by Andrew Knapp, Collections and Exhibitions Intern
Often, things that appear quite plain on the surface can reveal a much more interesting story when we take a closer look. In a box, locked away in the vault of Braintree Town Hall, we came across a large lump of rock known as a puddingstone. Puddingstone is a conglomerate, a type of rock made up of pebbles or small individual pieces of rock that have been cemented together with finer sediments such as sand or clay. This can give the rock the appearance of a Christmas pudding, hence the name puddingstone! In the south east of England puddingstones are mainly confined to Hertfordshire and are almost exclusively found as loose lumps of rock, as is our specimen. Because of its hardness it is a very common building material in old stone buildings, particularly churches, and can be seen in walls across the region. But this is only the most recent part of its story.
Our best chance of reconstructing what any region was like in the distant past lies in examining the rocks that were formed at that time. Because the processes that form and shape rocks happen to this day we can, by comparing these rocks to modern geological processes, get a good idea of what was happening on the surface at any period. And, by looking closely at the constituent parts which make up a rock, we can tell where these originated.
The first part of our puddingstone story is concerned with the large pebbles that are contained within. They are made of flint, and we know that flint is formed within chalk deposits as it is an extremely common rock across the south-east of the country. The major chalk deposits of England were laid down in the Cretaceous period, in a tropical sea around 90 million years ago. As the years passed by, these rocks were buckled by geological forces, pushing some to the surface.
Chalk is a comparatively soft rock and is easily weathered away by rain and waves, leaving behind the hard flint it contains. We can see this happening at the White Cliffs all along the south coast. Flint is irregularly-shaped in its natural state, but the constant pounding by waves on the seashore smooths and rounds flint into hard pebbles of the type that are common on beaches. The flint pebbles in our puddingstone are exactly of this size and shape, so we know that at some time they were pebbles on an ancient beach. We know that this ancient coastline existed some 56 million years ago during the Eocene epoch, and stretched from Portsmouth in the south to Cromer in the north. The climate of Britain at this time was hot and dry and as the seawater around the pebbles evaporated, a mineral called silica precipitated from it. This mineral bound the pebbles together, forming the puddingstone.
Millions of years passed before the next and final part of the story. Beginning around 2.6 million years ago, the Earth entered a cold period which resulted in a series of ice ages. During these times vast ice sheets covered much of the northern hemisphere, and glaciers scoured the underlying land as they crept slowly across it. The enormous power of glaciers enable them to pick up vast quantities of rock and sediment and carry them great distances, sometimes many hundreds of kilometres. As the glaciers melted, their rocky cargo was dropped onto the underlying land where it still remains to this day. Indeed, much of northern Essex is overlaid with great layers of these glacial deposits. Rocks like our puddingstone are much too large to have been moved by water, and we know that the only way they can end up far away from their source is if they have been moved by glaciers. Such rocks are called ‘erratics’ because they seem out of place in the local geology. They are found anywhere that glaciers once existed. The largest known examples are upwards of 15,000 tonnes, so our 15 kilogramme chunk of Eocene seashore is light work! Smooth patches on the surface of the puddingstone even reveal where the glacier has slowly ground the surface flat as it moved across the surface.
So from this one piece of rock we can see evidence for three distinct periods in the Earth’s history, at a time when it was very different to today. That’s pretty interesting, isn’t it?