Chapter 1 – The Post-Glacial Flooding Hypothesis
In the first book of this trilogy ‘The Post-Glacial Flooding Hypothesis,’ we looked at the new mathematical models that allowed us to calculate the amount of water that was released during and after the Last Glacial Maximum just over ten thousand years ago.
These models showed us that a minimum of 8.42 quadrillion tonnes of water was released on the UK at the end of the last ice age. This is equivalent of 98425.2 inches of rain falling on every square Inch of Britain’s landmass or the same as – One Inch of rain steadily falling every day for the next 270 years
The worst known flooding in British history occurred in 1947 when just six inches of rain (149mm) fell on up to 12″ of snow (so a maximum of 15″ of rain if melted) over three months. The flooding, which inundated nearly all the main rivers in the South, Midlands, and the Northeast of England, was notable for its origins, geographical extent, and duration.
It was impacting on thirty out of the forty English counties over a two-week period, when around 700,000 acres of land became flooded. Tens of thousands of people were temporarily displaced from their homes, and thousands of acres of crops were lost, and this was just 15 of the estimated equivalent 98,425 inches of water that was shed on the British landscape after the last Ice Age.
Raised Water Table
According to William Donn (Donn et al., 1962). The Fennoscandian and Great Britain ice sheet covered 4.7 106 km2, which is equivalent to:
• 8.42 106 Gigatonnes of water / 4.7 106 km2, which gives us 1.79 Gt per km2
• 1.79 Gt of water at a penetration rate of 43%, give us 0.77 Gt of water per km2
• 0.77 Gigatonnes of water by U.K. landmass 242,495 km² give us 186,804 Gt of groundwater
This water will be released at a rate of 1 – 12mm per annum and be at a depth of possibly 75km. Therefore, to release groundwater at a depth of 75km at an average rate of 6mm per annum would take 12,500 years – not the 1,200 years previously believed, which is just the surface water from the last stages of the meltwater ice.
This is the reason that rivers (like the Thames) still flow even after months of drought, as the groundwater is constantly leaking into the river, which was at its highest rate at the start of the Mesolithic, just after the great meltwater floods.
If this model is correct, we should be able to get verification via other empirical evidence, as shown in sea level water rises, to see if it has been constant over the last 12,500 years.
|Figure 1– Historical Sea Level Change|
Most geologists and paleoclimatologists, when talking about the end of the last ice age, refer people to the phenomenon called the ‘Meltwater Pulse’ – which is the rapid rise in sea level (20m) between 13,500 and 14,700 years before present, over a 400 – 500 year period. Although it is a tremendous value, it should be recognised that this ‘pulse’ as only 16% of the total sea rise since the end of the last ice age.
We have constructed a model of sea-level changes by combining both the Wadden Sea model (Vermeersen et al., 2018) and the NASA sea-level model to calculate the rise in sea level in the North Sea area and the discharge levels from the Rivers that flow into the area over the last 10,000 years since the Last Ice Age (Fig. 1).
When we put all the information about the Holocene together including the vast amounts of meltwater, Ice melt pulses, raised the water table and increased precipitation, then we are left with evidence of increased water activity, volume and consequential levels of Holocene rivers.
Table 1 – Wadden sea level rises and precipitation levels (* Meltwater Pulse)
|BCE||Change (mm) over – 500 years||Discharge Ratio rate – compared to today = 1||Adjusted Ratio for Discharge – Pulses||Precipitation levels mm pa|
Figure 2 is based on the above table which shows the Wadden Sea area (part of the North Sea) which indicates how the rising sea levels correlates to river discharge into this area and therefore size.
|Figure 2 – Discharges Rates from the Wadden Sea – the yellow graph is precipitation and the green is the water pulses above the estimated steady blue discharge from the water table|
Therefore the blue ‘discharge’ line (on the graph) is an indication of the volume discharge and hence the size and height of rivers over the last ten thousand years – the chart uses today’s discharge rate (950 AD) representing one unit, to give us a simple visual comparison to previous years as a ratio of size – hence 8050 BCE the rivers were 140 times larger than today.
Other Post-Glacial Flooding around the world
The flooding after the LGM was not only limited to Britain. We have shown in the first book of the trilogy that flooding occurred in Northern America – showing an increase of discharge from rivers such as the Mississippi increasing from today’s rate of 16,790 m3/s to 160,000 m3/sjust after the LGM, a rise of 853%, clearly showing an increase in river height.
We also see the same river level of increases in Germany and the consequence of these raised discharge rates and water levels in the historical flooding of the Black Sea some 2000 miles from the Ice sheet which turned the freshwater to a salt lake when it overflowed into the Mediterranean.
Britain’s Post-Glacial Flooding
In a Lewin and Macklin, 2003 study they showed that 147 channel floods occurred during the Holocene period due to the rise in the water table, which continued up to just 1000 years ago. Moreover, as pointed out within the paper “Constant Holocene sedimentation might be expected to produce straight-line cumulative plots and even probability levels, with deviations representing episodic or increasing/decreasing trends inactivity”.
However, the uneven preservation of alluvial units (Lewin and Macklin, 2003) base their ‘recorded’ alluvial archive in different ways, especially but not exclusively towards later Holocene sediments. Furthermore, 14C dated materials are also likely to appear bunched in relation to 14C production variability, which may or may not coincide with flood production, so that to an extent the spikiness of the illustrated plots represents more complex factors than episodic alluviation alone. These factors need to be borne in mind when interpreting the data presented.”
We then investigated the Thames as a case study to see how Britain’s largest river was affected in the Mesolithic and Neolithic periods. This was achieved by constructing a discharge model based on the sedimentary data supplied by the British Geological Society’s superficial maps of the areas and some borehole core sample.
The conclusion of this study was that the current average discharge of 65.8 m³/s was increased by 3723% within the watershed area, which allowed us to estimate that at its peak the Thames River discharged 2450 m3/s (0.0025 Gt /s or 1314 Gt per annum). About the same rate of one of the smaller rivers of North America during the same period, which begs the question as the Thames is the largest river in the country, would it not be affected mostly by the meltwater at the end of the last ice age.
To investigate, we need to look at a detailed excavation undertaken at the edge of the BGS superficial Alluvium flood map, to get some real evidence about what sediments are shown, which will allow us to better understand the dates Geologists have suggested in the past.
If we look at the Cross-Section profile of the Thames, we see that the area of Holocene superficial sediment effect, increases by at least one mile and is terminated by Boreholes TQ47NE344 and TQ58NW141.
Borehole TQ47NE344 – shows 5.95m of “Brown Silty Sand” before hitting Chalk and TQ58NW141 4.42m of “Loamy Sand and Stones” with a base of “sand and Gravel”.
This increases the Thames Flood Model from a discharged 2,450 m3/s to 12,250 m3/s, which reflects more accurately the North American Discharge Model.
Peat (Ultimate Evidence of Holocene Flooding).
The formation of bogs in the UK began 10,000 years ago at the end of the last ice age, when glaciers retreated northwards, leaving behind a landscape of shallow meltwater lakes and waterlogged hollows. An estimated 2.3 million hectares (9.5% of the UK land area) is covered by bog peatlands. 1
Peat (turf) is an accumulation of partially decayed vegetation. One of the most common components is Sphagnum moss, although many other plants can contribute. Soils that contain mostly peat are known as a histosol. Peat forms in wetland conditions, where flooding obstructs flows of oxygen from the atmosphere, reducing rates of decomposition.
Bogs are the most important source of peat, but other less common wetland types also deposit peat, including fens, pocosins, and peat swamp forests. There are many other useful words for lands dominated by peat, including moors, muskeg, or mires. Landscapes covered in peat also have specific kinds of plants, mainly Sphagnum moss, Ericaceous shrubs, and sedges (see bog for more information on this aspect of peat). Since organic matter accumulates over thousands of years, peat deposits also provide records of past vegetation and climates stored in plant remains, particularly pollen. Hence, they allow scientists to reconstruct past environments and changes in land use.
Peat forms when plant materia – usually in marshy areas, are inhibited from decaying entirely by acidic and anaerobic (lack of oxygen) conditions. It is composed mainly of marshland vegetation: trees, grasses, fungi, as well as other types of organic remains, such as insects, and animal remains. Under certain conditions, the decomposition of the latter is inhibited, and archaeologists often take advantage of this.
Peat layer growth and the degree of decomposition (or humidification) depend principally on its composition and on the degree of waterlogging. Peat formed in very wet conditions accumulates considerably faster and is less decomposed, then that in drier places. This allows climatologists to use peat as an indicator of climatic change.
Unlike sub-soils, such as head and alluvium, most peat bogs can be accurately carbon dated – this is central to understanding post-glacial flooding as the flooded areas of Britain would be major contenders to accumulate marshes and bogs that would create the peat sub-soils of today (Fig. 3).
|Figure 3– Peat growth – reflecting the post-glacial flooding of Britain (blue map)|
We can use peat not only to find the Holocene wet areas and raised river levels – but moreover, we can also use them to identify the size and flow of the rivers during the Mesolithic and Neolithic period.
|Figure 4 – Carbon dated peat samples showing when these peat formations were formed|
Blanket bog occupies approximately 6 % of the area of the U.K. today. The Holocene expansion of this hyperoceanic biome has previously been explained as a consequence of Neolithic forest clearance. However, the present distribution of blanket bog in Great Britain can be predicted accurately with a simple model (PeatStash) based on summer temperature and moisture index thresholds, and the same model correctly predicts the highly disjunct distribution of blanket bog worldwide. This finding suggests that climate, rather than land-use history, controls blanket-bog distribution in the U.K. and everywhere else. (Gallego-Sala et al.,2016).
The maximum rate of peat production is not as one would expect directly after the meltwater flooding at the end of the Last Glacial Maximum, because if the waters subsided quickly, then its peak Peat rate would occur in the early Holocene period. But what we see, is that the peak growth is 4000 to 6000 years later, as we see in Scotland (fig.4), the reason for this, is because the rivers were at their highest and fastest, early in the Holocene – not allowing the right environment for the growth of marshes and bogs. It is only when the rivers start to subside, that peats beginning to form, which is only interrupted, by occasional flooding, due to precipitation towards the end of the Neolithic period, and the onset of farming.
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