Backwell Environment Trust

 Geology

The geology of BET's nature reserves is dominated by Carboniferous limestone that was laid down some 300 to 350 million years ago in a warm, shallow sea from the remains of countless marine animals.

One hundred and fifty million years later during the Jurassic Period, mineral rich water began to be forced up from the hot depths of the earth through the many faults and fissures in the limestone which today has resulted in the accumulation of numerous metal ores in the bedrock.

The most abundant minerals to be found on the reserve are galena (lead sulphide), calamine (zinc carbonate), yellow ochre (iron oxide), barites (barium sulphate), malachite (copper carbonate) - and probably many more as yet unidentified.

Potato stones are quartz nodules either spherical or ovoid in shape with a somewhat fanciful resemblance to the humble potato.

During the Middle Ages, the quartz crystals in these stones were sometimes mistaken for real diamonds and were consequently locally referred to as ‘Bristol Diamonds’. They are quite widespread in our area and vary in size from less than 1cm to a maximum of about 20cms in diameter. When cut in half, potato stones reveal their amazing internal structure which is frequently made up of the convoluted, coloured bands you see here. As in this local example from nearby Stancombe Quarry, the chief colouring agents often come from a complex mixture of red iron oxides.

A Cut and Polished Potato Stone