mill Glen, Ochil Hills

I’ve plenty more to tell you about these valleys and the villages that they lead down to, so look out for the next Clackmannanshire post next Wednesday. Meanwhile on Friday I’ll post about a different part of Britain.