Chiltern Society PhotoGroup

 
 
 
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Hertfordshire Puddingstone in the churchyard: a conglomerate of flint pebbles in a silica matrix at the base of the Reading Beds, which were deposited in the Eocene (56 million years ago). The pebbles, eroded from the chalk, were rounded and sorted by water action (similar to pebble beaches). During a brief period when the sea retreated in a semi-arid climate, capillary action brought dissolved silica from underlying rocks. The water evaporated leaving the silica which hardened around the pebbles.
Author: J Harrison