{"id":3377,"date":"2017-04-07T03:27:07","date_gmt":"2017-04-07T08:27:07","guid":{"rendered":"http:\/\/rdnhs.org.uk\/blog\/?p=3377"},"modified":"2017-04-07T03:27:07","modified_gmt":"2017-04-07T08:27:07","slug":"nettlebed-25-march-2017","status":"publish","type":"post","link":"https:\/\/rdnhs.org.uk\/blog\/nettlebed-25-march-2017\/","title":{"rendered":"Nettlebed – 25 March 2017"},"content":{"rendered":"

On a fine sunny day 10 members joined Lesley Dunlop to explore the geology of Nettlebed Common.\u00a0 Lesley explained that on most of the Common the sands and clays of the Lambeth Group overlie the chalk bedrock.\u00a0 These remnants of a more extensive deposit were deposited about 60 million years ago in a warm estuary.\u00a0 They provided raw materials for the brick and pottery industry and there are many ponds in the former clay diggings formed because the impermeable clay prevents rainwater from draining into the chalk below. The Lambeth Group sands and clays are acidic so in more sandy areas the acid drainage \/groundwater can pass through fissures in the chalk beneath and eventually dissolve enough to produce a sinkhole.\u00a0 Our figure of 8 route started along The Green, after a short distance we turned into the wood to view an example of this.\u00a0 A large 4m x 5m sinkhole had opened up about 3 years ago after a wet January.\u00a0 The boundary of the chalk layer was visible, and some bricks on one side suggested the hole may have opened up previously and been filled.\u00a0 Other sinkholes have appeared in local gardens. We then headed back and up Chapel Lane;\u00a0 green alkanet, gorse and primroses were in flower.\u00a0 Continuing along the track we saw some of the ponds left after clay extraction for the brickworks, and recently cleared of overhanging vegetation.\u00a0 Our first Brimstone of the day was spotted. Heading west we crossed Mill Road and saw sand and clay workings alongside the track.<\/p>\n

After crossing the Recreation Ground we reached Priest Hill, an area with Nettlebed gravels and interglacial organic silts which is a SSSI for its geological significance.\u00a0 Lesley explained that this was a chalk landscape up to 80 million (m) years ago; and there is no chalk younger than this here although the end of the Cretaceous period was 65m years ago \u2013 there is a 20m year gap.\u00a0 This may be because this area had been uplifted above sea level.\u00a0 The exposed surface would then have been subject to weathering.\u00a0 The Lambeth Beds were deposited in the estuarine environment which followed. There is no other geological record in this area for another 50m years.\u00a0 The gravel deposits are dated at 2m years ago.\u00a0 Although there was glaciation as far south as Aylesbury there was no ice cover here, just permafrost, so the water passing over the surface would have caused some erosion.\u00a0 During glacial periods there is more surface run off, and the rivers cut down through overlying layers.\u00a0 Braided river systems were formed where the river regularly switched channels and deposited gravel.\u00a0 The Nettlebed gravel is the oldest associated with the Thames.\u00a0 It is probably not a terrace deposit, but possibly a gravel deposit across an area which was preserved because it dropped into a sink hole \u2013 other small gravel deposits exist locally. Subsequently the drainage pattern of the Thames changed and covered the Midlands.\u00a0 Then the terraces nearer the Thames were established with more local material.<\/p>\n

We followed the path to an area near a large beech tree where the 2m year old gravel (deposit) is exposed.\u00a0 The older rounded stones are flints which have been exposed on a shore line.\u00a0 The action of river water is not sufficiently intense to round off flints.\u00a0 A broken pebble showed a 2-3mm worked layer. We also saw some small creamy white quartz stones, they originate in veins of igneous rock, probably granite in SW England or Brittany. In the Triassic period erosion caused by flash floods washed pebble beds to the Midlands.\u00a0 These quartzite Bunter pebbles were carried down by the river systems and deposited in younger gravels here. Walking on we left the gravels and moved onto the Lambeth Beds again, with muddy patches due to bands of clay in the sand layers.\u00a0 We passed an open area previously used as a scout camp where heather was growing in the acid soil and mosses which prefer acid conditions were identified.\u00a0 Hard fern was seen on the sides of a large pit.<\/p>\n

Report by Julia Cooper and Rob Stallard<\/em><\/p>\n