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October 15, 2015 at 11:41 am #8657Anonymous
Geology Meeting on 14 October 2015. Notes on THE JURASSIC COAST OF DORSET
The Jurassic Coast is a World Heritage Site. It stretches from Orcombe Point near Exmouth in East Devon to Old Harry Rocks near Swanage in East Dorset, a distance of 96 miles. There are three key coastal areas where the Jurassic rocks can be studied in Dorset.
1. The cliffs around Lyme Regis and Charmouth.
During the 19thC Dorset quarrymen used the term Lias (a corruption of the word layers) to describe the Lower Jurassic (205 – 180 Ma) sequences of alternating thin limestones and bluish grey clays that form the cliffs in West Dorset. Here the Lias reaches a thickness of over 130 metres. It represents the sediments deposited on the margins of the Tethys Ocean which spread over much of the Permo-Triassic landscape of Britain. The Lias sea contained a rich fauna of ammonites and marine reptiles such as Ichthyosaurs and Plesiosaurs made famous by Mary Anning, the Victorian fossil collector who lived in Lyme Regis. Dark bituminous shales also occur in the Lias which suggest that the sea deepened from time to time. The shales contain iron pyrites, a mineral formed where there is a lack of oxygen in deep waters. The Bridport Sands occur at the top of the Lias sequence and mark a change in environmental conditions as sandy sediment was washed into the area.
2. The Isle of Portland (Upper Jurassic; Portland & Purbeck; 150 – 142 Ma)
The Isle of Portland forms the southern flank of the Weymouth anticline (eroded down to the Oxford Clay) and it is highest in the north and dips gently down to sea level at Portland Bill in the south. Although the Portland Stone extends across most of the area 75% of it is covered by the lower Purbeck Beds (quarry overburden). The Portland Stone is a fine grained white limestone that is well jointed and easily dressed into rectangular blocks. It became popular as a building stone in the 17th C when St Paul’s Cathedral was rebuilt after the Great Fire of London. Most of the quarries are on or near the coast so that the stone could be exported by sea. A fossiliferous limestone known as the Roach occurs at the top of the Portland Stone and contains ‘Portland screws and osses ‘eds ‘ otherwise known as Aptylexia (gastropod) and Laevitrigonia (bivalve). Large ammonites such as Titanites have been extracted from the Portland beds.
Chesil Beach is a 16km long shingle ridge (storm beach) that links Abbotsbury at the head of the Fleet lagoon to Portland. Longshore drift moves south eastwards bringing coastal material from the west.
3.; The coast around Lulworth Cove.
The coast from Durdle Door to Lulworth Cove and Mupe Bay is formed of a wall of Upper Jurassic strata that form a barrier protecting the softer Wealden Beds (Lower Cretaceous) from the continuous erosion of the sea which has already broken through in several places. Stair Hole adjacent to Lulworth Cove provides a good example of the way that the sea is excavating the soft Wealden beds behind the Portland/Purbeck wall. The famous Lulworth crumple in the Purbeck beds shows the effect of the Alpine earth movements (25 Ma) during the Oligocene. Lulworth Cove itself has been cut back through the sands and clays of the Wealden beds to the high cliffs of the massive Chalk ridge that extends eastwards into the Purbeck hills. Note the exposure of Upper Greensand (at the base of the chalk) near where the road reaches the cove and here the greensand really is greenish in colour! On the east side of Lulworth Cove is the Fossil Forest that consists of silicified boles which mark the base of conifers that grew during Purbeck times.
4. The Isle of Purbeck.
The most important structural feature of the Isle of Purbeck is the Purbeck monocline produced by the northward thrust of the Alpine orogeny. The chalk ridge of the Purbeck hills can best be seen around Corfe Castle that stands on a knoll between a twin water gap. Here the chalk is almost vertical hence the width of the Purbeck hills is relatively narrow Rule of thumb…the steeper the dip of a stratum, the narrower the outcrop. The underlying Wealden, Purbeck and Portland beds all dip steeply north near Corfe but then become horizontal across the Isle of Purbeck forming the southern limb of the monocline.The best place to see the actual curvature of the monocline is in St Oswald’s Bay to the west of Lulworth.
On the coast around St Alban’s Head the Kimmeridge Clay (154 -150 Ma) appears below the Portland Stone cliffs. Kimmeridge Bay is the type locality for this formation, consisting mainly of black shales and clays with some bands of limestone that form the Kimmeridge ledges. Some of the shales are bituminous but they have a high sulphur content giving off an obnoxious smell when burnt.
Swanage was the centre of the Purbeck Stone trade in the 18th & 19th C but in Medieval times Purbeck ‘marble’ was much in demand for cathedral interiors, eg.Westminster Abbey and Salisbury Cathedral. A beautiful example of ‘marble‘columns can be seen in Eldon Memorial Church near Kingston.
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