Dunkirk II Transgression

[Nathan Ross]

Supposedly, at some point during the 3rd century, rising water levels in the North Sea caused the coastal lands in north-west Europe and Britain to become flooded or inundated, and no longer capable of supporting habitation. This 'Dunkirk II Marine Transgression' may perhaps have led to all sorts of population movements and changes with ongoing effects into the migration era.

But what's the current thinking on this? When did the process begin, and how rapid was it? How much land was 'flooded', and what was the state of it afterwards? Was a large area of the modern Netherlands actually covered with seawater, or did it turn into salt marsh, or was it just the salinity of the water table that caused the population to retreat?

This map (from Cleary, The Roman West, AD 200–500: An Archaeological Study, 2013, p.47) seems to show the sea covering the land over a large area - how accurate would that be?:

   


Apparently Lake Flevo (the modern Zuiderzee) was still freshwater until the sea breached the isthmus in the thirteenth century. So surely enough dry land must have remained to hold back the seawater at this point?

[Walhaz]

I'm very interested to know more about this as well. Topography was often markedly different thousands of years ago, and anything we can add to our knowledge of the geographic changes in specific time periods helps us improve our understanding considerably. Unfortunately, I know very little about the topic. But I would wonder how long the rise in sea level would have persisted? Specifically to your question about the Zuiderzee, would it have been possible for the salt ocean to have come in and made it salt back in the 3rd Century, and then after the sea level receded it could possibly have gone back to being fresh, until the sea breach in the 13th Century? Over the intervening hundreds of years I would think that the Zuiderzee could have reverted to freshwater, but I can really only speculate.

[Robert]

The area was not submerged all of the time, but regularly inundated for periods of time. This need only happen once every few years (ruining crops, drowning livestock and make an area impassable) to cause the population to pack up and head for higher ground. For a society based on agriculture, there is no coming back from repeated flooding, as a missed harvest (and one heavy saltwater flood is all it takes to ruin the crops) is a year without food and income. The original Frisians moved south into the Frankisch area's in the Merovingian period. The Frisians of today are indeed immigrants from Pruissia, not descendents from the brave Frisians described by the Romans.

[Nathan Ross]

would it have been possible for the salt ocean to have come in and made it salt back in the 3rd Century, and then after the sea level receded it could possibly have gone back to being fresh, until the sea breach in the 13th Century?

I don't know either! There does seem to have been a lot of geological and hydrographic study done in the area, although most of it is highly technical and I don't know if it yields the kind of historical information we might like!

Robert's point might suggest that regular periodic inundation created large saltwater lagoons inside the Frisian islands, which in turn would have made Lake Flevo increasingly brackish over a long period before the final sea breach, perhaps.

The original Frisians moved south into the Frankisch area's in the Merovingian period.

Or perhaps before that? I know there are settlements recorded in North Holland into the fourth century, so presumably some scattered population was hanging on in these areas long after the initial sea-level rises (which Edith Wightman in Gallia Belgica dates to c.275 for the north French/Belgian coastline). Some Frisians appear to have been relocated as laeti inside the empire around that time, while others may have migrated or been resettled in Britain. But I think you're right about the process that led to these population movements.

[Robert]

It only takes a week of flooding to ruin the crops, so I do not think the sea actually spilled into Lake Flevo. It is more likely periodic spring tides pushed inland into the river bassins. An effect of this is also creating fresh water floods in the upstream area's of the rivers which drain into the sea. The salt water intrussion acting as a stopper to the normal river discharge. So flooding would be both salt AND fresh water. This effect can move upstream for tens of miles.

[Nathan Ross]

An effect of this is also creating fresh water floods in the upstream area's of the rivers which drain into the sea

Ah yes, good point! So the whole inland area could also have become steadily wetter and more marshy, without actually being covered by the sea.

[Flavivs Aetivs]

I thought the whole Marine Transgression theory had been disproven?

[Robert Vermaat]

Posted at the same time as Evan!!

Robert is right: one salt flood and your land is gone (comparable to the 'salting' of Carthage. Bracking water will do the trick too if your crops are sensitive enough.

Actually the Dunkirk II transgression floodings have been called into question recently. Especially the cause of the flooding as 'rising sea levels' as sole cause has been under debate since.

Unfortunately the internet is still full of the old theories.
The Dutch wiki page is reasonable: https://nl.wikipedia.org/wiki/Duinkerke-transgressies
Another link: http://www.geo.uu.nl/fg/palaeogeography/...ogeography

[Nathan Ross]

I thought the whole Marine Transgression theory had been disproven?

I know it's been questioned (which is why I started my original post with 'supposedly'!) - but I don't think there's currently enough evidence either way to say it's been 'disproved'. The map I added above is from a 2013 publication, which suggests the theory still has some currency.

Especially the cause of the flooding as 'rising sea levels' as sole cause has been under debate

Thanks - I suspected that might be the case. But I find the two articles you linked a bit hard to fathom (ha!) - if it wasn't rising sea levels, what might have caused the change? Or was there no change? What sort of condition might this land have been in, if not actually inundated?

[Robert]

I did not say an occasional salt water influx will turn Farmland into useless wasteland for all time. Zeeland was flooded several times, most recently in 1953, but agriculture recovered after the dikes had been rebuild and the area drained. Prolonged flooding with salt water will.
To understand the mechanisme of upriver flooding, it takes a bit of knowledge of how a delta bassin works. Like I said, a number of spring tides with winds from the north amplifying the PERIODIC rise of the sealevel can create a severe buildup of water in the rivers and extensive flooding. It does not really need to spell a rise in global sealevels, just a shift in weather patterns due to a shift in the airflow in the upper atmoshere (jetstream) and some bad luck. Ruined crops and areas turning mashy would be an incentive to look for greener pastures elsewhere, the vacume created by the Romans leaving could provide room towards the south.

[Robert Vermaat]

if it wasn't rising sea levels, what might have caused the change? Or was there no change? What sort of condition might this land have been in, if not actually inundated?

I think I recall that I once read that the Romans also used peat. So they dug their own holes? But a changing climate with heavier storms could also devastate unprotected coats, wideling the gaps through which the sea got inland.

[wayne oldfield]

Was there not a world wide climate change with a warm period (causing sea level rise in the 3rd to 5th centuries? I' m sure i have read one theory that it caused many cultures to decline / fall / migrate. Just a small rise in sea level would cause wide spread floodin in tidal zones and low lying areas while changes in rain fall patterns could trigger mass migration

[Flavivs Aetivs]

It's been established that the El Nino cycle caused a megadrought in Central Asia/Mongolia in the 360's, 430's, and 550's, with the first one being the most severe.

[Nathan Ross]

a warm period causing sea level rise in the 3rd to 5th centuries?

I'd always thought it was the other way round, actually, with the later Roman centuries being colder.

This pdf suggests that the climate got cooler in the third century before warming again in the mid fourth: "dendrodata indicate cooling beginning c.200; after a few warm years from 221 to 231 and sharp cooling from 243 to 253, gradual cooling prevailed until temperatures stabilised c.315 and shifted to warming c.365". The essay goes on to suggest that 'five major volcanic eruptions' between 235 and 285 may have exaggerated these climate shifts, and notes that the disruption this must have caused may have exacerbated the political and social crises of the period.

It is fascinating to speculate on how a minor and even imperceptible change in climate could have had such far reaching effects. Nothing here (as far as I can see) about coastline changes in NW Europe though...

[Michael Kerr]

Although I am no expert on the effects of El Nino on climate I am somewhat sceptical that the El Nino cycle has ever had much effect on climate change in Central Asia or even Europe or the Eastern United States as El Nino is basically a Pacific event affecting South America, Australia, New Zealand and Pacific island nations as well as regions on the western side of Northern America and the east of Asia which possibly may affect Mongolia.
The massive Himalayan Mountains and other mountain chains in Central Asia have always been the reason that Central Asia receives smaller amounts of rainfall compared to the monsoonal regions to the south and have always been under threat of droughts.
It seems to have become fashionable to blame El Nino for climatic changes everywhere. In Australia it is common to blame price rises in fresh food on El Nino. Central Asia has always had more than its fair share of droughts causing friction amongst the various nomadic tribal groups relying on herds of livestock and shrinking water and grassland resources for survival and Priscus mentions a Hun invasion of the East in either 395AD or 420 AD which he heard about on a diplomatic mission to Attila which occurred due to severe stock losses caused by a severe drought but whether these droughts were caused by El Nino is unsubstantiated. mile:

Regards
Michael Kerr