Studland: the South Haven peninsula
John Chaffey looks at the geology that formed this unique environment
Published in March ’16
The South Haven peninsula lies opposite Sandbanks on the other side of the entrance to Poole Harbour. Although only a few square kilometres in area, it occupies a unique place in Dorset’s coastal landscapes. Much of the eastern part of the peninsula has only been won from the sea in the last 400 years. Its coastal and inland sand dunes are the largest in Dorset, if not along the south coast of England, and are, unusually, east-facing. As the sand dune lands have grown at the expense of the sea, a large freshwater body of water, Little Sea, has become landlocked and isolated from the coastal waters of Studland Bay.
Much of the western part of the South Haven Peninsula is a continuation of Studland and Godlingston Heaths to the south. To the east ridges of sand dunes and their intervening damp ‘slacks’ have gradually accumulated from the sea.
The old sea-cliff forms a natural dividing line between the area of plateau heathland on the western side of the South Haven Peninsula and the land to the east that has been formed by accretion from the sea in the last 400 years or so. The old cliff-line is probably best seen behind Middle Beach Studland, where it is still quite an imposing feature. It extends away northwards and is encountered again at Knoll Beach, where the road from the main Ferry Road descends the old cliff-line to the National Trust café and shop. It continues, much degraded, northwards on the western side of Little Sea. It is best developed where the National Trust bird hide sits on its crest overlooking the lower lands around Little Sea. It then declines rapidly in height so as to be barely visible where the path runs from Ferry Road to Studland Bay.
The plateau heathland to the west is underlain by sands and clays of Palaeogene age, which are exposed in the low cliffs on the Poole Harbour side of the peninsula. This harbour coast extends from Brands Bay northwards to Bramblebush Bay and the Sandbanks Ferry. The little headlands of Redhorn Quay and Jerry’s Point extend out towards the channel of Redhorn Lake. An early map by Ralph Treswall (published 1585-6) shows the western part of the peninsula extending further north as a shingle spit as far as Stone Island, a mass of gravel about halfway between the Sandbanks Ferry terminal and Brownsea Island. Today, Stone Island is only exposed at low tide; rising sea levels may been responsible for its diminution in size over time.
Old maps give us some idea of the way in which the eastern part of the peninsula has grown from the sea. Maps by Saxton (1575), Camden (1607) and Speed (1611) show the peninsula as a narrow strip of land bounded on the east by the old sea-cliff that is seen today. Little Sea first appears on a map dated 1721 as a tidal inlet opening into Studland Bay. It was partly enclosed by a sand dune ridge to the north (Third Ridge identified by Cyril Diver in the 1930s) and another one growing north from Studland (today’s Southern Heath) By the end of the eighteenth century Little Sea had become a lagoon and today’s Second Ridge had begun to form. Sheringham’s map in 1849 shows the beginnings of today’s First Ridge, but Little Sea was still invaded at the highest tides. By 1886 Little Sea had been virtually cut off from the sea. Today Little Sea is connected to Shell Bay by the drainage channel known as Central Cut, and the smaller body of water, Eastern Lake drains northwards via New Cut.
It will be evident that the growth of sand dunes to the east of the old cliff line has been a steady one. Captain Cyril Diver was the first to produce a map which showed the pattern of the sand dune ridges in the 1930s, and it is still regarded as the standard map of the South Haven Peninsula. However, in the last eighty years change has continued. Since Diver’s time a new, seaward ridge has begun to develop, and now it is the main dune ridge fronting the waters of Studland Bay, known as Zero Ridge. Over the last few years (towards the end of the 20th century) there have been signs of the development of a new ridge towards the northern end of Studland Beach near Pilot Point and the Training Bank. This has been known variously as Zero minus One Ridge, or Zero Zero Ridge. It now boasts a growth of the dune-maintaining marram grass. Severe winter storms can curtail the growth of new dunes, but they usually recover. The high wave conditions of early 2014 eroded away Zero Ridge north of the Heather walk entrance, but recovery began in the summer months.
For sand dunes to grow as they have done on the eastern side of the South Haven Peninsula, there appear to be three main requirements. Firstly, there needs to be a steady supply of sand. Secondly there must be a mechanism for transferring this sand to the shore and thirdly there has to be a plant association that will fix the sand and ensure the growth of dunes.
The issue of the supply of sand for the dunes is an interesting one. The erosion of sand from the Palaeogene formations in the cliffs between Sandbanks and Hengistbury Head did provide a supply of fine to medium-grained sand. It is possible that this supply of sand may have moved out to Hook Sand offshore from Sandbanks. It has been suggested that, in the past at any rate, wave and current activity may have moved the material from Hook Sand across the southern end of the Swash Channel that gives vessels access to Poole Harbour, thus aiding the growth of the dune system over the last 400 years. However, with the completion of the promenades between Poole and Southbourne this particular supply of sand will have ceased. Furthermore, with the deepening of the Swash Channel, it will have become more difficult for sand to be moved across the Swash Channel. This may well account for the fact that the dunes at Middle Beach Studland are now rapidly eroding, lacking any replenishment. Some sand is available from the erosion of the cliffs to the south of Middle Beach, but this supply is not thought to be significant.
Waves are an important mechanism for bringing the sand onshore. Although the prevailing wind in Studland Bay is from the south-west, there are strong winds from the south-east as the leading edge of a depression moves up the English Channel. With a considerable fetch (the distance over which winds blow across water) to the south-east large quantities of sand can be delivered to the shore at these times.
The colonisation of the sand dunes at Studland has been well studied. Once the sand has been moved onto the beaches of the Peninsula, it is colonized by a variety of dune grasses. The most important ones are sand couch grass, sea lyme grass and marram grass. This latter is most important because its long interwoven roots can accumulate sand up to a metre’s depth in a year, as is witnessed by the growth of the dunes of Zero Ridge. Once the dune ridge is fixed, it is then colonized by a range of other sand-loving plants, such as sea spurge, sea bindweed, sea rocket, sea holly and sand sedge. A cross-section of Zero Ridge will reveal sand couch grass on the seaward side, deep-seated marram grass on the crest of the dunes and on the inland side a range of other plants dominated by sand sedge.
The dunes of South Haven Peninsula also show the interesting phenomenon of invasion, as well as colonisation. Important species invade the dunes from the open heathland beyond the old cliff-line. Ling heather has invaded the early ridges, and is in the process of invading Zero Ridge on the landward side: bell heather has also invaded the older ridges but in the damp ‘slacks’ between the ridges cross-leaved heather prevails. On the older ridges gorse is another ready invading species and has become dominant in some casesa. Scots pine also has begun to appear, with some specimens close to Zero Ridge. The contrast between the older dunes with more organic matter and the younger ones, has led to the former being known as grey dunes and the latter as yellow dunes, relatively poor in organic matter. The damp slacks between the ridges have their own distinctive plant associations with not only the cross-leaved heather, but also purple moor grass, various reeds and rushes, bog myrtle and birch and alder. Sallow-birch carr has developed in places in the slacks. Some of the slacks, and other ill-drained areas have become rather waterlogged, and present an almost swampy appearance, particularly where Central Cut reaches the seaward dunes at the northern end of Shell Bay. Oak, birch and holly have colonized much of the older sandy areas at the foot of the old cliff-line.
Such an ecosystem as the heathland and dunes on the South Haven Peninsula requires careful and sensitive management. Within the system there are rare birds such as the Dartford Warbler and the stonechat, and their habitats (gorse and heather) need protecting, particularly for nesting purposes. All six of the common British reptiles are found on the South Haven Peninsula, and measures are taken to protect their vulnerability, particularly the sand lizard and its ‘citadels’ where they are concentrated. Birch scrub is regularly cleared to encourage more open growth of heather. Pine-cutting reduces the spread of the Scots Pine, and sallow-birch carr is regularly cut to encourage other aquatic plants.
Erosion of the dunes is a major problem, more particularly at the southern end of the South Haven Peninsula where recreational pressures are greatest. The National Trust has now adopted a policy of strategic retreat, since direct dune protection from erosion is costly and ineffective. Coastal retreat is measured at about a metre a year and gabions and timber palisades do not afford the protection that is needed. Serious damage was done to beach access at Middle Beach Studland in the winter of 2014: temporary solutions were put in place. Serious erosion of the dunes remains a problem, although quieter conditions of summer aid recovery. ◗