This nodule was formed 160 million years ago in the Midlands. It was brought to this area by an ice sheet around 440,000 years ago.
Septarian nodules are one of the few building stones available locally and were used to build Colchester Castle and the Town wall.
Septarian nodules also formed in this area in the London Clay deposit. They date from the Eocene period and are around 50 million years old. It is believed that septarian nodules were formed when masses of mudstone were buried in the clay. Over millions of years they gradually dried out and shrank forming a network of internal cracks. These cracks then filled with crystals of calcite and other minerals.
Some of the most curious objects in the natural world are the nodules, or concretions, that are found in clay strata that was originally mud laid down on the sea floor millions of years ago, and of these, septarian nodules are perhaps the most attractive and bizarre. In Essex, septarian nodules occur in the London Clay which dates from the Eocene period but nodules of Jurassic age occur in the till, or boulder clay, having been brought south to Essex by the ice sheet from the Midlands.
Exactly how septarian nodules, or septarian concretions, were formed remains a mystery. Over millions of years they were entombed in the clay and gradually dried out. As they did so they would shrink from the centre outwards, forming a network of internal cracks, which filled with crystals of calcite and sometimes other minerals such as barite. No traces of the cracks are visible on the surface of the nodule. The cracks are called 'septa' from the Latin word septum or ‘partition’, which gives the nodules their name. The most famous septarian nodules in the world are the Moeraki boulders on New Zealand’s Otago coast. These two metre diameter spherical boulders are a popular tourist attraction. Research on these boulders, which occur in Palaeocene marine mudstones, suggest that the cracks formed as the concretions grew and the whole process took several million years.
Smaller septarian nodules from British Jurassic clays such as the Kimmeridge Clay are capable of being cut in half and polished to reveal the colourful network of cracks. Those from the London Clay usually break open readily and sometimes reveal beautiful groups of barite crystals.
The septarian nodules from the London Clay are generally referred to as ‘septaria’ and at certain horizons in the clay they are free of internal cracks or cavities and have been collected from the Essex coast for use in building construction. Notable examples are Colchester Castle and Roman wall and countless churches on the coast. Septaria are also called ‘cement stones’ as they were formerly collected at Harwich for use in making ‘Roman cement’ before the invention of Portland cement.
Concretions were eroded from the clay and subjected to weathering often revealing the honeycomb-like pattern of infilled cracks, sometimes with the calcite 'veins' standing proud of the surface. Some septarian nodules can be very large; a specimen probably weighing at least a tonne is on display in the grounds of Saffron Walden Museum (complete with scratches obtained while it was trapped in the ice sheet). Attractive polished sections of septaria have also been used for the fire surround in the museum’s Ceramics and Glass Gallery on the first floor. They can also be seen elsewhere in the area, for example in the Stable Block in the grounds of Audley End House where there is a large specimen that has been sliced in half to make an unusual table. Because they were formed in the mud on the sea floor, the local septaria sometimes have traces of fossils on their surfaces, particularly ammonites that lived in the Jurassic seas.