Field meeting report: geology in Rochester, Kent, Saturday, 15 February, 1997

Field Meeting Report: geology in Rochester, Kent, Saturday, 15 February, 1997 E. Robinson ROBIKSON, E. 1997 Field Meeting Report: geology in Rochester. Kent. Saturday, 15 February 1997. Proceedings ofthe Geologists' Association, 108.327 328. The rock types offered as kerbstones, cobbles or walling in a well-known eighteenth to nineteenth century town range from a diversity of granites to a wealth of examples of the local stone from the Medway Valley - Kentish Ragstone. Shop fronts of later date introduce Portland Stone and Bath Stone brought in by sea. and exotic stones such as larvikite and gabbro when transport was freer and taste invited a wider range of choice. On the basis of what can be seen on the streets and in the walls of the cathedral, there is a clear opening for a geological town trail and cathedral wall games.

Department of Geological Sciences. University College London> Gower Street. London WC I E 6BT

On the understanding that there is good geology to be found in any average town if you choose to look. 45 members of the Geologists' Association gathered at Rochester Railway Station on Saturday. 15 February. Rochester provided the opportunity for those who regularly do inner city walks to meet up with members of the Kent Local Group who had previously tested themselves at Dover Castle and at Faversham. For both. Rochester was an opportunity 10 recognize familiar building stones, but with some differences from what can be considered 'normal'. An early example in this last category is provided by the kcrbstoncs, Alongside the usual diorites ('bluestones') from the Channel Islands, and the feldspar-rich Cornish granites, there occurred dull red granites, some with brick-red feldspar phenocrysts, which were identified as originating from the north coast of Brittany. Together, they made up an assemblage of cobbles and kerbs tones which were available to Kent Coast and Thames Estuary towns through coastal shipping movements in the nineteenth century. dressed stone being in part a 'ballast' cargo for ships returning from the delivery of manufactured goods to the London docks. It was a pattern which had been seen in Faversharn a few miles to the east. High Street Rochester is the main axis to a town retaining a variety of eighteenth and nineteenth century town houses with conversion to shops at street level. This conversion constantly requires you to look upwards as often as directly ahead, to take in the age and adaptation of buildings. Brick and timber-framed buildings can sport panels of polished larvikite at pavement level. Younger buildings were often stylistically sympathetic to older frontages. including a recently redundant Post Office which demonstrated all the features expected of Portland Stone etched by sixty years of weathering. Nearby, another vacant building offered the sombre brown weathered surfaces of Bath Stone to add to the standard building stones of any English townscapc, Rochester is dedicated to its associations with Charles Proceedings ofthe Geologists' Association, 108,327-328.

Dickens so, appropriately. the Charles Dickens Centre midway down the High Street is a focus of attention for good quality sixteenth century red brick (Eastgate House), with a 'tudor' doorway framed in scaly-weathering Reigate Stone from the Upper Greensand of the northwest Weald. This fine-grained glauconitic sandstone is much easier to carve into mouldings and decorative details than the local Kentish Rag (Lower Greensand) but tends to suffer when exposed to wind, rain. and frost, as seen here in this doorway. In contrast, the nineteenth century paving slabs of Coal Measure sandstone demonstrated their hard-wearing qualities but at the same time. revealed patterns of ripple mark and current scouring in the worn down surfaces reduced by foot-tread. A discussion on the nature and distribution of sarsen stones was triggered by a series of on-end stones lining the entrance to a paved garden behind the Dickens Centre. The stones are strikingly contrasting in their exposed surfaces. Their almost flint-like character is demonstrated by the subconchoidal fracture - a feature of fine-grained sarsen stones. What caught the eye. however, was the strongly marnmilated surface of one face; the other remaining flat and featureless. Sarsens are known from the North Downs (Kit Coty's stones) and from the lower Tertiary sands which overlie the Chalk towards the Estuary and the Isle of Grain. It remains for the Kent Group to discover further examples of such mammilated stones in one or other of the nearby sand deposits. A short step from the Dickens Centre, the High Street is crossed by the medieval town wall. This gave a chance to examine the principal local building stone, Kentish Ragstone. Blocks of this gritty grey sandy limestone. some including bands of dark chert. others, shells of Exogyra, were seen and recognized as a stone difficult to dress into well-formed facing stone. This was a good introduction to a major building stone used in the twelfth century Cathedral of St Andrew which forms the core to the walled town. The north transept and what JS called Gundulfs Tower offer sight O() 16-71\78/97 S I o.oo © 19'17Geologists' Association

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Fig. 1. St Andrew's Cathedral, Rochester.

of walling of mixed character, blocks of ragstone bonded with flint and medieval tile, the better dressed quoins to walling being in a fine-grained brown sandstone thought to be from the Weald. Facings to walls and doors and window tracery involve the distinctive yellow limestone from Normandy - Caen Stone, which was demanded by the

Norman master builders employed by Gundulf in the twelfth century. Thousands of tonnes of this stone were introduced into southeast England by Gundulf and his contemporaries involved at Canterbury and in London (Gundulf had a hand in the Tower of London), raising the thought that this was no mean feat of organization and transport through Channel seaways which are still a problem to navigate on account of tide set and currents Victorian architects introduced several new stones into the fabric, notably grey-green Chilmark Stone from Wiltshire (recognized by its cross-bedding and colour tone), Bath Stone of a rich golden colour on the cloister face to the cathedral, and finally, Weldon Stone from Northamptonshire in the northwestern tower to the west front. Interest, however, lies in shafts to the west front which show a strong crenulate banding along the length of the column. These are stones referred to as 'onyx marble', presenting something of a mystery in that term, but by nature, a form of spring deposit. Cut and polished examples of this kind of stone are known from Roman villas and temples, the materials usually coming from sources in Algeria or Tunisia. On these grounds, the columns here and in the nearby cloister arcading, are thought to have been 'recycled' from a pre-existing Roman building in Kent and introduced for their contrasting character to stand alongside columns and carved capitals of Caen Stone. The fullest analysis of the stones of the cathedral, including the monuments inside, have been written up by Bernard Worssam and Tim Tatton-Brown (1993), but it remains that several sectors of the cathedral walling, especially those revealing older rubble coursing, would make admirable examples for 'wall game' exercises for schools if sketched and run off as worksheets. This, and a town geological trail to match the existing 'heritage' trail produced by the Information Centre of Rochester, were tasks which we left for the Kent Local Group for the coming year.

REFERENCE WORSSAM, B. C. & TATION-BROWN, T. 1993. Kentish Rag and other Kent Building Stones. Archaeologia Cantiana, CXII, 93-125.