Geology sans frontières

Chalk: a shared geology

The Upper Cretaceous-aged Chalk Group (commonly known as ‘the Chalk’) is perhaps Europe’s most iconic geological unit. Besides forming the famous white cliffs, the Chalk:

• is a major aquifer, supplying millions of people with drinking water and sustaining rare ecological habitats
• is important for energy as it hosts oil and gas reservoirs in the North Sea, provides the foundation for offshore wind farms and hosts numerous shallow geothermal-energy schemes
• has the potential act as storage for hydrogen and CO2
• is a raw material for cement
• hosts many major civil engineering and infrastructure schemes across northern Europe including, in the UK alone, the recently approved Lower Thames Crossing, HS2, the Channel Tunnel and the new ‘super sewer’, the Thames Tideway Tunnel in London

Yet the Chalk is also one of the most misunderstood geological units. It is a common misconception that it is a uniform rock unit with very little structure or faulting. In reality, it has significant variations in physical properties and is often faulted.

Recent geological mapping in the Chilterns and Yorkshire Wolds, undertaken by BGS in collaboration with the Environment Agency and water companies, has shown how geological discontinuities in the Chalk affect groundwater flow. These discontinuities facilitate the development of dissolutional conduits and rapid flow pathways, creating a very heterogenous aquifer. This heterogeneity generates major challenges for the water industry, civil engineers and planners.

As an aquifer, the Chalk is under pressure, both in terms of water quality and resource. The demand for water, especially in more populated areas of southeast England is increasing, but there is also a need to protect rare chalk stream habitats and maintain river flows. Climate change, drought and contaminants such as nitrate exacerbate the problem. Addressing these issues require a greater geological understanding of the aquifer and how groundwater flows it.

These challenges aren’t just restricted to the UK: France has similar problems in the extensive chalk outcrops across the north of the country, as do the Netherlands, Denmark and Belgium.

Current research

A good understanding of the Chalk Group is needed to better predict groundwater flow and engineering ground conditions. This requires good quality geological maps and 3D models fit for the 21st century, using all the available information including field data, geophysical borehole logs, geophysical surveys and biostratigraphical data. In the UK, geological maps used to show the Chalk Group with just three subdivisions; we now divide the chalk into nine individual mappable formations, reflecting their differing engineering and hydrogeological properties. These more detailed maps are able to show much more geological structure, and are more relavent to engineers and hydrogeologists.

Recent groundwater modelling in BGS work has focused on building the national scale British Groundwater Model, simulating groundwater flooding, and projecting the impact of climate change on chalk streams and public water supplies. Other geological surveys are also modelling groundwater, for example, to investigate the impacts of nitrate and other contaminants on the chalk aquifer.

Likewise, more detailed understanding of the Chalk Group has helped civil engineers better predict ground conditions on major infrastructure projects. For example, BGS maps and models help identify zones of weak faulted ground that might be an issue for tunnelling or road cuttings. Similarly, BGS helped characterise flint content in the Chalk in the Thames Estuary to help design cutting heads for the Lower Thames Crossing tunnel-boring machines. Lessons learned from major projects in the UK can be equally applied in northern France and vice versa.

Common problems: common solutions

A key remit of geological surveys and research institutions is to work to find solutions to geological problems. BGS has teamed up with other northern European geological surveys and research institutes to discuss common interests in the Upper Cretaceous Chalk. Twenty-eight participants from 13 different research institutions and geological surveys, including geologists, hydrogeologists, biostratigraphers and engineering geologists, gathered for the inaugural EuroChalk workshop in the town of Étretat on the French coast. Étretat hosts spectacular chalk cliffs and rock arches made famous by the painter Claude Monet. These amazing outcrops admirably demonstrate how variations in the Chalk influence the local hydrogeology and cliff stability.

Outcomes

Exactly how does the Chalk vary across northern Europe? Many productive discussions were had during the workshop and several common themes were identified. Key to understanding the variability of the chalk is the application of a unified Chalk Group stratigraphy, so variations and changes in rock properties across the Anglo–Paris basin can be better understood and predicted.

Another area of common interest is the development of karst features in the chalk such as sinkholes, dissolution pipes, caves and dissolutional conduits. These are important not only from a hydrogeological perspective, but also as an engineering hazard. The French geological survey, BRGM, has been leading the way here, undertaking numerous tracer tests and identifying sinkholes across Normandy. A similar approach is being taken by BGS, learning from the French.

A third area of interest is incorporating lithological variability and karst into groundwater models. At present, many groundwater models treat the Chalk as a single porous medium, often modelled as just one or two layers. This ignores much of the complexity within the group. Much discussion was had on how to best approach modelling groundwater in the Chalk at a range of scales.

Next steps

This first meeting generated a huge amount of interest and enthusiasm. The next steps are to translate this into concrete actions. Essential to this is identifying potential funding sources for common projects, such as stratigraphical correlations across the Anglo–Paris basin. A special issue on Chalk Group stratigraphy in a relevant journal is another possibility.

Another workshop is being planned for 2026, either in Maastricht or in south-east England.

Thanks

Thanks to Ophélie Faÿ (University of Mons) and Eric Lasseur (BRGM) for organising the event.

About the author

Dr Andrew Farrant is a geologist and karst geomorphologist based at the BGS. He is the Regional Geologist for Southeast England and has extensive experience mapping the Chalk across southern and eastern England.