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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,
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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.
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Man has made use of the
local geology; particularly the
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.
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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 illustrations (right).
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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
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of the county. There are some distinctive highpoints along the escarpment such as the hills at
Melbury, Bulbarrow, Hod, Hambledon and Eggardon. The shallow sloping dip 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
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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
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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
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contrasting with the more exposed seaward side of this distinctive feature which is separated
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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
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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
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and Bournemouth. The resistant headlands along the coast form distinctive landmarks, with
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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
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stacks of Old Harry Rocks to the east of Ballard Point are the remnants of the former chalk ridge
which linked the Isle of Wight.
2
The geodiversity of Dorset is expressed in both the landscape, and the quarries opened for
building stone and other mineral resources. However, the use of local stone has greatly declined
over the last one hundred years and geodiversity has suffered as a result. Many old quarry
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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 Hamstone
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distinctiveness of the built environment.
The loss of geodiversity 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 geodiversity. On the other hand, in landscape terms, quarrying can
be damaging. Is quarrying in Purbeck part of the landscape or damaging to the landscape? What
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large parts of Dorset local stone is no longer available, which has a direct impact on the quality of
new buildings and the character of the built environment.
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It would be impossible to examine all of the rock types and their expression on the landscape and
the built environment. Therefore the following attempts to describe only the most prominent or
striking examples.
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Softer clays form vales and are often surrounded by escarpments formed from harder rocks. The
Marshwood Vale is derived from a huge dome shaped fold in Lower Jurassic aged clays. These
clays also formed the rocks that are seen on the coast between Lyme Regis and Seatown. The
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drained; here the soils are thin and leach readily, supporting heathland vegetation.
The valley of Portesham through to the lowlands behind
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To the north,hard Portland Limestone and Purbeck Beds form the
Weymouth Ridgeway and these have been quarried for local
stone and dry stone walling. Between Worbarrow Bay and
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by Purbeck limestone to the south and the chalk to the north,
which has been folded into a near vertical orientation. The River
Hooke has cut its valley into Middle Jurassic aged Fullers Earth
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the valley at Kingcombe. To the north the overlying chalk
downland creates a dramatic contrast to the low-lying valleys.
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The soft Lower Jurassic Bridport Sand gives rise to a
distinctive landscape of steep, V-shaped valleys and sunken
lanes, around Loders, Mapperton and Powerstock, near
Bridport and northward, to the Yeovil area. A thin band of hard
Inferior Oolite limestone caps the soft sand and this often
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This gives rise to a striking change in landscape
character, usually from clay vales to steep slopes leading
up onto the chalk plateau often dissected by steep dry
valleys. Although the chalk is not hard, it is a massive
rock that has resisted the forces of weathering over
millions of years. The edge of the plateau forms striking
features; Eggerdon Hill and Fontmell Down are classic
examples.
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possibly the most extensive and easily recognised rock in
England.
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leading onto Salisbury Plain is all formed by chalk.
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The Tertiary sands, gravels and clays in the east of the
county give rise to heathland. Sands and gravels form
dry, leached soils that support the heath, while clays form
boggy areas.
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an ancient Tertiary (around 40 million years old) land surface.
The rocks were raised up and planed away by erosion. Later
the rivers cut into this land surface leaving remnants of the
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The area north and west of Weymouth lies within a huge
anticline or dome shaped structure. The rocks rise out of the sea
in the south, forming the wedge shape of Portland, and dive back
into the landscape to the north. The result is a series of parallel
limestone hills and clay vales that run east to west from the
shores of the Fleet to Osmington.
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Across Purbeck, from Durdle Door to Ballard Down, the rocks
have been heaved up by earth movements into a huge kink.
Either side of the fold the rocks are virtually horizontal but
within it, they are vertical. Where the chalk is caught up in this
fold, it forms the ridgeway extending from Lulworth to Ballard
Down and out across to the Needles on the Isle of Wight.
In places rivers have breached the ridgeway, making an ideal
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During the last Ice Age, areas such as the Marshwood Vale
or the slopes either side of Abbotsbury hill would have looked
similar to the coastal cliffs with landslides cutting into the
hilltops and mudslides extending into the valleys. With the
onset of less harsh weathering, these landslides have
reached a stable angle but the bumpy ground and crescent
shaped scars extending into the hilltops can still clearly be
seen. These ancient landslide slopes can be seen around the
edge of the Marshwood Vale (Sliding Hill at Bettiscombe is a
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below the Bellstone on Eggerdon and both sides of
Abbotsbury Hill Fort.
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permeable and water normally sinks through it. So, how did
the valleys form? During the last Ice Age the ground was
permanently frozen, but the winter snow melted in the
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valleys.
The Valley of the Rocks is a particularly unusual example
where the top of the slope was originally capped by Tertiary
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pebbles (conglomerate) that have been cemented together to
form ‘Puddingstone’. Frost action during the Ice Age slowly
forced these massive rocks down the slope to the bottom of the Valley where they lie today. Similar
rocks can be seen in the stream next to the church at Portisham.
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