Harrow and Hillingdon Geological Society


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We spent a pleasant evening in Guildford with Diana Smith starting with a detailed account of the history of the town. Although there were people in the surrounding country it didn't become a settled town until Saxon times. The initial trade was wool. The town is near to good sheep country and much of the wealth came from spinning, dying and weaving. It became famous for Guildford Blue which was dyed with woad. Later it had a mint where royal coinage was struck.

Eventually the wool trade died out but, with some engineering work on the River Wey, to make it navigable as far as Weybridge and the Thames, Guildford became an important trading centre.

We did a tour of the High Street, somewhat marred by the fact that Guildford was hosting a rather large cycling event that evening as part of the Guildford Festival. We finished in the oldest part of the town. Diana did a magnificent job talking over the very loud speakers and we saw various types of building stones, a few of which are shown below...

Bike race
Unfortunately the bike racing started at the same time as our Geological walk. Diana managed to talk over the loud speakers and we saw most of the stones. The other side of the street was unreachable. Maybe we should go back another day to look at that side...
Flints and Sarsens
The wall of the remaining medieval part of Holy Trinity church (rebuilt 18th Century) is made of good quality flints and roughly dressed sarsen stone (both local materials). Small gaps in the construction are filled with thin galettes of flint (see enlarged picture).
A classic example of granite facing stone. You can clearly see the grey, glassy quartz, some white feldspar, a great deal of pink feldspar and black biotite mica. The large crystals are formed by long, slow cooling of the molten magma at depth.

This is a collapsed limestone breccia originally polished to a high finish. It is now weathered to a matt finish, although the inside of the column (away from the weather) still retains its high polish.

The small fossils are the remains of echinoids.

Serpentinite and Borrowdale slate

On the left is serpentinite, thrust up from deep in the mantle. This stone probably came from Greece. It is heavily brecciated which of course gives it a lovely pattern for a facing stone.

On the right is an example of Honister Green Slate from the Borrowdale Volcanics in the Lake District. This is not a true slate, but is composed of volcanic tuff. However it does contain mica, and so has a micaceous cleavage, which makes it useful as a building stone.

Mixed fossils
Fossiliferous limestone set in the floor of a shop doorway. Originally it was difficult to see the detail in the beautiful white stone, but years of dirty shoes have done us geologists a favour by bringing out the lovely detail.
This is a detail of the stone above; the large white dot towards the top right. It is a large foraminifer.(Sorry, no larger picture is available)
Feldspar crystal

This large feldspar crystal is in one of a series of large granite blocks set into the road in Victorian times. The blocks were set wheel-width apart so that the constant traffic didn't gouge large ruts into the road.

The crystal shows good zoning formed during crystallisation, and a reaction halo around the edge.

Unfortunately the very best crystal now resides under a yellow line...

The tower of St. Mary's church
This is the oldest part of Guildford. The church tower dates back to Saxon times and exhibits a rare form of buttressing. This can be seen in the enlarged picture where a small red arrow points out the slightly raised row of flints. Others can be seen if you look carefully.



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