Chalk scenery, Yorkshire

Chalk is a pure white limestone made almost entirely of the microscopic remains of tiny plankton (called coccoliths) that lived in the warm seas of the Cretaceous Period between 70 and 100 million years ago.

As the high power photograph to the right shows (from an electron microscope) shows, there are lots of gaps between the remains so that chalk is very porous. Water can drain down quickly from the surface into cracks or joints. This is why most chalk downland areas are dry, with few trees, and valleys that only contain streams during very wet winters.

Water pumped up from the chalk provides much of the tap water for eastern and south-east England.

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