Tiny fossils identify chalk for mosaic cubes

Tons and tons of chalk would have been needed in Roman Colchester to make lime for mortar. There is no local chalk so where it did come from? Alison Tasker from Leicester University has been researching ancient chalk tesserae (white mosaic cubes) to see if she can find the source of the chalk they were made from. It’s a new avenue of research and Alison is making great progress. As part of her work, she looked at eight white tesserae which we  had excavated about fifteen years ago on the site of the temple at Gosbecks.

Chalk is fine-grained white sedimentary rock consisting of calcium carbonate made up of minute fossil fragments of marine organisms. These were sensitive to their environment which means they can be used to date the formation of the chalk in which their remains lie. Alison has identified Gavelinella usakensis which links it with chalk formations in Dorset, Wiltshire, Hampshire, south Sussex, the Isle of Wight, the  Swaffham and Thetford area in Norfolk, and the Margate area in Kent. Chalk of this age does not outcrop in the Lincolnshire/Humberside area or in the Chilterns so these areas can therefore be ruled out as the source of the chalk used to make the Gosbecks tesserae.

More work remains to be done and Alison concludes that chalk from Norfolk and Kent warrant study since these outcrops, ‘although limited in extent, contain softer chalk and are within the easiest reach of Colchester’. Interestingly geologist, Kevin Hayward, sourced the greensand used to build Colchester Roman circus to the Medway area in Kent. The predominant building stone in use in Colchester in the 1st century AD was septaria but this seems to have given way to a large extent in the next century to Kentish greensand which presumably was brought to the town by sea around the Essex coast and up the Colne. Given the scale of this trade, its extension to Kentish chalk seems highly plausible.