Geology

The chalk here in Hampshire was deposited during the Cretaceous Period (145 - 66 million years ago). During this period we were covered by a shallow sea teaming with life. Shells and skeletons of these dead ceatures sunk to the sea bed and formed a thick, calcareous deposit - the Chalk. 

Over the following millennia this chalk was buried under numerous layers of younger sediments but a series of ice ages, the last occurring between c. 115,000 - c. 11,700 years ago, meant that ice sheets moved south and eroded these sediments, re-exposing the chalk. Just how far south the ice sheets extended still divides geological opinion but the fact that melt water played a major role in forming the low lying chalk downland we enjoy in Hampshire today is undisputed.

Chalk is an aquifer - a rock body that both stores and transmits groundwater. The British Geological Survey says that the chalk beneath our feet is approximately 400 m thick and deposited during the Coniacian Age (89.8 - 86.3 million years ago). Core samples taken from recent boreholes indicate that our chalk is from the Santonian to Campanian transition (86.3 million years ago). This transition or boundary may account for more faulting in the chalk - that can speed up groundwater transmission. Is this what makes our catchment so reactive to groundwater levels?

Chalk hydrology is a complex topic but simply put the volume of water stored in an aquifer is dependent on groundwater levels, which in itself is dependent on rainfall. 

All chalk layers or formations have unique properties so hopefully a geological survey of the Bury Hill chalk quarry may help validate this theory (the banner image on this page is of the chalk face at Bury Hill chalk quarry).