DUNNET BAY S.S.S.I. - HISTORICAL AND
DEVELOPMENT OF THE DUNE SYSTEM AT DUNNET
On the seaward face of the dunes there is active erosion of the dune fronts. The sea-level around this part of the coast is estimated to be rising at l mm per year (Sparks and West 1972), a net result of isostatic and eustatic changes. On a beach with a 5 deg. slope, this would give a minimum advance of 1.l cm per year by the sea.
Apart from eroding the dune fronts the rising sea-level will reduce the width of beach sand exposed between tides and shorten the time during which it can dry. This will mean a wind of higher velocity will be required to lift moist sand and carry it into the dunes.
There are several active blow-outs along the dune ridge, which are moving fresh dune sand towards the road. Prom aerial photographs (Band 1981) the distance moved between 1946 and 1975 has been calculated. The largest moved at a rate of 2m per year over this period.
Dune systems are dynamic and future changes in sea-level and wind patterns could have dramatic effects on the development of the ridge at Dunnet.
ORIGINS OF THE SAND AND SOIL ON DUNNET LINKS
Additional minerals found in the Links soils, which are not found in soils derived from either parent rock, are aragonite and calcite/dolomite. These are relatively unstable forms of calcium carbonate and being of marine organic origin demonstrate the route which the sand particles have followed after erosion from the rock.
It has been suggested (Ritchie 1972) that many dune systems may owe their origins to large quantities of sand and sediment carried by rivers to the sea in the immediate post-glacial period and in some areas of igneous or metamorphic rock, where there is little active erosion now, it is felt that reserves of off-shore Flandrian deposits provide dune building material.
At Dunnet, the local sedimentary rock is more susceptible to present day erosion, with Upper Old Red Sandstone being the softer of the two types. The proportions of each rock type in the soil could simply represent current erosional factors modified by (a) the different proportions of each rock in the vicinity and (b) the susceptibility of each type to erosion.
If Flandrian deposits are the main sand reserve, there could be long-term implications for the Dunnet dune system. In time rises in sea-level could place the sediments beyond the reach of any local wave or current activity and input of sediments could fall.
On the other hand it is also suggested (Ritchie and Mather 1970) that Dunnet Head acts as a large groyne preventing long-shore transport of local sediments and trapping them in the Dunnet Bay area, if this is the case then recently eroded sediments may continue to supplement sand supplies to the bay.
SEA-LEVEL CHANGES AND DUNE DEVELOPMENT SINCE THE LAST GLACIATION
In the warm Flandrian climate which succeeded the final period of the last glaciation, melting glaciers and isostatic rebound of the continental crust caused dramatic changes in sea-levels which are still continuing at a much reduced rate today.
Using world-wide (eustatic) measurements of sea-level changes it is possible to gain some idea of the movement of the sea into the bay at Dunnet over the last 10,000 years. Locally these levels will have been modified by sediment movement into the bay and isostatic movement of the Highlands. Nevertheless, there will have been an overall rise in water-level with time. The changes would have been swift at first gradually decreasing until today when isostatic and eustatic adjustments are almost in equilibrium. Most of the large scale changes were over by 1,000 B.C. Superimposed over the basic trend will have been fluctuations due to smaller climatic changes e.g. increased cold, wet weather from AD 1,250 - 1700's with glaciers re-advancing and ice between Denmark and Norway.
At Dunnet the sea as it rose would have encroached further into the bay, possibly even pushing a series of earlier dune ridges before it if there was sufficient sediment available.
In the period 8,000 - 6,000 B.C. much of Britain became covered by frost, from evidence of wood in peat (birch and willow spp.) and pollen diagrams for Caithness (Peglet 1979, Durno 1958), it is probable that the county was never warm enough for deciduous or pine forests, pollen for these species was low and probably represents an input from distant sources. Instead the area appears to have been covered with birch and willow scrub and this was probably the type of vegetation which grew on the land in front of the advancing sea and which is now preserved in peat under the beach sand at Dunnet.
About 5,000 B.C. the weather became wetter. Changes are seen in the pollen types at this date which is known as the Boreal Atlantic Transition (BAT). The initiation of peat growth in some parts of Britain is attributed to man's intervention'- "The slash and burn" policy coupled with increased grazing preventing tree regeneration.
However in Caithness it could be attributed to increased precipitation leading to water-logging on a very flat landscape, certainly pockets of peat existed within the county from a very early date, in 5,500 B.C. there was an increase in pollen from Ericoid plants particularly Calluna vulgaris (Durno 1958). Pollen indication of man's interference was first thought to be indicated by the elm decline, seen at 3,000 B.C. in a Caithness lake core (Pegler 1979), but it is now more generally thought to be linked with an increased appearance of pollen from weeds of cultivation, in particular Plantageo lanceolata, which occurs around 2,000 B.C. in Caithness and matches dates for evidence of man's increased activity in the Orkneys.
Shallow depths of peat are found on the beach at Dunnet, below high water mark and it is also found under the base of the dunes. Often associated with this peat are lengths of wood and it appears that the transition from scrub to peatland is preserved here.
The depth of the peat found is shallow, the greatest depth found so far is 45 cms. The peat appears highly humidified and lacks marked layering showing vegetational changes. There is no apparent acrotelum (active layer) normally found on peat bog surfaces, suggesting the upper layers may have been eroded at an earlier date. The peat is sandy implying a possible input of sand from the shore during peat development. A gradual input of sand would have altered the drainage and mineral content of the peat and therefore the flora and this may have slowed or stopped peat formation but the transition to pure sand is sudden suggesting a swift advance of the sand and/or missing upper layers.
In conflict with the idea of a straight forward advance of the sea into the bay is the suggestion that the area is a raised beach(Mercer 1980-81). The siting of archaeological remains around the 10 m contour at Dunnet and Sandside Bay has been suggested as an indicator of a higher sea-level at some previous time. Added to this are rounded beach pebbles found within the dunes and reports of the large shells, Lutraria lutraria, being found in deposits inland along the Burn of Midsands.
If the dating of the hut circles is taken as Late Bronze Age to early Iron Age, the 10 m high watermark would have occurred around 1000-500 B.C. In opposition to the raised beach theory are archaeological remains below this contour, these are however undated and in the case of two cairns behind the dunes, it isn't clear whether they are small fixed dunes or ancient sites, only one stone slab has been found and no proper investigation attempted, other sites at the southern end of the dunes aren't as easily explained away.
There are several ways in which the sea-level could have been higher at some time:(a) a change in the pattern of isostatic rebound, (b) a dramatic rise in sea-level which could have required a temperature increase, wetter conditions and a retreat of the ice sheet or (c) a combination of both.
Current isostatic changes are lifting the Central Highlands at 3 mm per year, this is a higher rate than the surrounding continental crust and causes tilting downwards of neighbouring areas. Such a tilting over a period of time would alter the gradient of the land behind the bay and land at 10 m now could have been at a lower level previously.
Large, swift rises in sea-level from melting of ice age glaciers were over by c. 1000 B.C., although the climate around 1,000 - 500 B.C. was conducive to at least temporary rises in sea-level due to increased wet and cold. There was a period of storms and flooding abandonment of low laying areas and renewed peat growth. In an area such as The Links this could have turned much of the central section into a damp, marshy environment which wouldn't have been suitable for habitation.
On existing evidence it can be seen that a straight forward advance of the sea into the bay in response to eustatic changes, is an over- simplification as is the unmodified idea of a 10 m raised beach due to isostatic uplift.
R E F E R E N C E S
|This article first appeared in the Caithness Field Club Bulletin April 1988|