The first point of interest on this part of the walk is Argos on the corner of Eastgate Street and Brunswick Road. If you take a look at the stone facings either side of the entrance, you'll see what is called Shap Granite. This beige/pink stone is Igneous which means it was formed as molten rock - Magma - cooled. Crystals then formed to produce an interlocking meshwork. It was formed some 400 million years ago in the Devonian period when the landscape was very different from how it is today - imagine lots of volcanic activity and a very barren landscape and you'll get the picture. If you take a close look at the stone you'll see large pink crystals embedded in the rock - these were formed in very hot melts rising from the interior of the Earth. The Shap Granite you can see here formed very slowly within these melts, which is why the crystals are so big. This granite comes from the eastern side of the Lake District.

Discover more... Geology enthusiast Alan McKay explains about the Shap Granite on the building. Watch the video: 56k Dial-up | 256k Broadband To watch video content on the BBC you will need to have a program called RealPlayer installed on your computer. Download it for FREE by clicking here

Take a short walk along Brunswick Road, crossing to the other side. You will see a grand Victorian building which is the City Museum. This building offers another kind of stone which is called Oolitic limestone. This stone was quarried in the nearby Cotswold hills and gives us a really fascinating insight into how the land looked around that period, some 170 to 225 million years ago. It was in the middle of the Jurassic period when dinosaurs roamed the Earth! The area was, again, a very different place to behold. For a start, it was probably around the equator and it was also under water. Gloucestershire, at the time the Oolitic limestone was beginning to form, was most likely made up of tropical shallow sub-tropical lagoons where gentle wave action 'rolled' the shell debris to create fine Ooliths - if you look very closely at the stone, you'll see small rounded grains and these are the result of wave action rolling the shell fragments back and forth.

A Vug on the Museum wall

You'll also notice that several of the limestone blocks have a hard toffee-like coating on them. This is what's called Tavertine and is basically calcium carbonate - it's the same stuff that makes up stalagmites and stalactites in cave systems. It's formed by rain water which has seeped through cracks in the stone and deposited the calcium carbonate over millions of years. If you examine the wall closely, you'll see a hand-sized hole in one particular stone block where crystals have formed - this feature is called a Vug (right). A Vug is a hole that's partially lined by minerals - in this case Calcite crystals.

Discover more... Geology enthusiast Alan McKay explains about the Oolitic limestone used on the City Museum and an unusual feature called a Vug. Watch the video: 56k Dial-up | 256k Broadband To watch video content on the BBC you will need to have a program called RealPlayer installed on your computer. Download it for FREE by clicking here

Returning back to Eastgate Street and heading left, you'll notice a large glass-topped structure in front of Boots - if you look inside you'll see the ancient remains of the East Gate. This was one of the main entrances through the defences of Gloucester and it stood here for more than 1700 years. The first East Gate was built by the Romans in AD 58 and it had a tower on each side of the passageway. The East Gate survived undermining and a heavy bombardment during the Siege of Gloucester in 1643. The gate survived into the 18th Century as a school and prison until it was demolished in 1778. The viewing window overlooks what used to be a medieval horsepool.

Larvikite crystals

Walking along Eastgate Street, the next point of interest is the Lloyd's TSB bank which you will see on your right. This building, built in 1898, also features another example of Igneous stone formed by rock cooling from a molten state. The rough-hewn stone is called Scandinavian Granite and is an example of what's called intrusive igneous rock. This means that it was formed deep within the Earth and a feature of this is the formation of large interlocking pink feldspar crystals. The smoother black-grey blocks above the granite are called Larvikite (pictured right) and they feature iridescent feldspar crystals - these have formed this way because of impurities in them. These stones would have been formed around the Devonian period when the surface of the Earth was much more volcanically active than it is today.

St. Michael's Tower in The Cross

Moving on to The Cross - the meeting point of Eastgate, Northgate, Westgate and Southgate Streets - you'll see a church tower to your left. This is St. Michael's Church (pictured right) and it offers another chance to learn more about what the area would have looked like about 170 million years ago. The tower itself is constructed from Oolitic limestone mined in the Cotswold hills or even Robinswood Hill, which isn't too far from the city centre. However, closer examination of this limestone reveals yet more evidence that Gloucestershire was either coastal or completely underwater at that time period. If you approach the wall nearest the coffee shop, you can see a great example of what's called cross-bedding - this is the action of currents pushing shell fragments up an incline on the sea floor, depositing them down the far side of the incline. This happened repeatedly, forming bands of shell fragments which indicate the ancient currents.

...You've just completed the first stage of the walk. If you cross over to the HSBC Bank on the corner of Westgate Street and Northgate Street, you'll be ready to begin the next stage.