Portesham Quarry Earthcache EarthCache

Safety

Take care with any loose rock. Do not hammer chert because of very dangerous splinters. Be careful not to disturb cows or other livestock and respect the property of the farmer and land-owner. Is is possible to fall from the edges of rock faces so be observant.

The old quarry, at Portesham Farm, east of Portesham, has become overgrown since the 1890s when it was worked both for building stone and for lime-burning. In late Victorian times it was of particular interest because of a large stromatolite ("tufa") feature in the Purbeck Formation, reputedly once thought by the quarrymen to be the remains of a fossil elephant. A Purbeck silicified tree was also found at the time of quarrying. Since then the exposure has deteriorated, The front part of the quarry as seen from the information board shows the Portland Stone, overlain by the lowest Purbeck, as described below. A further quarry face is hidden by trees to the right and above the first. It may include the Cypris Freestones of the Lower Purbeck, The hill slope below the quarry is cut down past the Portland Sand into the Kimmeridge Clay. The Clay was mined for a short time during the early 20th century. History The quarry has been used since the 12th century, from the evidence of dated buildings. The Abbey at Abbotsbury, started in the 12th century has ashlar (large squared blocks) of the fossiliferous Portland limestone described above containing ooliths, shells and crystalline calcite. Athelhampton house was recorded as being built in the 16th century from Portesham quarry. The churches in Portesham and Abbotsbury, Waddon Manor, Corton Farm and chapel all have windows of the Portland Stone, while their walls are of the Lower Purbeck. The thinner, laminated Lower Purbeck is used for building in Portesham, with a little in Abbotsbury. Stone from this, or other Ridgeway quarries eastward through Upwey to Poxwell, has been used in the Weymouth and Dorchester area, and northward through several river valleys. Both stones were also sold further afield in the 19th century, once the railway was built. The railway was built to transport the iron ore from Abbotsbury, about 1874, and an incline came from the quarry to the railway. The quarry was re-opened to build Hardy’s monument. On the top of the hill the disturbed ground shows that the Purbeck limestone was more extensively quarried than the Portland. Buildings here date from the 13th to 17th centuries. Landscape The Portland and Lower Purbeck Ridgeway stretches from Portesham, between Weymouth and Dorchester to Poxwell. There were several quarries along its length, used for building stone. The Weymouth anticline to the south shows clay vales and limestone ridges, working south from Portesham they are the Kimmeridge clay vale, the Corallian limestone and sandstone ridge, the Oxford Clay vale, the Cornbrash limestone and Forest Marble limestone ridges. The Forest Marble is in the core of the anticline, and the succession is repeated in reverse order as you move through Weymouth and onto Portland.

THROMBOLITES - "STROMATOLITES" The large cylindrical thrombolite

Thrombolitic [i.e. "stromatolitic" but not laminated] limestone occurs around a mould of a coniferous tree. There is no doubt that this was a tree because it has recorded silicified wood within it, and traces are still present. In addition a silicified trunk has been found nearby at the time of quarrying. Silicified trunks and moulds occur commonly on the Isle of Portland and at the Fossil Forest Lulworth, although they are best developed there above the Great Dirt Bed, and not so well-developed at the Lower Dirt Bed level. The thrombolitic coating is thickened in a series of encircling rings occurring at intervals of about 15 cm. The regularity and orientation of these suggests that the tree was standing vertically in water at the time of their formation. A depth of about 4·6 m. is implied by the length of the algal sheath. The basal part of the now-missing fossil tree has left an impression in the thrombolitic limestone. This could be just an impression or rotted timber, but it appears more likely that it is the mould of part of the bark of the tree. If this was a bark mould, then the bark seems to resemble the bark of a swamp Cypress tree, like the modern Taxodium distichum. Further in the interior of the mould can be examined. It can be seen that the tree trunk had well-defined straight grooves. Straight grooves are common on various modern trees, including Cypress trees. Purbeck fossil trees have been described as Cupressinoxlyon ("proto - Cypress wood") by Professor Jane Francis. It is not known whether this particular tree was of Cupressinoxylon type. Araucaria-type debris is known from the Portesham Charophyte chert a little higher in the succession, and it seems likely that both Cypress-type and Araucaria-type trees were present in Purbeck environments. Whether anything resembling the Swamp Cypress existed there is not known. There was a swamp with horsetails (remains in the Portesham Charophyte Chert), and there was a freshwater lake at times. There are still so much unknown about Purbeck environments.

Question 1. At the listed Co-ords look at the Information board and cliff then tell me the 7 Layers.

Question 2. List 2 typical fossils of the Portland Limestone?

Question 3. What is the width and length of this object and describe its apperance?

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