Dyke Construction – Hydrology 101

When it comes to Dyke Construction and the function of ‘linear earthworks’ we call ‘Dykes’, there is massive confusion amongst both professionals and amateur archaeologists about how such structures could function when they are dry today?

The perception of these ‘Dykes’ is they are ‘rivers’ like the Thames following uphill over hills or Victorian Canals with locks and gates regulating the flow of the water – which are both equally nonsensical as a prehistoric structure. Basic Hydrology that most people (should be but not necessarily) learnt at school is that water is under the ground – not just a little water but 30% of all the fresh water on the planet.

This abundance of ‘groundwater’ is evident as it is the source of ALL rivers and supplies the Wells that have been dug since the beginning of time when rivers were absent. Even today, if you go into your garden and dig a hole, it will eventually fill with groundwater, whether in a valley or on top of a hill or mountain.

How and why water is on hills is very challenging for individuals as most people have a simplistic view of water being flat and sitting at ground level – but the earth is a far more complicated structure as this is the reason that it took centuries for people to recognise that we lived on a sphere and not a ‘flat-earth’ as such concepts as gravity are hard to comprehend…. for some!

The reality is that ‘streams’ of water are encapsulated within the bedrock allowing ‘mountain springs’ to start rivers at a great height as groundwater is under pressure and erupts to the surface from BELOW and does not flow up or down the hill internally – but will flow downhill AFTER it escapes from the soil.

Wells work even on top of hills as the groundwater is encapsulated in the bedrock and soil

The above illustration shows that if wells are dug halfway up a hill where there is a water pocket then they will fill – if we join up these wells then the entire ditch will also fill with water – sourced from the ground.

The Engineering and Hydrology

The central aspect that must be remembered when considering the reasons behind the construction and maintenance of these earthworks (Dykes) is that the environment was so much different in the Mesolithic Period, which changed rapidly when entering the Neolithic and then even more changes in the Bronze and Iron Ages.

Once the ice sheets had melted and the climate began to warm, the landscape gradually changed from open tundra to dense woodland. By around 8000 BC, pine and birch dominated the woodland cover. These were slowly replaced by lime, elm and oak with some hazel. By 6500 BC, pine and birch woodland would only have been found on the thinner limestone soils of the uplands.

With up to 90% of the land covered in woodland of one sort or another, the Mesolithic people needed all the open ground they could find to hunt larger animals like deer using their flint tipped bows and arrows. The lakes also provided plenty of minor game such as birds and fish.

Britain 8000 BCE – a flooded landscape with 90% tree coverage

We know that the hunters were here because archaeologists have collected thousands of their flint artefacts from sites around both lakes. Recent fieldwork and excavation by Bradford University around Malham Tarn has thrown more light on the people who used it as a hunting base. It seems that in the later Mesolithic, people were camping out on areas of slightly raised ground close to the shore of the Tarn. Geophysical survey work has shown that at one of these camp sites, there are several possible hearths.

Charcoal has also been found in Mesolithic contexts in the wetlands above the Tarn. It seems likely that the hunters burned back the edge of the woodland in order to create more open ground for their prey to graze on. This would also have favoured the growth of hazel since, unlike other woodland trees, hazel grows back quickly from a burnt stump. With hazelnuts being a significant winter food source at this time, the people may have had this aim in mind too. People had begun to alter their environment, and it was the beginning of the end of the wildwood in the Neolithic Period.

Consequently, at the time of the construction of Dykes the water table was still high, and rivers and wetlands dominated the landscape.  When looking at the landscape of these Dykes (particularly our case studies of Offa and Wansdyke) we notice that the earthworks is not consistent or continuous.

Fell walkers who have followed these features on foot have trouble accepting that these were canals that have been abandoned long ago and hence are just a shadow of their former self.

But if we look at other known abandoned canals from just a mere 100 years since their abandonment, we see there look remarkably the same, and even today, people find it hard to accept these once were part of a massive ‘super highway’ of the Victorian era that linked cities of trading together – like our ancestors dykes.

The gradients of some of the valleys these features enter have given walkers great concern that if water had been within the ditch, it would all run away to the bottom of the valley, leaving the canal ditch dry and useless.

No not Offa or Wansdyke but Grace Dieu Priory, Leicestershire – Dyke Construction

The problem with this analysis is that the walkers rely on OS maps that show these dykes as continuous features – but the reality if we look at the scheduling of these monuments through Historic England, this is far from the truth.

As we have shown in the case studies, most of these earthworks stop at the top of the valley hill and continue on the other side as if there was something in between?

What we find is that there is indeed something, and it’s called water, as, at the time of construction, the river levels were higher, and these valleys would have been flooded (as shown in section one of the book). So, they would paddle across the riven.

Moreover, what we see added at a later date are extensions to the original dyke to follow the falling river levels down the valley in sections and to a different specification to the above initial earthwork.

This can be shown on the section of Offa’s Dyke just outside Chepstow, where the dyke enters the valley but seems to stop at the top and then other partitions are added later.

Offa’s Dyke Nr Chepstow – showing the Canal is not continuous as shown on OS Maps

In above GE photo, we see that the extracts of Offa’s Dyke that enters the dry river valley changes in character except for one aspect – the width of the bank.

So, what makes the width of the bank so important?

The width gives us a clear view of how over time, the use of this earthwork changed.  What we see today is not what was initially built in prehistoric times – then the ditch was of greater importance, and then as the water table fell over many millenniums, the bank became of great significance and adapted.

Typical Dyke profile – Dyke Construction

The bank needs not to be so vast unless it has changed from being a towpath (only 2 – 3m wide) to a road that took two-way traffic?

Interestingly it is now the same width as a standard Roman Road (5m – 10m). We see from our Offa example that the road is 6m – 14m and only 0.4m to 1m in height.  This suggests that the Dykes purpose changed in later use, and looking at the 1800 OS map; this is confirmed as Offa’s Dyke is marked as ‘ancient road’.

Offa’s Dyke an ancient road? – Dyke Construction

This would explain why the ditch became more shallow down the dry valley, and on the Historic England monument reports there are a copious number of ‘Pits’ were found next to Bank, indicating that the contents of these pits were used to widen the road later than the original ditch.

We can only speculate that the ditch, which is only half to a third of the size of the ditch outside the dry river valley area, was still used as a canal or was entirely abandoned eventually for a road when the water table diminished?

Looking at how the Victorian engineers used locks to go up and down hills does give us an alternative possibility to this road use.  For we have this strange mystery of why only ‘separate sections’ survive in dry river valleys – this example is not an exception but more of the rule.

Is it possible that this sectioning was quite deliberate, and if so, what could be the reason? Well, a clue may come from an unusual source – Stonehenge.  If you look at the excavation reports, you see Stonehenge is not a continuous ditch but a series of pits and chalk walls.  These walls maybe use to stabilise the level of the water and if so, is the same engineering design being found here?

Modern LOCK solution over hills – Dyke Construction
Prehistoric LEVELS solution – where you drag the boat overland to the next level

If we cut small sections of Dyke and leave a small wall in between the cuttings, you have your modern Canal lock system – the only defence is that you would need to drag the boat over the surface between segments of the canal by leaving a low grove in the wall.

isolating water levels is not rocket science – here is a rice field on a mountain

Or if this option was not possible, they have used the tried and tested method of hauling boats up and down hills – by hand.

Even today boats are dragged to higher river and lake levels – Dyke Construction

Hydrology

To understand how these canals worked in hillsides of Britain where today they are dry and barren, you need to appreciate the landscape after the last ice age.

As we have already started the environment was coved mostly (90%) with woodland and trees.  This is because there was an abundance of water on the land as the water table was increadabily high.

This made the landscape almost like a latter day tropical rainforest rather than the grassy plains we see today.

Mesolithic Period has 90% woodland and tree coverage – even over hills

The reason for the high water table is a direct consequence of the last ice age which at its maximum about 30k years ago had most of Britain under 2 miles iof ice cap. 

The melting of this 361.8 gt of water (see first chapter for details) or 67,000 inches of water per square inch – flooded the soil which it could not absorbe and so it leak out for thousands of years at all elevation levels.

Offa’s Dyke as we see it today – Dyke Construction

This shows why rivers were at their highest level in history in the Mesolithic period and how easily it would be to find the water table if you dug a well or in this case a ditch even 7,000 – which is the current estimated date of the construction of these dykes.

Offa’s dyke in the Mesolithic – Dyke Construction

This leaking of water into the environment can be found in not only the SEA LEVEL changes (Table 1) that shows this constant flow of groundwater into rivers then into the sea rising sea levels – but also in other measurement such of the age of water in the groundwater aquifers.

These dates show that water entered the groundwater table in vast quantities in the Ice Age – but stopped for six thousand years – so did it stop raining for 6,000 years? Or was more water coming out than entering the ground?

Age of Water, showing that the most water was placed in the landscape soils during the ice age and it seeped out for 6,000 years before the rainfall penetrated the land again – Dyke Construction


 
The reason for the construction of Dykes in the past is shown by the sheer volume of ‘Linear Earthworks’ found. There are 1497 Scheduled Dyke sites found covering the entire british landscape – from the Known Offa and Wansdyke to the East Coast and now we have found that the Vallum connected to Hadrian’s Wall was also once a prehistoric Dyke that was reused by the Romans to convey the stone to the Walls.

The idea that these features are Medieval (although they may have been reused at that period) in origin is impossible as they are found as far as field as Southern Ireland (a meear 147 Dykes) and on both the Shetland and Scilly Isles – too wide spread to be these so called ‘saxon’ boundary/ defensive markers.

The full results of our most detailed investigation is due to be published in 2022. But extracts can be found including LiDAR investigations on this site and our video channel.