Clay, chalk, and crops: Archaeological evidence of the Agricultural Revolution

When archaeologists dig, we often find ourselves uncovering ditches and pits — and our ongoing work ahead of the construction of Sizewell C is no exception. One of the first questions we ask is, ‘Why did people dig all these pits?’ The answer can vary depending on the period, location, and even who you ask. Near Sizewell, we’ve uncovered many post-medieval quarry pits. On the sandy gravels of the Suffolk coast, it’s clear that some were dug to extract sand and gravel for construction. But intriguingly, we’ve also found a group of pits targeting clay-rich soils.

Many post-medieval clay quarry pits in East Anglia are found to specifically target chalk-flecked ‘marly’ clay. This type of clay was used by farmers for ‘marling’, which is the practice of spreading chalky clay onto fields as a form of manure to improve crop yields. We usually find such agricultural clay quarries located around the margins of fields, close to where the clay was needed without leaving a big hole in the middle of the field itself. Some of these pits survive into the modern day as ponds, having filled with water over the years after being abandoned.

The chalky clay that these quarries were targeting is part of the Lowestoft Formation, a thick layer of material left behind by the Anglian Glaciation, which was at its height around 450,000 years ago. This material, known as glacial till, was ground up and pulverised by the advancing ice sheet as it moved eastwards across the midlands, before being deposited across large areas of East Anglia, including Suffolk. The chalk (calcium carbonate) within this clay is what gives it its beneficial character, providing calcium for plants and reducing soil acidity. Perhaps the most famous example of marl, is that that makes up the lower part of the cliffs of Dover.

Although the practice became more widespread during the so-called Agricultural Revolution of the post-medieval period, ancient accounts also mention its use during the Roman period in Britain. In Book 17 of his Natural History, the Roman writer Pliny the Elder writes that in Gallia and Britannia some of the locals use chalk and chalky clay, ‘which they call marl’ (‘quod vocant margam’), to fertilise their fields. One type of such material they refer to as glisomarga, which he tells us is a mixture of chalk and clayey soil (‘est […] creta fullonia mixta pingui terra’), the application of which can make a field fertile for 30 years before it needs to be reapplied.

Image: an imaginative 19th-century portrait of Pliny the Elder

However, the practice became much more widespread during the post-medieval period. Arthur Young, an agricultural writer who compiled a county-by-county survey of farming practices in the late 1700’s and early 1800’s, discussed it in depth! In Chapter XII of his General View of the Agriculture of the County of Suffolk (written in 1797), he tells us that, ‘There are various sorts of clay’, but that which is ‘strong and full of chalk […] is the best; the use of it prevails very much at this time’.

The process by which it was applied first involved prepping the field to be marled, which was done by sowing it with grass seeds for two consecutive years. After this, chalky clay was dug out of pits and spread over the field using tumbrels and carts near the end of summer. It was then left alone until the following spring, when it was ploughed. Youngs tells us that the benefitsto the field could last for twenty years before it needed another application of chalky clay, very similar to Pliny’s assessment made over 1,700 years earlier!

In Youngs very through account he even goes into detail on the finances of excavating quarry pits, writing that, “In the eastern part of Suffolk [that’s where we are digging!], if the clay pit be not very distant, the price of digging, carrying, and spreading, is 50 shillings for 120 loads”, although in one grumpy footnote he complains that if the workmen are not closely supervised, “the opportunities they have for perpetually deceiving him” in terms of payment increase.

Not all farmers were willing or able to incur the cost of digging clay quarry pits or leaving a field unploughed for three years straight, and Young reports the words of one of his correspondents, who wrote that, ‘if a farmer lets his land lie a proper time after claying […], it will be near three years before he gets any of his money into his pocket again from the first year’s claying’.

Another issue was that the chalky clay of the Lowestoft Formation also contains patches of gravel and stones, which had been picked up and mixed in by the glacier during its advance; in a list of potential losses that might be incurred by a farmer, Young includes, ‘Loss by the stones thrown out by the labourers in the pit, the farmer seldom making half the money of them again’, estimating that there would be about one cartload of ‘useless stones’ ‘to 70 loads of clay’.

Despite the delayed return on investment and the nuisance of gravel mixed in with the clay, farmers near Sizewell clearly saw the value in marling. Our excavations have revealed the remains of their clay quarry pits, evidence of their efforts to improve crop yields and work the land more productively.