Geology Group Diary (12)

Welcome to Pembrokeshire U3A Forums Geology Geology Group Diary (12)

  • This topic is empty.
Viewing 1 post (of 1 total)
  • Author
  • #8991

    The Geology Group met at Merlin's Bridge Village hall at 10.30am on Wednesday 14 September 2016. Please note that the following notes will be available in hard copy at the next meeting.  The topic for the session was….
    The oldest rocks in South Devon are found in the most southern part of the county between Start Point and Bolt Tail. The area is known as the Start Point Complex and is formed of alternating zones of grey mica schists and green hornblende schists. The schists have been produced by regional metamorphism involving high temperatures and pressures. The mica schists show strong foliation and are formed from pre existing sandstones and shales whereas the hornblende schists were originally basaltic lavas and tuffs. The Start Point complex is faulted against the Meadfoot Beds in a line running E-W through Salcombe. This fault zone has a counterpart along the northern margin of the Lizard complex in Cornwall. Both areas appear to have been metamorphosed and thrust from the south against the Devonian land mass during early Variscan earth movements. Therefore although the schists were metamorphosed during the Lower Devonian (417-391 Ma) they are derived from much older rocks. Details of the Start Complex thrust structure are clearly seen at Outer Hope near Bolt Tail where a melange of brecciated boulders lies in the centre of the fault zone separating the schists from the Meadfoot slates to the north.
    Another more recent feature of the South Hams (undulating land south of Dartmoor and between the rivers Dart and Plym) is the 140 metre marine erosion platform. This is a plateau surface that was cut during Pliocene times when the sea level was much higher than today.  A raised beach with degraded cliffs stand about 10m metres above present sea level around the coast particularly near Prawle Point. This relates to the end of Pleistocene times as sea level rose as the ice sheets melted. Much of the material below the raised beach is formed of periglacial head; angular rock fragments in sandy clay (solifluxion deposit). At Hallsands just north of Start Point there are the ruins of a small fishing village standing on the raised beach. The village was destroyed in the winter storms of 1917. This was a disaster that should never have happened. It was the result of extensive shingle dredging to provide material for extensions to the naval dockyard in Plymouth which began in 1897. The dredgers removed the protective shingle barrier in Start Bay which had previously absorbed much of the energy of the easterly gales.

    The large estuary of the Salcombe river bisects the South Hams; its branching outline is characteristic of a drowned river system known as a ria. This was produced as a result of rising sea level around 10,000 years ago when the Pleistocene ice sheets were melting.

    The Meadfoot slates are Lower Devonian in age and they can be seen at Tor Cross where the sea has exposed the intricate fold structure of the original bedding and its relation to axial planar cleavage. Immediately north of Torcross is Slapton Ley, a freshwater lagoon protected from the sea by a shingle spit that has grown southwards across the mouth of the River Gara. Meadfoot Beach in Torquay is the type locality for the Meadfoot beds but Middle Devonian (391-370 Ma) limestones form many of the cliffs around Torquay including Hope’s Nose. Here on the south side of the headland there is a superb example of a recumbent fold above a gently inclined thrust plane. A well preserved raised beach with pebbles and shells embedded in sand stands about 10 metres above sea level on Hope’s Nose. London Bridge is the name of a natural arch near to Dyer’s Quarry; where formerly limestone blocks were quarried and exported by sea. Today the floor of the quarry exposes a rich fauna of both single and colonial corals. Triangle Point marks the southern end of Meadfoot Beach. It is a faulted block of limestone where the exposed inclined bedding plane is covered with corals and stromatoporoids. These are mounds of onion like layers which have been secreted by creatures as a protection.
    The nearby Kent’s Cavern is an example of a cave system that has been formed by underground streams dissolving the limestone and removing calcium carbonate in solution. Kent’s Cavern proved to be a rich depository of animal bones when the cave earths were excavated during the 19th century. The bones of sabre toothed tigers, hyaenas and woolly rhinoceros indicated that these animals were probably hunted by Neolithic people who inhabited the cave from time to time.

    The railway line between Teignmouth and Dawlish follows the coast where Brunel inaugurated his atmospheric railway in 1846. The construction of this coastal railway opened up some excellent cliff sections in the dune bedded sandstones of Permian age (290-252 Ma). At Coryton’s Cove on the south side of Dawlish honeycombed weathered  breccias (alluvial fan deposits) are overlain by aeolian sandstones  (desert dune formations). Cross stratification can be seen in the cliffs north of the town where each set of dunes has been eroded as the wind direction changed and a new set of dunes deposited above on a new surface. Several sea stacks can be seen along this stretch of the coast including the Parson and Clerk near Dawlish. Dawlish Warren is a sandspit that has been built out from Langstone Rock by longshore drift across the Exe estuary. The sand dunes on the seaward side of the spit give some protection to the salt marsh and nature reserve on the inner side.

    Budleigh Salterton is a pleasant seaside town to the east of the Exe estuary. On the pebble beach Millais was inspired to paint ‘The Boyhood of Raleigh’. The Elizabethan explorer was born at nearby East Budleigh c 1552. Geologically the cliffs at Budleigh Salterton are desert sandstones of Triassic age (252-241 Ma). The pebble beds were laid down by ephemeral braided streams in a desert basin. Oxidation of iron minerals to ferric salts in arid environment gives red colour to the rocks. Stream channels can be seen in cross section in the cliff face. Cobbles, boulders, and pebbles are set in gravel and silty sand. The pebbles are mainly quartzites but there are also clasts of vein quartz, schorl (quartz + tourmaline), porphyry and sandstone. The top of the formation is marked by a distinctive yellow sandy band and a thin layer of black pebbles. These mark an aeolian abrasion surface on which lie polished wind faceted pebbles known as dreikanters. The overlying Otter Sandstones are formed of wind blown desert sands. They contain rhizoconcretions that are carbonate cemented concretions around the roots of primitive Triassic plants.
    NB. (i) The Devonian rocks of South Devon are all marine whilst in the rest of Britain Old Red Sandstone was being deposited under arid continental conditions.(ii) The Permian and Triassic rocks make up the New Red Sandstone that was deposited under desert conditions.

Viewing 1 post (of 1 total)
  • You must be logged in to reply to this topic.