West, Ian M. 2013. Osmington Mills - Introduction: Geology of the Wessex Coast (part of the Jurassic Coast - World Heritage Site). Internet field guide. http://www.southampton.ac.uk/~imw/osmill.htm. Version 18th December 2013.

Osmington Mills, Dorset, Corallian - Part 1 - Introduction

Ian West,

Romsey, Hampshire
and Visiting Scientist at:
Faculty of Natural and Environmental Sciences,
Southampton University,

Webpage hosted by courtesy of iSolutions, Southampton University
Aerial photographs by courtesy of The Channel Coastal Observatory , National Oceanography Centre, Southampton.

Home and List of Webpages | | Osmington - Pt. 1 - Introduction |Osmington - Pt. 2 - Osmington Mills to Ringstead |Osmington Pt. 3 - Bencliff Grit |Osmington Pt. 4 - Osmington Oolite | |Osmington Pt. 5 - Osmington Mills to Black Head | Osmington - Pt. 6 - Corallian Fossils | Osmington - Pt. 7 - Bibliography | | Ringstead to White Nothe | | Poxwell Quarry and Anticline (with borehole into Corallian.

Bran Point between Osmington Mills and Ringstead, Dorset. Here the Middle White Oolite of the Osmington Oolite descends to the beach

Oil Sand in the Bencliff Grit at Osmington Mills, Dorset, 3rd November 2007

Ian West points out Arenicolites burrows in the Middle White Oolite of the Osmington Oolite Formation at Bran Point, Osmington Mills, Dorset

Second year students from the National Oceanography Centre, Southampton, mostly oceanographers, crowd around a Bencliff Grit concretion at Osmington Mills, Dorset, 2006

View eastward from Red Cliff Point - labelled

Black Head, Osmington Mills, viewed from near Redcliff Point

Osmington Oolite reefs west of Osmington Mills

Oil seep on the ledges above the Bencliff Grit, near Bran Point, Osmington Mills, Dorset, 2006

Go to another Osmington Guide?

Osmington - Pt. 1 - Introduction
Osmington - Pt. 2 - Osmington Mills to Ringstead
Osmington - Pt. 3 - Bencliff Grit
Osmington - Pt. 4 - Osmington Oolite
Osmington - Pt. 5 - Black Head
Osmington - Pt. 6 - Corallian Fossils
Osmington - Pt. 7 - Bibliography
| Ringstead to White Nothe |


Access and Parking
Bencliff Grit
Bibliography and References
Car Parking
Goldring Study of Bencliff Grit
Oil exploration
Oil sand
Oil seep
Oolite, Osmington
Oolite, Osmington

Introduction - General

View eastward from Red Cliff Point Osmington Oolite reefs west of Osmington Mills

Redcliff Point to Bran Point, near Osmington Mills, Dorset - geological map of Arkell

Aerial photograph of Osmington Mills, Dorset, with the  Osmington Mudslides of 1910-1914 shown, photo courtesy of the Channel Coastal Observatory

Aerial photograph, courtesy of the Channel Coastal Observatory of the coast between Osmington Mills and Bran Point, Dorset

Aerial view of the ledges between Bran Point, Osmington Mills, and Ringstead, Dorset, courtesy of the Channel Coastal Observatory

Old, pre-1999, photograph of the cliffs between Osmington Mills and Bran Point, Dorset

The cliffs of Corallian strata between Osmington Mills and Bran Point, Dorset, 2006

The cliffs around Osmington Mills provide excellent exposures of Upper Jurassic, Corallian strata and some Kimmeridge Clay. The sections are fossiliferous, have numerous trace fossils and many sedimentary structures. It is a good place for the amateur geologist. It is also a very good training locality for students, particularly for sedimentology and palaeontology, and is good for student exercises at making graphic logs and undertaking simple geological mapping. The cliff sections are about 6 km (about 4 miles) east of Weymouth, with access by a narrow tarmac road to a car park on the cliff-top at the Smuggler's Inn, Osmington Mills.

The locality is by road about 15 minutes from Weymouth, 1 and a quarter hours from Southampton, about 2 and a half hours from London. Study the Ordnance Survey 1:25,000 Outdoor Leisure Map 15, Purbeck and South Dorset, see map reference SY 735817. There are several caravan parks and holiday centres close by, including the Osmington Bay Holiday Centre.

The Corallian limestone, sandstone and clay strata, about 60m thick in the cliffs at Osmington Mills, are Oxfordian in age. The Oxfordian Age of the Late Jurassic lasted from 157.1 to 154.7 millions years ago. The Corallian Group was deposited in the later part of this, after the upper Oxford Clay of the earlier part. The conditions were generally shallow marine and the ancient latitude and climate Mediterranean. Dorset was then at about 37 degrees north and it was probably rather warmer than the present Mediterranean because coral reefs were present in southern England at this time. There are some corals in the Osmington - Ringstead cliff sections but they are not abundant here. Dinosaurs were living on the nearby land and ichthyosaurs, plesiosaurs and ammonites were present in the sea.

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2. Safety and Risk Assessment

Some general information regarding safety on Dorset geological field trips is provided and you are requested to read this if going to localities described here.

Almost everywhere on the Dorset coast care must be taken with regard to hazard of falling rocks. The stretch of coast from Osmington Mills to Bran Point must be considered carefully with regard to cliff and weather conditions before proceeding along it. In dry weather in summer it is much like any other Dorset cliffs and there is no excessive risk, although, of course, a rock can fall at any cliff anywhere.

On rare occasions, particularly during heavy rainfall or following it, or after frost, the high cliff between Osmington Mills and Bran Point can be dangerous, especially the steep western part (the oil sand exposure). Debris may fall and the foot of the cliffs should not be approached in these conditions. Bran Point and eastward is rather safer than the main cliffs. While debris can fall at Black Head, the cliffs there are not so steep. Obviously, assessment will be made at the time of a visit. Safety helmets should be worn when adjacent to a cliff. A minor hazard is that of falling on slippery sea-weed covered rocks on the shore.

The tidal range is not great and in normal weather conditions the coast is nearly always accessible. Low tide is better for geology because the shore reefs are exposed. Obviously beware of rare storm wave conditions.

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Access and Car Parking

Students meet in the car park Smugglers Inn, Osmington Mills

View from Osmington Mills - Portland

At Osmington Mills (SY 734816) there is a private car park, belonging to the Smugglers Inn, at the cliff top at the emd of the stream valley. There is a toilet block at the roadside.

From the car park there is a the view of the Isle of Portland across Weymouth Bay. The distant breakwaters of Portland Harbour are also visible. This embayment of the sea, formerly open to the east, is above easily-eroded Kimmeridge Clay. In the bank at the back of the car park there are some limited exposures of Osmington Oolite (the Middle White Oolite), which show cross-bedding. The effects of landslides can be seen from the western end of the car park.

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Stratal Succession

Succession of the Corallian strata in the Osmington Mills region, Dorset

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To the Beach

There have been minor cliff-falls recently and the route to down to shore is liable to vary. The original steps were alongside the privately owned slipway, built long ago by coastguards and in World War 2 used for a lifeboat (Bruce, 1989; 2001). There is usually a steep path down from the car park. An easier route to the shore passes around the back of the Smugglers Inn.

The beach is rocky with boulders and ledges. Somewhere around here was the landing place for the very successful smuggler French Peter and the ledges which form a natural dock, might well have been used for landing contraband (Bruce, 1989; 2001). Note, incidently, that there is also "Frenchman's Ledge" halfway between here and Bran Point, to the east.

Footnote: Smuggling at Osmington Mills

In the 18th Century Osmington Mills was a remote wooded coombe with a rough track inland to Osmington village, down which smugglers would have brought pack-horses for transporting their goods . The coast has retreated to a small extent since then but would not have been much different. The pub, the Smugglers' Inn was much smaller. It was known originally as the Crown, and then later as the Picnic Inn (Morley, 1994) . In the 1790s it was the headquarters of a foreign smuggler chief, as this was an excellent well-hidden place to land fine goods from France. French Peter, as he was locally known, was really Pierre Latour or la Tour (Morley, 1994). He had a fast, lightly armed cutter, L'Hirondelle (the swallow). His local accomplice was the landlord of the Smugglers' Inn, Emmanuel Carless. The Board of Customs had, at one time, five cutters in Weymouth trying to catch him and there was a price on his head. Presumably he had to use various different parts of the coast to advoid capture, and, perhaps, he was one of the users of the nearby Smugglers' Path on White Nothe ( a steep chalk path, scene of a dramatic capture of a man in league with the preventative officers in the novel Moonfleet by J.M. Falkner, 1898).

Morley (1994) repeats a story of French Peter and the Crown Inn (the Smugglers' Inn) and this is summarised here. Latour anchored his cutter off the beach here at Osmington Mills one day and made his way up the cliff path, which you have just descended, to the Inn. The landlord who knew him well offered him gin instead of the brandy which he normally drank, and giving signs that all was not well, pointed to the great fireplace, which you can still see in the pub. French Peter, realising that a Revenue Officer was hiding, announced that he had a chill in his bones and asked for a fire in the hearth to warm him. While the Revenue Officer had been in the pub in advance the landlord had told him frightening stories about French Peter. Smugglers certainly could deal with the enemy in a particularly vicious way because on one occasion a Preventative Officer was thrown over the cliff near Durdle Door. The Officer was soon smoked out of the chimney and fell out through the fire to the laughs of the smugglers. He was given a brandy and was allowed to get away defeated! Latour was very successful with his smuggling and it is said that later he married the landlord's daughter and settled in France, living on his fortune.
(For more on smugglers in Dorset see Hathaway (1994) ) ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

At the foot of the cliff turn left for the main cliff section to Bran Point and Ringstead (or, alternatively, turn right and go across Tank Bay to Black Head ). If you are going east, as most people tend to, then you will see fossiliferous Nothe Grit, really a grey sandy clay with much calcium carbonate. Look for the bluish-grey shells of the large round oyster Gryphaea dilatata and other fossils. At the first exposure, near to the steps down the cliff, this sandy clay is very weathered and if you look carefully you will see the glinting surfaces of small selenite (gypsum) crystals. Oxidation of pyrite (iron sulphide) produces sulphuric acid which, in turn, reacts with any calcium carbonate around to produce calcium sulphate.

Nothe Grit and the cascade

The Cascade over the Preston Grit or Trigonia huddlestoni Bed at Osmington Mills, Dorset, in November 2006

Old photograph of the Cascade over the Preston Grit, Osmington Mills

A short distance further east there is a small cascade or waterfall over the Preston Grit. A house gives an impression of having been built almost on the stream. The older photograph shows watercress flourishing in this carbonate-rich water which then flows across the beach between the limestone boulders, which make convenient stepping stones. This stream, like many others in Dorset, has been rejuvenated by the effect of coast erosion so that there is small recently formed, steep-sided valley within a broader (Pleistocene) one. The stream bed is not quite, however, adjusted to the present sea-level, but is about a couple of metres above. In this respect it is similar to the stream at Lulworth Cove and at Chapman's Pool. Others such as at Freshwater Steps, near Kimmeridge, and Portland Bill, are much higher and finish in waterfalls. These may be an indication of extensive coast erosion which has caused the truncation of the stream some distance inland of the point where it was at sea-level. This has some significance with regard to rates of coast erosion. Why the Osmington Mills stream is so low requires further thought. The matter is complicated, though, because sea-level is still rising long after the main phase of the Flandrian Transgression. Possibly, the Freshwater Steps waterfall is anomalously high, whereas this one is normal for Dorset.

View from Osmington Mills - Portland

If you go further east along the beach, you will notice the rough overgrown cliff on the left. This is an old landslide.

Continuing eastward you will approach a small headland, seeing as you go some Nothe Clay at the foot of the cliff. The beach here consists mainly of limestone pebbles from the Corallian, with occasional boulders from the Osmington Oolite and the Bencliff Grit. The small headland (map ref. SY 739814) is the start of the main Osmington Mills to Ringstead cliff section and large nodules from the Bencliff Grit are abundant. In the distance, and where you will often see a cormorant drying its feathers, is the remains of the wreck of the Minx. This was just a coal barge which lost her moorings on Porland in 1927 and is wrecked on the part of Pool Ledge which is known to the locals as West Maze (Bruce, 1989).

This field guide now continues, with description of the coast beyond, as: - Osmington - Osmington Mills to Ringstead.

Alternatively, proceed westward on the beach from the Osmington Mills car park towards:

Black Head.

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Brachiopods in the Basal Kimmeridge Clay at Osmington Mills

Finding Torquirhynchia inconstans in the inconstans Bed at Osmington Mills, Dorset, 2nd November 2008

One of the most well-known of the Dorset brachiopods is the asymmetric rhynchonellid - Torquirhynchia inconstans (J. Sowerby). In older literature this was referred to by the synonyms Rhychonella inconstans and Rhactorhynchia inconstans. In theory it should be found in the basal Kimmeridge Clay, Pictonia baylei Zone, at Ringstead, Osmington Mills, Black Head and at Portland Harbour. The exposures were much better in the past and it is not easy now to find it at all these localities. Fortunately, the inconstans Bed is, at present (2008), well-exposed about 200m. northwest of the Slipway at Osmington Mills. The specimen shown above was found there. They are not numerous, but another was discovered nearby within a few minutes. There seems to have been some recent erosion of the cliff by storm waves. The equivalent of the Ringstead Coral Bed is underneath but does not usually contain corals at this particular locality.

See Brookfield (1978) for discussion of the possibly intertidal environment in which this brachiopod lived. Note that this fossil is uncrushed and therefore unlike most fossil remains in the Kimmeridge Clay.

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End of a sedimentology field trip at Osmington Mills, Dorset led by Professor Dorrik Stow and Ian West; at the Smugglers Inn, Osmington Mills

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I particularly thank Professor Dorrik Stow and his many sedimentology students who have appeared in field photographs during their field trips. I also appreciate discussion with various students who have undertaken research projects on these cliffs. I very much appreciate the kind help and cooperation of the Channel Coastal Observatory in making available excellent aerial photographs of the region of study. I am grateful for the support of the School of Ocean and Earth Sciences, National Oceanography Centre, Southampton University. The hosting of the website is by Information Systems Services of Southampton University.

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To continue the Osmington Corallian Field Guide go to the section of interest:

- Osmington - Osmington Mills Introduction

- Osmington - Osmington Mills to Ringstead.

- Osmington - Bencliff Grit

- Osmington - Osmington Oolite

- Osmington - Black Head

- Osmington - Corallian Fossils

- Osmington - Bibliography

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|Home and List of Webpages Osmington - Pt. 7 - Bibliography | | Ringstead to White Nothe |

Copyright © 2013 Ian West, Catherine West, Tonya Loades and Joanna Bentley. All rights reserved. This is a purely academic website and images and text may not be copied for publication or for use on other webpages or for any commercial activity. A reasonable number of images and some text may be used for non-commercial academic purposes, including field trip handouts, lectures, student projects, dissertations etc, providing source is acknowledged.

Disclaimer: Geological fieldwork involves some level of risk, which can be reduced by knowledge, experience and appropriate safety precautions. Persons undertaking field work should assess the risk, as far as possible, in accordance with weather, conditions on the day and the type of persons involved. In providing field guides on the Internet no person is advised here to undertake geological field work in any way that might involve them in unreasonable risk from cliffs, ledges, rocks, sea or other causes. Not all places need be visited and the descriptions and photographs here can be used as an alternative to visiting. Individuals and leaders should take appropriate safety precautions, and in bad conditions be prepared to cancell part or all of the field trip if necessary. Permission should be sought for entry into private land and no damage should take place. Attention should be paid to weather warnings, local warnings and danger signs. No liability for death, injury, damage to, or loss of property in connection with a field trip is accepted by providing these websites of geological information. Discussion of geological and geomorphological features, coast erosion, coastal retreat, storm surges etc are given here for academic and educational purposes only. They are not intended for assessment of risk to property or to life. No liability is accepted if this website is used beyond its academic purposes in attempting to determine measures of risk to life or property.

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Dr Ian West, author of these webpages

Webpage - written and produced by:

Ian West, M.Sc. Ph.D. F.G.S.


at his private address, Romsey, Hampshire, kindly supported by Southampton University,and web-hosted by courtesy of iSolutions of Southampton University. The website does not necessarily represent the views of Southampton University. The website is written privately from home in Romsey, unfunded and with no staff other than the author, but generously and freely published by Southampton University. Field trips shown in photographs do not necessarily have any connection with Southampton University and may have been private or have been run by various organisations.