Dale Fort Blog Number 6

30 04 2012

  History of Dale Fort Part 3

The First Humans

  An important local early site is The Nab Head, 3 miles north of Dale.  This site has yielded lots of flint chippings and other artifacts leading to the conclusion that there was a Mesolithic (c12000 years ago) industry located here producing flint tools.  There is no local source of flint and it has been suggested that the raw material was imported from Ireland, 50 miles or so to the west.  Similar finds have been made at Long Point and Great Castle Head 1.5 miles west of Dale Fort.  The fields near Castle Beach are also a rich source of Mesolithic microliths and flint cores.

A piece of worked flint, a microlith and a beach flint core (from which the sharp shards were struck off)

It seems likely that there would have been a human presence on what is now Dale Point.  What did it look like?  If you stand on Dale Point and watch the biggest ships coming and going and look at the buoys marking their channel, you can see the course of the ancient River Cleddau.  The sea would have been further away than it is today.  You would be standing on a hill probably with wooded slopes. Looking down the incline you might see your dinner running around on the fertile soil of the flood plain of the ancient Cleddau. Today’s large ships follow the deepest channel available.  The river cut this before sea level rose to its present height roughly 8,000 years ago.   Around that time populations increased in Britain especially near the coasts. The most obvious evidence of early human activity at Dale Point is the ditch and bank cutting off the narrowest part of the peninsula.  Evidence from this site indicates a history dating back to the early Bronze Age (c4000 years ago). The site has the longest history of occupation of any of Dyfed’s peninsula forts.

In late 1991, when Dale Fort’s sewerage system was being rebuilt, a (deranged) proposal was made to build a sewage treatment plant on the ancient site.  This would have required all sewage to be pumped to the highest point of the Dale Peninsula. The works would have destroyed any archaeological evidence and so CADW (= KEEP – the Welsh equivalent of English National Heritage) decided that an emergency archaeological investigation was appropriate.

A group of archaeologists excavated the site at speed.  Forsaking trowel and toothbrush they employed a mechanical digger to skim off the topsoil and several weeks of study ensued.  Two flattened surfaces of what may have been habitations were discovered. A further flattened surface was possibly the remains of a track.  Several ‘pot-boilers’ were also found.  These are stones, cracked or shattered by rapid cooling as a result of being heated in a fire and dropped into cold water in cooking vessels, thus heating the water and cooking the food.  Hot stones may have also been used to make a type of a sauna bath.  Some fragments of Beaker pottery and some bronze and iron objects have also been found at the site.

The first defensive structures are thought to date from the later part of the Bronze Age  (c2600 years ago).  The mass graves of earlier times began to be replaced by individuals interred in tumuli or burial mounds. The skeletal remains of the earliest settled peoples indicate that in general they led longish lives and had peaceful deaths. From then onwards things began to change.  Celtic peoples (originating in the Iberian Peninsula?) were in the process of dispersing over most of Europe.  They brought with them tools and weapons made from iron. Skeletons of this period are younger and more often show signs of violence.  They also had transport in the form of the horses and they had invented the potters’ wheel.

Dale Promontory Fort looking North from the South Rampart

Revetting

Revetting

How long the Celtic Diaspora took to reach the settlement at Dale is not known.  However, some time after 2600 years ago, the existing defensive bank was improved and strengthened with internal structural stonework. Radio-carbon dating done on material found between the Bronze and Iron Age levels gives a date of 790 BC.  These are the earliest physical remains carbon-dated from any similar site in Dyfed.

   Professor Grimes (centre right, grey hair) and his slaves around 1967

Professor Grimes (centre right, grey hair) and his slaves around 1967

The structure was the subject of many archaeological studies by the late Professor Grimes and his students. The annual “Courses for All” archaeology week was for years a popular part of the Dale Fort programme.  Visitors today will observe the remains of the Iron Age rampart as a bank of moderate height and a ditch of moderate depth.   It would have been a much more formidable defence when it was constructed, the bank three or four feet higher and capped with sharpened stakes, the ditch three or four feet deeper.  The large post-holes on either side of the central entrance indicate that the wooden entrance gate was a formidable barrier.

 

 

A further archaeological dig was inadvertently begun on the 6th November 2005 when builders of the new St. David’s block created a compound for their materials and machines just inside the rampart.

They stripped off the top layer of earth with a digger and covered the site with hardcore, thus destroying any archaeological evidence that might have been present.  The National Park Authorities were very upset at the partial destruction of an ancient monument.  The builders however had asked for and been given their permission to excavate the site.  How could this be?  It seems that in order for Grimes to dig the site without breaking the law, it had been necessary to temporarily deregister it as an ancient monument.  The last Grimes dig had taken place in 1988 and the site had not been re-registered.  Permission had been granted because it was not listed in the ancient monuments file.   On the orders of CADW, in July 2007 the stripped off surface was examined by workers from Cambria Archaeology.  It took 2 days just to remove the hardcore.  Evidence of a small structure of unknown function was found near to the bank.  Little else was found in that dry summer, until it rained.  Then, just inside the rampart, slightly north of the entrance, just visible on the wet surface appeared two concentric circles.  The diameter of the inner one was 12 metres.  The archaeologists realized that they had found the site of a huge circular hut, surrounded by a drip-gulley (drainage channel).  This had been missed by Grimes and only found now because of a bureaucratic blunder, an accidental act of vandalism, and a chance change in the weather.  The sheer size of the foundation makes one wonder whether it might have been a permanent habitation for someone important.

The ring of dots shows the approximate position of the huge hut circle Pete Crane (archaeologist) is not giving a fascist salute but explaining the site to Steve’s course members on A Nibble of Pembrokeshire

The builders had piled up the earth into a 21st Century defensive rampart to prevent thieves from stealing materials and machinery.  CADW felt that these new defences might bamboozle later generations of archaeologists and so they made them (as best as they could) put it back where it had come from.  The stripped surface is now used by our students for a variety of botanical studies.

Dale’s promontory fort is one of many on the Pembrokeshire coast.  It was believed that these easily defended positions were used mainly as refuges. The local population would live and work in the locality around the fort.  When enemies approached, they would retreat with their animals and valuables behind the rampart, close the gates and encourage them to go away.  Discoveries like the site of the large building on Dale Point have led to the suggestion that at least some of these sites might have had permanent occupants.

Return to the blog for the next exciting installment which might be some myths and legends from the Age of Saints (Dale Fort’s buildings are named after local saints, so it’s sort of relevant)





Dale Fort Blog Number 5

25 04 2012

This article is about a large brown seaweed that is currently spreading up the coast of Wales.  I did a PhD on it many many many years ago.

Sargassum muticum in Britain

by Steve Morrell

Sargassum muticum (taken from the “Wanted” poster produced in 1975 to raise public awareness of the species)

Historical Background

 Sargassum muticum is a large brown alga native to Japan. In 1973 Bill Farnham of Portsmouth University found it growing at Bembridge on the Isle of Wight.  The Bembridge Ledges are a Site of Special Scientific Interest for seaweeds and the presence of a large non-native species was of both interest and concern.  Biologists were worried that it might replace native species.  Boat users, fishermen and recreational users of The Solent were also concerned.  They thought that it might disrupt their activities by blocking engine cooling systems, clogging fishing equipment and so on.

There was some evidence from the Pacific coast of the USA that the introduction of Sargassum there had resulted in the loss of native species as a result of shading and abrasion by Sargassum fronds.  This, together with the SSSI status of The Bembridge Ledges, convinced a meeting of 15 marine biologists (held at Portsmouth Polytechnic in 1973) that an attempt should be made to eradicate the alga.  It was conceded that it would be a somewhat optimistic undertaking but it was thought worth having a go.  If nothing else useful information about a poorly understood field would be gained.

The media were informed and delighted in the story of an alien species invading our native shores.  The local MP asked a Sargassum related question in the House of Commons.  As a result, for a few years Sargassum became quite famous (as seaweeds go).

A small army of volunteers was assembled from the local Naturalist Trusts and they set off to try to remove the alga from Bembridge.  It was a difficult shore to work on because the plants were growing in deepish lagoons for the most part.  The basal parts were attached very firmly to the substratum and if left could easily regenerate.  Bits of fronds that became detached would float off and help spread the species further.  It soon became clear that such efforts were doomed to failure and no serious attempt was made to eradicate the species after about 1979.  Clearance efforts on the south coast ceased entirely after about 1982.

Until 1975 its distribution was limited to The Isle of Wight and the Solent.  By 1976 it had been seen as far away as the estuary of the River Yealm in Devon and on the Cherbourg Peninsula in France.

The spread of Sargassum in Britain seemed to suffer a hiatus in the 1980s, as it seemed to be difficult for it to survive on the generally more exposed west coast. It took it a long time to round Lands End and become established in a suitable place.  There was no evidence of Sargassum beyond Land’s End up to 1986 when the author was employed to look for it.

In 1997 however, some drifting fronds were spotted at West Dale Bay, Pembrokeshire implying a resident population somewhere reasonably nearby. That year, a small, attached population was found at west Angle Bay, Milford Haven, Pembrokeshire, by a CCW (Countryside Council for Wales) survey team. These plants were removed before they became fertile but since some of the specimens were at least two years old and likely to have reproduced it may have been too late.

A new population was found established in a pool at nearby Broadhaven (North) in 2000, this was also removed by the CCW.  Despite this, I found cast off Sargassum fronds that same year on Broadhaven Beach.  They were fertile fronds bearing mature receptacles, so it looked like there was an as yet undiscovered population or populations somewhere else in the locality.  An attached population of Sargassum was found at Black Rock, Dale in Pembrokeshire in August 2001

The population appeared to be well established, 93 plants were counted mostly well developed with primary and secondary laterals, air bladders and receptacles (not hairy yet, i.e. no developing germlings).

Figure 1.  Sargassum in a pool at Black Rock

Figure 2.  Close-up of part of a frond

Figure 1 shows a pool at Black Rock containing a thriving population of Sargassum.

Figure 2 shows part of a frond from one plant. Cigar-shaped receptacles (fruiting bodies) and air vesicles (bladders) can be seen.

 

Feeling the basal parts seemed to reveal old lateral scars (evidence of last years growth). It was estimated that some of the bigger plants were up to three seasons old and may have settled about the same time that drift Sargassum was found at West Dale in 1997.

There were some large (70cm or so) plants that did not have scars so presumably were year one plants. Some of these may have settled from the drift material of June 2000 (lots of it, found by the author at Castle Beach Bay, roughly 2km East of the site) or offspring of the first settled plants. A few short plants of 10-15cm were found that had small holdfasts relative to primary laterals (like newly growing germlings). It was thought that these were probably late/slow developers rather than plants settled this year. In total, 9 rockpools in this area had Sargassum in them.

In February 2002 I discovered another attached population in Milford Haven at Musselwick near Dale. The site was visited on an exceptionally low tide and is accessible on foot only about twice a year.   We were able to obtain a limited number of quadrat samples in the time available and estimated a population density of about 2 individuals per square metre.  By about 2006 it had established itself at Castle Beach and Jetty Beach so Dale Fort was surrounded.  What it does not seem to have done (so far) is dominate the intertidal as it has done elsewhere.

Morphology and Reproduction

Sargassum muticum has a long-lived basal structure that looks a little like a small tree (up to about 10cm high), this bears lateral branches.  The main (primary) laterals can be anything up to 8 metres long and they bear secondary laterals.  When held up out of water you can see these secondary laterals hanging down like washing on a line.  The laterals have leaf-like structures (laminae) and air bladders (vesicles) and between these emerge the reproductive structures or receptacles.  The receptacles are cigar shaped and covered in pits (male and female conceptacles) within which develop the eggs and sperm.

During spring tides the eggs and sperm are released from the male and female reproductive parts (conceptacles).  The eggs are held close to the receptacle on jelly like strings and the sperms swim actively to fertilize them.  Over the next couple of weeks the fertilized egg develops into a multicellular germling.  It is still attached to the receptacle at this point and has developed a short thallus (1-2mm) and tiny rhizoids (root-like structures).  It looks a bit like a diminutive hot-air balloon at this point.  The receptacles begin to look hairy because they are covered with tiny germlings.

At the next set of spring tides these partially developed germlings drop off and because they are quite hydrodynamic “glide” away a short distance from the parent (probably no more than a metre).  The rhizoid initials are extremely sticky and as soon as they land they stick to the substrate.  Now they have an advantage over other macro algae.  They are already half developed so can grow like the clappers and avoid getting grazed up by herbivores.  (If they fail to drop off they stick to the receptacle, this eventually rots and they float off but can’t reattach).  Releasing at spring tides will give them a longer period of calm conditions to attach and start growing.   Thousands of germlings are produced so it’s very likely that some of them will strike it lucky and land in a suitable spot.

Sargassum muticum fronds are capable of reproducing even when detached from the parent plant because of the many small air bladders which keep them afloat and therefore alive.  During the late Summer and Autumn the primary laterals are cast off from the perennating base.  So long as they remain afloat they can continue to grow and reproduce.

A key element in its success is the fact that it is the only monoecious, self-fertilizing species of its genus. Male and female gametangia are present on the same receptacles.  This means that fronds can continue to grow and reproduce as long as they remain afloat.  Germlings can be released over anywhere the loose frond floats over.  Just a single receptacle can produce many offspring.  Should they settle in a suitable spot (wet and not too exposed) they will attach and develop into adult plants next spring.

This explains the rapid spread and extraordinary success that Sargassum experienced on the south coast of England since its introduction on the Isle of Wight.

  Sargassum is capable of surviving attached to stones while buried in soft mud and can tolerate variable salinity (down to 17 parts per thousand in Lake Vlissingen in the Netherlands). It might find conditions in Milford Haven eminently suitable for its growth.  Having said that, the population at Black Rock near Dale has virtually disappeared.  I have been unable to find anyone claiming to have removed it.  Maybe some factor or factors unknown caused its demise?  If anyone has any information I’d be interested to hear it.  Most of the stuff you read about Sargassum seems to take the popular press viewpoint, i.e. It’s entirely a bad thing.  This is not necessarily the case.  On the South coast of England large areas that where once mudflats now have a major primary producer.  Not only that, its highly complex morphology provides an array of attachment surfaces for epibionts of all kinds.  An early study by biologists from Portsmouth Polytechnic found 80 animal, 52 plant and 9 fungal species.  I well remember from my own work in the early 1980s the stunning variety of sea squirts, bryozoans and other creatures that lived on the plants from late Summer.  It is well known that the structural complexity of ecosystems plays a major part in determining their diversity.  Sargassum muticum can certainly enhance the physical complexity of a system.  In my own studies on the ecological effects of clearance programmes I found that epibionts of Sargassum survived the clearance better than non-target species.

In summary, I’d say that Sargassum has been given a harsh press.  It’s an interesting attractive plant that supports a vast diversity of other species.  It’s certainly not going to go away, so we’d better get used to it.

Here is my short video of how it all works:

Watch out for blog number 6 for more interesting stuff you didn’t know you wanted to know.





Dale Fort Blog Number 4

18 04 2012

The History of Dale Fort (Part 2)

This post is the second in a series that describe the history and pre-history of Dale Fort.  It deals with the rocks the buildings are actually constructed from.

Although Dale Fort is made mostly from rock quarried no more than 20 miles away, its stones are very different to the Old Red Sandstone they sit on. The main body of the fort comprises grey limestone blocks.  They were formed when the Rheic Ocean (see Blog Number 3) expanded and covered most of what was left of the Caledonian Mountains.  At that time, our part of what was to become Britain was located around the equator, so the sea was warm.  These conditions encouraged the deposition of calcium carbonate.  Ultimately this formed the limestone of South Pembrokeshire.  300 million years later these deposits were quarried from West Williamston on the Carew River and transported down the River Cleddau to be used in the construction of Dale Fort.

Some buildings and the battery at Dale Point are capped with granite.  This comes from Lundy Island in the Bristol Channel and is the youngest of Dale Fort’s rocks. It probably emerged as an intrusion (a hot up thrust from below (Ooh err missuss…) around 30 million years ago, in association with the Flimstone Fault which runs along the line of the Bristol Channel.

The remnant of the courtyard fronting the former Officer’s Quarters (St. Brides) is paved with thick slate flags. These probably came from Abereiddy or Rosebush in North Pembrokeshire. They were formed around 500 million years ago, when fine sedimentary shales were metamorphosed into slate as a result of heat produced by earth movements and the intrusion of molten material from below.

The bricks used in the construction of the fort probably came from the works at Bridgewater in Somerset and Goodwick near Fishguard.  The gravel and sand used for concrete and mortar probably came from the glacial deposit at The Gann, a mile or so from Dale Point.

 

Don’t miss the next historical blog which will deal with the first humans





Dale Fort Blog Number 3

3 04 2012

The History of Dale Fort (part 1)

This post is the first in a series that will describe the pre-history and history of Dale Fort.

We begin with the rocks the place sits on:

The Dale peninsula is made of sedimentary rock.  It was formed when particles of material worn from other rocks, settled out from water as mud or sand and were compressed over an immense period of time to make new rock.

400 million years ago, the American and European continental plates collided.  Our part of what is now Britain was somewhere near the edge of the continent formed by the merging of the two landmasses.  Geologists call it the Caledonian Continent and the mountains raised are known as the Caledonian Mountains.

The Caledonian Continent was roughly twice the area of Australia. The parts of it that straddled the equator were extremely hot and wet.  Among the mountains there were lakes and saltpans; there were no plants and no soil.  What is now Wales was around 1500 miles south of the equator on the southeast coast.  Over the next 50 million years, the continent drifted northwards, (at a speed roughly equivalent to the rate at which your fingernails grow) until the part that was to become Britain was on the equator.  During this time thousands of metres of rock, stained red with iron (III) oxide wore away from the mountains.  They were washed as sediments down the rivers.  Not all the sediments were to reach the sea, some settled out at the edges of wide, sluggish rivers and in their associated lakes.

The Dale Peninsula (Dale Point below middle right)

To the south of the mountains, nearer the sea (the Rheic Ocean) the rivers slowed and massive deltas formed.  A small fraction of these sediments, much compressed, is what makes up the old red sandstone of the Dale Peninsula today.

It is possible to detect what appear to be bands or layers in the rock which show that deposition may have been seasonal. There are also surface cracks in the rock, which may have formed during periods of drying.  The extent of the Rheic Ocean seems to have varied, extending northwards flooding the land and then retreating southwards, re-exposing the land and river deltas.  These marine incursions leave their trace today in the form of occasional bands of green rock in the mostly red sandstone.  Old red sandstone is stained red because it contains an iron compound, which is fully oxidized (like red rust). The sandstone consists of a matrix of this compound (iron (III) oxide) holding together harder particles of quartz.  When the sea flooded the southern part of the Caledonian Continent, the red sediments were altered chemically by the seawater and the iron (III) oxide was reduced to iron (II) oxide which is green in colour. This produced the green sandstone seen in exposures at Castle Beach and running more or less east west underneath Dale Fort.





Dale Fort Blog Number 2

2 04 2012

This is a short note to tell readers about some short You Tube films that I’ve made to try to demonstrate some techniques that our students sometimes find troublesome.  I’m hoping that this will be the beginning of lots of these because I think the potential is huge and there doesn’t seem to be much of this kind of thing on the web.

I hope that you might find them useful/mildly entertaining.

1. Starlings at Mabesgate

This is an amazing murmuration that took place earlier this year by my house

The link is

2. Error Bars in Excel 2007

I couldn’t find help in the Excel help system and it was driving me mad, so when I worked out how to do it, I made this film so you don’t have to go mad.

The link is:

3. How to Measure Heights on Seashores

Who hasn’t spent the long Winter nights dreaming of a film like this?  It shows you a quick, easy and surprisingly precise method of height measuring on any sloping ground.

The link is:

If you put the titles into the You Tube search engine you’ll also get to the films.

Look for the next blog where I’m thinking of beginning of a History of Dale Fort serial.