Geology dominates the landscape, wildlife and local character of Dorset. From clay vales, limestone scarps, chalk downland to sandy heaths and even ancient landslides and erosion surfaces, a great diversity of rock types, erosional processes and structures have created the unique, variable and distinctive countryside. This ‘geodiversity’ underpins the biodiversity of the county from chalk downland to limestone grassland, heathlands, wet flushes and vegetated sea cliffs, it is the range of rock types and erosional forces acting on them that have created the habitats for the county’s celebrated wildlife.
Man has made use of the geology; particularly the local building stones and mineral resources. The character of the picturesque towns, villages and, in places, dry stone walls, is due to the use of a diverse range of local building stones. Aggregate, sands, gravels, clays and oil are all important economic resources to the county and are obviously based on the underlying and geological resource.
The clear relationship between geology, topography and landscape character is seen in the following diagrams.
The central parts of Dorset are dominated by an extensive swathe of chalk running North East to South West and through the South of the County round Weymouth to Purbeck. It extends North East towards Salisbury Plain in Wiltshire. It is an elevated, spacious landscape with a prominent escarpment which dominates and defines the undulating farmlands to the North, West and South of the county. There are some distinctive highpoints along the escarpment such as Melbury Hill, Bulbarrow, Hod, Hambledon and Eggardon Hills. The shallow dip slope of the chalk borders the heathland landscapes of the Poole Basin. This undulating lowland contains the remnants of a once extensive area of heathland; Hardy’s Egdon Heath and is based on thin and impoverished sands and gravels. It is drained by the alluvial basins of the Frome, Piddle, Stour and Avon, four of Dorset’s main rivers which flow out to sea through extensive stretches of pasture, marsh and mudflats associated with Poole and Christchurch harbours.
To the north of the chalk escarpment, the Blackmore Vale is an extensive, flat clay vale bordered by limestone ridges to the North West and deeply undulating foothills beside the escarpment to the south and east of the vale. The vale is drained by the alluvial basin of the Stour and its tributaries the River Lydden and the Caundle Brook. To the West of the county the landscape is more varied reflecting the complex geology, with steep distinctive greensand ridges separating clusters of deeply incised valleys and broad rolling farmland. In the middle of West Dorset, the Marshwood Vale, formed on the marls of the Lower Lias, is a secluded, bowl shaped depression, almost hidden by the surrounding complex landforms. These landforms include the broad arc of heathy greensand ridges and some of the highest points in the county such as Pilsden Pen and Lewesdon Hill, the highest at 915 feet, are found here. These undulating greensand ridges are outliers of the Blackdown Hills to the east in Devon and this landscape continues south up to the cliffs where it is truncated abruptly by the sea and down into the rolling landscape of the Axe valley to the north.
The lowlands to the West and east of Weymouth are different again to the West Dorset landscapes. A series of broad, hogback shaped limestone ridges alternate with spacious shallow clay valleys. The landforms all follow an east-West alignment and the landscape has a steady consistent rhythm. This ‘ridge and vale’ landscape flows down to the quiet edges of the brackish Fleet lagoon contrasting with the more exposed seaward side of this distinctive feature which is separated from the sea by the sweep of Chesil Beach. This distinctive shingle bank links the rocky limestone peninsula of Portland to the mainland.
The far South eastern corner of the county has a unique and remarkable geological context. The Isle of Purbeck is renowned for the variety and structural clarity of its rocks and landforms. A high chalk ridge, which once linked Purbeck to the Isle of Wight, separates the heathlands of the Poole Basin from the secluded clay valley of the River Corfe to the South. A further ridge and distinctive plateau, this time of limestone, separates this valley from the sea.
The sequence of cliff and bays along the 87 miles of Dorset’s coast clearly reveal the complex geological structure of the landscape inland. The rocks have been carved by the powerful forces of coastal erosion and deposition leaving a legacy of unique features like Lulworth Cove, Durdle Door, Chesil Beach, the under cliffs and landslips around Charmouth and the beaches of Poole and Bournemouth. The resistant headlands along the coast form distinctive landmarks, with Golden Cap at 626 feet being the highest point along the entire South coast.
Other features represent remnants of previous coastal processes, indicating the long history of physical landscape evolution. An example of this, mentioned above, is when the sea rose following the ice age leading to the severance of the chalk ridge joining the Isle of Purbeck to the Isle of Wight and the flooding of the upper reaches of the Solent River to form Poole harbour. The chalk stacks of Old Harry Rocks to the East of Ballard Point are the remnants of former chalk ridge which linked the Isle of Wight.
The geo-diversity of Dorset is expressed in both the landscape and the quarries opened up for building stone and other mineral resources. However, the use of local stone has greatly declined over the last one hundred years and geo-diversity has suffered as a result. Many old quarry sites have become overgrown or filled in. Some quarries are designated as Sites of Special Scientific Interest or Regionally Important Geological/Geomorphological Sites (RIGS) but without management they will continue to decline. The loss of local quarries also impacts on the built environment as local stone is no longer available. As a result Purbeck Stone or Ham Stone are typically substituted. The use of artificial stone further weakens the character and local distinctiveness of the built environment.
The loss of geo-diversity is, without doubt, the greatest issue for geology across the Dorset countryside but it could be different. Greater use of local stone could maintain the character of towns and villages and create geo-diversity. On the other hand, in landscape and visual terms, quarrying can be damaging. Is quarrying in Purbeck part of the landscape or damaging to the landscape? What does it do for local character and the economy? Clearly a balance has to be struck but across large parts of Dorset, local stone is no longer available, with a direct impact on the quality of new buildings and the character of the built environment.
Key geological features
- it would be impossible to examine all of the rock types and their expression on the landscape and the built environment and therefore the following attempts to describe only the most prominent or striking examples