Newsletter, Winter 2011
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NEWSLETTER, WINTER 2011 MIMRAM VALLEY, SATURDAY APRIL 17, 2010 By Linda Hamling We met Mike Howgate, our leader for the day, at the garden centre in Codicote. He led us first through some woodland opposite, where bluebells and wood anemones were just coming into bloom, and we then emerged into a large field where Mike gave an introduction to the day. During the Anglian glaciation about 450,000 years ago, ice spread across the eastern part of the Chiltern (Chalk) scarp and diverted the River Mimram from its preglacial course, which was probably through Codicote, into its present position to the west of the village. The ice deposited mainly glaciofluvial sands and gravels over the Chalk. The British Geological Survey described these as kame deposits, because they form mounds and ridges. |One side of the field was bordered by a prominent gravel ridge, and in the opposite direction we could see numerous mounds covered by woodland. In a small scrape in the middle of the field, we found Bunter quartzite pebbles, which the ice had brought from the Midlands, and in the past there have been numerous sand and gravel pits in the area. Continuing south-westwards across Codicote Heath, and passing numerous other small exposures of the sands and gravels, we descended the steep slope into the present Mimram valley. After joining a path running parallel to the river, we had a view up a dry tributary valley, which was cut in gravels on the eastern side but in Chalk to the west. Boreholes in the area have shown that the sands and gravels are very variable in thickness, but locally reach 40 ft. Where the path met the Mimram, we left the river’s main course to follow the mill-race to Kimpton Mill (TL 198185). North of the mill, a watercress bed is fed by the mill leat, and beside the nearby lane, a cottage was built for the river keeper, who had the duty of ensuring fair use of the river for all its users. We then retraced our steps to the garden centre, where we had lunch. From there we drove to Nine Wells Watercress Farm (TL 180213), which is run by the Sanson brothers. First we bought bunches of delicious cress cut fresh from the beds, before a tour. A railway line salvaged from the Western Front in WWI runs along the length of the beds, enabling the cress to be harvested. A manicured hedge acts as a windbreak. The nine wells are boreholes taking water from the Melbourn Rock 200 ft below to feed the beds. Mike partially covered the head of one borehole with a brick to show the strength of the flow. Overflow from the watercress beds provides most of the water feeding the Mimram. One of the Mr Sanson’s told us that his great grandfather had drilled the wells. Little frogs leapt around our feet as we talked, and we also saw our first Brimstone butterflies of the year. We then drove on to Deacon Hill (TL 126299), which is on the north scarp of the Chilterns 1 near Pegsdon. The hill is capped by basal Upper Chalk disturbed by flint workings, and below us to the north, spurs of the escarpment were capped by outliers of Melbourn Rock and Totternhoe Stone. Further north we could clearly see the vale formed by the Gault Clay towards, in the middle distance, the Lower Greensand escarpment, and beyond that the northern side of the valley of the Great Ouse. Beyond Pirton to the northeast is another small outlier of Totternhoe Stone, from which stone was quarried for Pirton church. The Chalk scarp in this area is dissected by several steep-sided dry valleys, the origin of which is still debated. Spring sapping and gelifluction or nivation in periglacial conditions have all been invoked to explain them. One valley is known as Roaring Meg, because it contains a stream fed by a strongly-flowing spring rising probably from the Melbourn Rock. When Pirton first became a separate parish, Ralph Lindsay Loughborough, its vicar, was shocked at the dilapidated state of the church, and began collecting funds to repair it. He successfully appealed for 50,000 shillings (£2500) to be raised by donations of one shilling per person, and used the money slowly to rebuild the tower and parts of the chancel and nave. The stone was quarried by volunteer labour from the small outlier of Totternhoe Stone, which was part of the parish glebeland. The stone was hardened by exposure for six months before use. In his daily diary, Ralph Loughborough recorded fossils found during the quarry work. These included giant ammonites, Parapuzosia austeni, up to 1.5 m across, and a fossil fish Ctenothrissa radians, which was put on display in the church but sadly stolen. However, we were able to examine two of the ammonites, which had been built into the south wall of the church. In the porch, a piece of the dark brown ferruginous sandstone from the Lower Greensand had also been built into the wall. We sat in the church while Mike described the environment in which the Totternhoe Stone fossils had lived, and then went outside to examine the remains of the adjacent Norman Motte and Bailey castle. We thank Mike for the really interesting day exploring the fascinating geology of this part of Hertfordshire. NORTHERN IRELAND, MAY 8-15, 2010 By Linda Hamling and Jean Gardner Despite problems with delays to flights caused by Icelandic volcanic ash, we made our way successfully from Stansted to Belfast International Airport, where we collected our transport for the week, two luxurious Ford Galaxies. From the airport, we drove eastwards to Larne and then northwards along the magnificent Antrim coast road to our hotel for the week in Carnlough. H.V.Morton described this road as finer than the Corniche in the south of France, and we were later to hear from our leader, Phil Doughty, about the geological difficulties encountered during construction. The hotel had once belonged to Winston Churchill, who inherited it from his second cousin, the Marchioness of Londonderry. Sunday, May 9. On our first day, we drove north long the coast before deviating inland to cross high moorland, where we stopped to look at Loughareema or the Vanishing Lake (fig. 1). The main depression contained a small amount of water, but at other times the whole area is transformed into a large lake fed by streams at times of heavy rain. With no surface exit, the lake drains rapidly into the underlying Ulster White Limestone (= Chalk), but the changes in lake level cannot be explained by variations in the groundwater table, so it is thought the swallow hole that drains the lake periodically becomes blocked with peaty debris. When the 2 Fig. 1. Loughareema or the Vanishing Lake. Fig.2. Carrickarade Island and rope bridge. 3 hydrostatic pressure on this plug becomes too great, the plug is suddenly dispersed, thus draining the lake into the Chalk aquifer beneath. From Loughareema, we continued northwards into the Carey River Basin. Here Phil pointed out a series of flat-topped terraces, which originated as deltas created by glaciofluvial gravels deposited by outwash streams flowing into a lake ponded between the high land and the retreating ice sheet to the north. Previously the gravels were exploited for aggregate, but extraction had been terminated to preserve these interesting geomorphological features. At the charming harbour of Balintoy, Phil outlined the geology of Northern Ireland for us. Within a 20 km radius of Portadown, there is more diverse geology than in any other part of the province, though there are no Cambrian rocks. The Lewisian forms the little island of Inishtrahull, and in Balintoy were to see Ulster White Limestone and the Tertiary basalts that cover a large part of N. Ireland. When the Atlantic was opening around 60 Ma, there were volcanic outpourings of two types of lava, the Lower Basalts and the Causeway Basalts. The ‘carapace’ of Lower Basalt has preserved the Chalk, which developed a karstic landscape before it was covered. In a subtropical to warm temperate climate, the upper surface of the Lower Basalt was weathered to form a laterite, in which iron and aluminium were concentrated by weathering and leaching of silica and alkali metals. This reddened horizon, known as the Interbasaltic Formation, provides an easily recognizable reference point in the sequence. As the ice sheet melted after the Last Glaciation, the land rose by glacio-isostatic adjustment, but did so irregularly in relation to the eustatically rising sea level, thus producing a series of raised beaches around the present coast. In the middle of Balintoy harbour, the basalt has been downthrown to the north along a fault, so that Chalk occurs to the south and basalts outcrop on the seaward side of the harbour. In the car park, a sea cave exposed a fault which had brecciated the Chalk; we came upon another on the beach as we walked westwards. Just offshore were seastacks of Lower Basalt, and further out were other stacks formed of the tholeiites of the Causeway Basalt. Soon it became boggy underfoot, where we crossed an outcrop of Liassic (Lower Jurassic) clay emerging from beneath the Chalk, and marked by a line of springs. The springs have caused rotational slipping, as indicated by the landward inclination of seastacks composed of Chalk; one stack had a natural arch, whose appearance gave rise to the name Elephant Rock. Belemnites and crab burrows were found in the fractured Chalk exposed on the shore. In White Park Bay the Chalk is partially covered by large sand dunes.