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Oyster shell in the Portland stone

The first stop on this part of the walk is the HSBC bank on The Cross. This building is made from Portland Stone, a white limestone from the Isle of Portland off the Dorset coast. This might not have much to do with Gloucestershire but the excellent fossils embedded within the stone provide yet more proof that the southern part of Great Britain was under water at some point in time. Portland stone formed in the Jurassic period and you can see oysters and other types of shell in various building blocks. The whole area of the South West seems to have been made up of shallow, clear, blue-green waters in a warm climate.

Discover more... Geology enthusiast Alan McKay explains about the Portland stone and the oyster shells. Watch the video: 56k Dial-up | 256k Broadband To watch video content on the ÃÛÑ¿´«Ã½ you will need to have a program called RealPlayer installed on your computer.

Walking down Northgate Street you'll see The New Inn public house on your right. This building dates from around 1430, and was built as an inn for pilgrims who came to visit the Cathedral. The Inn was home to a vast retinue of knights, gentlemen and yeoman who lodged at the Inn to pronounce the accession of Mary Tudor to the throne. Announcements such as this were usually made on The Cross, but in bad weather the Inn would have been preferable.

St. John's Spire

Continue along Northgate Street until you reach St. John's Church. Take a left along St. John's Lane and then, as you approach the back of the church, bear right along Cathedral Via Sacra. As you make your way around the path you'll see a strange sandy coloured spire which rises about 4 metres from the ground. This is St. John's Spire and rumour had it that this was the spire of a church that got swallowed up by the Earth! Sadly, this story isn't true because, as you might have guessed from the name, it was the former spire of St. John's Church. What's interesting about this is the effect of forces of nature on the Cotswold Ragstone. As you will see from closer inspection of the stone, it has weathered badly after centuries of exposure to the elements. Also the rods of iron which were inserted into stone to strengthen it turned out to have completely the opposite effect. The iron oxidised or rusted, expanding and subsequently quickening the deterioration of the stone.

Discover more... Geology enthusiast Alan McKay tells us about St. John's Spire... Watch the video: 56k Dial-up | 256k Broadband To watch video content on the ÃÛÑ¿´«Ã½ you will need to have a program called RealPlayer installed on your computer.

A short distance back the way you came is Cathedral Way (Via Sacra). If you head along this pathway you'll reach the historic cathedral. The cathedral as you see it today began life around 1089AD. It's largely constructed from Oolitic limestone, which we've already learned about. This stone was mostly quarried at Painswick and Minchinhampton. If you leave the path and approach the South Wall you'll see another excellent example of cross-bedding which tells us the direction the water was flowing at the time the stone was first being formed.

Discover more... Alan McKay talks about the features of the cathedral's South Wall. Watch the video: 56k Dial-up | 256k Broadband To watch video content on the ÃÛÑ¿´«Ã½ you will need to have a program called RealPlayer installed on your computer.

Pavement patterns

Moving on to College Green, just opposite the main entrance to the cathedral itself, the flagstones offer up a unique snapshot of natural forces in action. The slabs just outside Nos 3 and 4 College Green were probably quarried up in the Forest of Dean and are made from Carboniferous Sandstone. The ripple imprints you can see are from the sea bed some 300 million years ago. The paving stones show us two things. The dappled patterns you can see on some slabs indicate oscillation ripples that were probably the result of tidal forces in action - you can actually see these sort of patterns on beaches today. The second type of pattern are linear ripples and these were likely to have formed on a river bed where the current was flowing one way. These slabs show the action of the water on a sandy surface before it was buried and subsequently compacted over time.

Discover more... Alan points out some fascinating geological features of the pavements around College Green. Watch the video: 56k Dial-up | 256k Broadband To watch video content on the ÃÛÑ¿´«Ã½ you will need to have a program called RealPlayer installed on your computer.

Heading inside the cathedral itself, you need to make your way over to the altar. At floor level you'll see a pinky grey limestone step. If you look closely you'll see that the step is crammed full of fossils of varying sizes. These are fossils of animals called Crinoids or Sea Lilies. These creatures lived exclusively on the sea floor and date back to Jurassic times some 200 million years ago.

Discover more... Alan shows us an example of crinoid fossils inside the Cathedral. Watch the video: 56k Dial-up | 256k Broadband To watch video content on the ÃÛÑ¿´«Ã½ you will need to have a program called RealPlayer installed on your computer.

Move into the Cloisters through the eastern door from the main hall of the cathedral and follow the passageway around. The Cloisters are famous not just for the Oolitic limestone in the beautiful fan-vaulted ceilings but because movie magic transformed them into the corridors of Hogwart's School of Witchcraft and Wizardry in the Harry Potter films. Follow the Cloisters around and back into the main hall once more via the western door.

Brachiopods

Directly on your right as you pass through the western door you'll see a series of memorial plaques on the wall. The middle of the three, the Charles Rudhull Monument, offers an incredible geological sight. The black stone is Silurian limestone from the Silurian era (409-439 million years ago). This age saw a rise in sea level after the Ordovician glaciation saw many land areas inundated by shallow seas. Coral and shellfish flourished in areas of shallow water that were free of heavy deposits of mud. What's interesting about this Silurian limestone is the fact that a great number of shellfish, called Brachiopods, were encased in the limestone, trapped forever as a marker of geological time. Brachiopods were much more abundant in seas of the Silurian Period. They have a shell made of two halves, which you can see if you look closely at the memorial. They didn't move very much, which is probably why they got fossilised in such great numbers!

Discover more... Alan finds fossils of shellfish called Brachiopods inside the Cathedral. Watch the video: 56k Dial-up | 256k Broadband To watch video content on the ÃÛÑ¿´«Ã½ you will need to have a program called RealPlayer installed on your computer.

Leaving the cathedral, bear right past the Church House and through an archway to the right of the Community House. You should find yourself in Miller's Green now. A wall right next to a telegraph pole offers a selection of intriguing local stones from different sources. The yellow stone is the now familiar Cotswold Oolitic limestone from the Jurassic period - it has the telltale white fossil fragments embedded within. There are also red sandstone from the Devonian period (363-409 million years ago) - these red sandstones were quarried in the Forest of Dean and were formed in river plains with arid climate soils. The green stone is Silurian limestone, also from the Forest of Dean and these, too, were formed in river plains and shallow seas. The final type of stone to be seen in the wall is the blue-grey Lias limestone from the Cotswolds.

Discover more... Alan shows us a wall with some interesting geology in Miller's Green. Watch the video: 56k Dial-up | 256k Broadband To watch video content on the ÃÛÑ¿´«Ã½ you will need to have a program called RealPlayer installed on your computer.

Andesite

Remaining in Miller's Green, if you follow the path back towards the archway you'll see a line of large pebbles to your left. One of the stones (pictured right) has an interesting history. It's called Andesite and it's an example of an extrusive igneous rock formed in a volcanic eruption. Andesite magma erupts from volcanoes as thick lava flows which hints at the volcanic activity of the Precambrian period through to the Ordovician age.

Discover more... Alan finds an example of volcanic eruption debris in Miller's Green. Watch the video: 56k Dial-up | 256k Broadband To watch video content on the ÃÛÑ¿´«Ã½ you will need to have a program called RealPlayer installed on your computer.

Heading back through the archway into College Green once more and bear right through another archway. In front of you will be the monument to Bishop Hooper. Queen Mary Tudor ordered Bishop Hooper to be burned alive in front of the cathedral in 1555. All this because he refused to change his Protestant opinions when England was officially a Catholic country.

Discover more... Alan talks about what can be found at Bishop Hooper's statue. Watch the video: 56k Dial-up | 256k Broadband To watch video content on the ÃÛÑ¿´«Ã½ you will need to have a program called RealPlayer installed on your computer.

St. Mary-de-Lode church

A little way further on from the monument to the unfortunate Bishop Hooper is St Mary-de-Lode church. The slates on the Chancel roof are made from a variant type of Oolitic limestone - this variation is more fissile and can be split into stone sheets and was cheaper than obtaining slate from Wales. However the stone was very heavy and its placement on a roof needed to be carefully thought out. The larger and heavier tiles can be seen at the lower part of the roof while the smaller, lighter tiles were placed at the top. Grading these slates in this way made it possible to use them in building. It's an example of how mankind adapted the materials around him to serve a useful purpose.

Discover more... Alan tells us about another variety of Oolitic limestone used on the roof of St. Mary-de-Lode church. Watch the video: 56k Dial-up | 256k Broadband To watch video content on the ÃÛÑ¿´«Ã½ you will need to have a program called RealPlayer installed on your computer.

...You now have the option to continue the shorter walk which finishes back at The Cross or join the longer walk which takes in the geomorphology and wildlife of Alney Island.

Click here if you want to continue with the shorter walk.

If you want to join the longer walk, click here but remember the clothing recommendations set out on the introduction page.