Kimmeridge Bay Dolomitic Limestone (Lead author: GR)
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Geology: Although the Kimmeridge Clay Formation largely comprises of oil shales and mudstones it also contains a series of limestone bands with a greater or lesser dolomitic nature. Dolomite was described as a rock by the French naturalist and geologist Déodat Gratet de Dolomieu (1750–1801), first in buildings of the old city of Rome, and later in the mountains now known as the Dolomite Alps of northern Italy. Dolomite may refer to the constituent mineral calcium magnesium carbonate CaMg(CO3)2 or to the parent rock, which may also be termed dolomitic limestone or dolostone. The mineral dolomite has a similar rhombohedral structure to calcite (CaCO3) except it has alternate layers of Ca2+ and Mg2+ ions separated by CO32- ions.
However, in anaerobic conditions suphate concentrations may fall due to the action of sulphate reducing bacteria, allowing the formation of dolomite. Microbial cell walls and extracellular polysaccharides may also provide nucleation sited for dolomite crystallisation. The resulting high concentrations of sulphide (S2-) will also promote the precipitation of Iron(II) sulphide (FeS, pyrite), reduced iron(II) carbonate (FeCO3, siderite) and ankerite (Ca(Fe,Mg,Mn)(CO3)2. The oxidation of these iron containing minerals, on exposure, gives hydrated iron(III) oxides which give the characteristic ochre colour to the Yellow Ledge Dolomitic Limestone. Hence the Yellow Ledge Stone may be described as a ferroan (ferrous ion rich) kerogenous dolomitic limestone consisting of 80% ferroan dolomite, 4% calcite with clay and kerogen (waxy organic material which when heated for a long period during deep burial can generate oil). The top surface occasionally reveals not completely crushed ammonites, indicating some early cementation.
Ammonite fossil, retaining some 3D structure, on the upper surface of the Yellow Ledge Stone Band. Suggests that dolomitisation occurred relatively soon after deposition. Ⓒ Geoffrey Rowland
An interactive map of Kimmeridge buildings and quarries mapped so far. The Layer control (top-right) allows you to show/hide different map layers. Ⓒ Geoffrey Rowland
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Dolomitic limestones occur thoughout the geological record. In the Dorset succession the Triassic Mercia Mudstone Formation, Jurassic Kimmeridge Clay and Portland Sand Formations and Cretacous Purbeck Formation all have dolomitic components. Present day environments where dolomite formation occurs are scarce. Conditions that would be expected to promote dolomite formation, high concentrations of calcium (Ca2+), magnesium (Mg2+) and carbonate (CO32-) ions, are usually associated with high sulphate ion (SO42-) concentrations which bind Mg2+ ions and keep them in solution. This has resulted in the 'Dolomite Problem' which apparently contradicts 'Uniformitarianism,' an underpinning principle of Geology.
The Yellow Ledge Stone Band (YLSB), dipping eastwards on Hen Cliff until it forms Yellow Ledge on the foreshore. Note the fault defining one edge of the Yellow Ledge and running diagonally up Hen Cliff. The joints result in the YLSB eroding into 'convenient' blocks, useful for building and forming a natural 'rock armour' for the base of Hen Cliff. Ⓒ Geoffrey Rowland
Blake's Bed No.42 (named by the Reverend Blake) lies a little under the Yellow Ledge Stone Band at the base of the Upper Kimmeridge Clay. The bed is a very ferroan dolomite or an ankerite and it contains not only small quartz sand grains but also common glauconite grains. Glauconite, an iron silicate mineral, is not present in the other dolomites and its usual origin, in the Mesozoic, is on current-swept sea-floors, where conditions are appropriate both for ferrous and ferric iron. It is not normal in the deeper and suboxic to anoxic Kimmeridge Clay sea floors. This bed has been quarried by the Romans to provide dark grey tesserae for their mosaic floors at Fishbourne, near Chichester. It is easy to recognise because of the glauconite and other petrographic features.
The dolphin mosaic at Fishbourne Roman Palace. Image in public domain.
Blake's Bed 42, a dark grey, glauconitic dolomitic limestone used in mosaics by the Romans. Surrounded by YLSB blocks eroded from the cliff above. Blake's Bed 42 marks the base of the Upper Kimmeridgian Clay, approximating to the start of the Tithonian (Bolonian or Volgian) about 152 million years ago. Ⓒ Geoffrey Rowland
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Kimmeridge Yellow Ledge Band Dolomitic Limestone: Buildings
Because of oxidation of its iron content on exposure, the Yellow Ledge Dolomitic Limestone is not very weather resistant and has not been widely used for building. However, where it is exposed on the foreshore, it displays almost rectilinear jointing and breaks away into conveniently regular blocks. It has been used in a few constructions, close to its exposure in Kimmeridge Bay.
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Old Pier: To the south-east of Kimmeridge Bay is the remains of a pier built from Yellow Ledge Dolomitic Limestone blocks by the oil shale company Wanostroct and Co in 1860. The company lit the streets of Wareham with gas from the Kimmeridge oil shale. Backed by the French Ambassador to London, they secured the contract to light the streets of Paris and built a small port at Kimmeridge. Sea wall: Close to the site of the Old Pier, Yellow Ledge Dolomitic limestone has been used for the sea wall protecting the car park close to the Fine Foundation Marine Centre. 
Sea wall of Yellow Ledge Dolomitic Limestone blocks. Photo: GR
It is the only large building constructed of Yellow Ledge dolomitic limestone, possibly from the beach but, more likely, from a small cliff quarry immediately to the west. It also contains some brick and pillars of Portland Stone. The dolomite is very yellow and it is likely that blocks that were already weathered were used for construction. Because of the threat from coastal erosion and the tower's progressive collapse, from 2006-2008 it was taken apart and rebuilt by the Landmark Trust about 25 metres inland from the cliff. 
Clavell Tower, before being dismantled, moved and rebuilt. Yellow Ledge Stone Band rubble blocks are exposed under the damaged render. The colonnade is of 'unusual' Portland Stone which may have been sourced from the local quarry on Smedmore Hill. Ⓒ Gordon Pipe 2006
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The Old Pier, constructed of YLSB blocks originally consolidated with hydraulic cement derived from Grey Ledge and Cattle Ledge silicaceous dolomitic limestones. Ⓒ Geoffrey Rowland
Clavell Tower: This folly and lookout was built in 1831 by the Rev. John Richards who assumed the name Clavell when he inherited the Smedmore Estate. It has historic connections with Thomas Hardy and was the inspiration for P. D. James' novel, The Black Tower. 
Clavell Tower built of rendered Yellow Ledge Dolomitic Limestone and Portland Stone pillars. Photo: GR
Yellow Ledge Dolomitic Limestone foundations at the original location of Clavell Tower. Photo: GR
Fisherman’s' boat houses (see image left), built from Yellow Ledge Stone Band (YLSB) ferroan dolomitic limestone rubble blocks. Located close to the Fine Foundation Marine Centre on the east side of Kimmeridge Bay. Ⓒ Geoffrey Rowland
Photos taken by Gordon Pipe during the rebuilding, as part of his study. Due acknowledgement is made here to Gordon's report :Pipe, Gordon, 2007: The Clavell Tower Story [A report on the history, dismantling and re-erection of Clavell Tower at Kimmeridge, Dorset], Project for Bournemouth University.
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Safety Notes
- From the east side of Kimmeridge Bay it is possible to walk eastward by the cliff top, past Clavell Tower, but there is no route down to the beach.
- At beach level it is possible to proceed east but only providing the tide is low and conditions are favourable. It is easy to be cut off on the beach here and only if the tide and weather conditions are safe should one proceed along the beach.
- The danger of falling rock is great and cannot be over-emphasised. Safety helmets are useful, especially for parties, and much care should be taken to keep out as far from the cliffs as possible.
