All the rocks in this area are sedimentary, in other words they were deposited in water. (‘Rocks’ include soft layers like sands and clays as well as harder rocks.) In general the rocks in the Guildford area belong to two geological periods: the Cretaceous (65-135 million years ago) and the Tertiary (2-65 million years ago). The Cretaceous rocks in this area lie to the south of Guildford and the Tertiary to the north (see sketch map). The Cretaceous period saw the last of the great dinosaurs and the Tertiary (the start of recent life) was the period when mammals began to flourish.
The Cretaceous and Tertiary rocks were formed when our local geography was very different from today. About 135 million years ago the area that is now the Weald lay at the northern end of a great freshwater lake. The Weald Clay we now find there represents the muddy bottom of this lake. Later, the sea encroached over the whole of our area until finally all Britain was submerged. The sandy area stretching from Hascombe to the Chantries was formed from the sands of this sea. The sediment from its bed formed the chalk, on which the main part of Guildford is built. About 25 million years ago the sea retreated, leaving the sediments to become hard and compacted. About half way through the Tertiary period earth movements raised the Wealden area into an elongated dome running from east to west. This remained above sea level, and has been eroded away until now only its edges or limbs are left, forming the North and South Downs (see diagram).
The area north of Guildford, however, subsided and was part of a great river delta or estuary during the Tertiary period. The muddy and sandy deposits on its bed now form the London Clay and the sandy heaths, which occupy much of the district north-west of Guildford. The several Ice Ages over the last million years have also helped to shape the landscape because, although the ice never came south of the Thames, sea levels altered drastically at times causing period flooding and affecting the development of rivers and their valleys. These high water levels occasionally left gravel or sandy ‘drift’ deposits on top of hills such as those at Netley Heath. River flooding has also caused alluvium and silty mud to be deposited in both former and present day river valleys.
Weald Clay has been used extensively for bricks and tiles. Most works have now closed although they are still active at Cranleigh. Ironstone nodules found in the Weald Clay were smelted for iron from a few centuries BC until the 18th century, and in places thin layers of Limestone full of winkle-like shells were quarried; an example of which can be seen in the paving behind Guildford House.
Greensand (Sandy Cretaceous Beds). The Hythe and Folkestone Beds contain commercially useful sands and Ironstone, sand cemented into dark, heavy rock by percolating water rich in iron salts. These can be seen in the St. Catherine’s Hill cutting on the Portsmouth Road. Both Ironstone and Chert (a toffee-coloured stone similar to flint and found in the Hythe Beds) have been used for cobblestones; there are chert cobbles near the Museum door at Castle Arch. Bargate Stone, a sandstone much quarried around Godalming in the past, may be seen in the keep of Guildford Castle and many Victorian houses in the area.
Chalk was used for building in the Middle Ages, as in the walls round Guildford Castle and the pillars of St. Mary’s Church nearby. Many large chalk quarries remain, but none are worked. Chalk was also the raw material for lime, used as mortar and for fertiliser. Flint (a form of silica) is found in the chalk and, besides providing prehistoric man with material for his tools, has been used in buildings from Saxon times until this century (the walls of St. Mary’s Church are a good example). Iron Pyrites also occurs in chalk, usually as rough brown lumps with a brassy appearance on broken surfaces. The Chalk also prevents oil and gas, from beds below, reaching the surface and is now being drilled for oil in this area.
Tertiary Beds. The London Clay, Guildford’s most important Tertiary formation, was used for brickmaking but none of the works in this area are active. River gravels of Ice Age or more recent date are commercially valuable and there are many gravel pits in the Wey Valley.
Fossils. The best source of fossils is chalk. Fossils can be found in old quarries, and sometimes in chalky soil in fields or gardens. The most common types are sea urchins. Fossils dating from the Ice Age, such as mammoth remains, are sometimes found in river gravels. There is a good collection of Bargate fossils at Farnham Museum, and a large general display at Haslemere.
Further Reading. The most useful guides to local geology and fossils are the British Regional Geology Handbooks for ‘London and the Thames Valley’ and ‘The Wealden District’, published by HMSO. Further information on fossils can be found in the British Museum (Natural History) Mesozoic and Caenozioc handbooks. The Hamlyn Guide to Rocks, Minerals and Fossils can also be used as a general guide to identification.
Guildford Museum, Castle Arch, Guildford, Surrey
(01483) 444750
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