The Lithostratigraphic Framework of the Hertfordshire County of England - Essay Example

The Lithostratigraphic Framework of the Hertfordshire County of England Introduction The Hertfordshire County of England has a young history geologically. The lithology of this area is largely composed of Quaternary and Tertiary deposits, with some glacial movement effects at its northern reaches. With the sandy conditions of the shores at its eastern side, you might say Hertfordshire is between a small rock and a big hard place. This report, a compilation of text and online sources, with some independent analysis, will discuss the lithostratigraphical framework of this area both from an historical perspective and from current (Halocene) developments, with geotechnical parameter values of the Quaternary deposits (see Figure 3 page 9). Formations, a principal mapping unit (enquire.com 2006), will be briefly described as to their importance in this report. The status of current ground conditions will also be presented as they affect the analysis of the area. Then geohazards, or expected/probable problems, will be considered. The objective of this report is to provide information that subsequent investigations could use. As in other Home Counties, Hertfordshire shares a common pattern of development from the 17th century, except for its construction of the New River in the Lea Valley that supplies London with water. The majority of settlements built in the times of the Normans were in the northeast. The Abbey of St. Albans, a major landowner, influenced land management strongly, including the formation of hunting parks, which Hertfordshire has had in abundance. Agriculture displaced many of these parks, and then they were wooded again. A boom in the 16th century of country-house building came after lands were confiscated by the Crown. The number of palaces from the 1700’s were many because of the “rich soil and wholesome air” that attracted wealthy people. (enquire.com 2006) Other building came along the river valleys, slowly. But the industry of malting and brewing in the late 19th century created an era of strong development. The decades since the mid-1900’s has seen major changes in the landscape of Hertfordshire. The development of Portland cement in the early 1900’s, that uses gravel deposits in its composition, brought about quarries and industry related to it (HGS 2005). Quarries have also been necessary for researching the strata of the land. Modern construction has been largely in the south/southwest part of the county. This report, focusing on Stevenage and Ware, is for development purposes in the northern sections of the county. Near Ashwell is the Steeple Morden Plantation Quarry that allows geologists to see the layers of rock. In trenches cut into the quarry, the color contrasts between Melbourn Rock and the soft chalk is obvious. The gray Plenus Marls and Melbourn Rock that were once thought to be the base of the Middle Chalk (HGS 2005) is now considered the marker horizon while shelly detrital rock is above that and smooth chalk above that. The eight beds of the Plenus Marls can be traced to the Paris Basin and used as a marker horizon across Europe, “indicating a change in anoxic conditions” (HGS 2005). There is a lack of clay-with-flint at this quarry. Quarry section showing chalky outwash gravel of Anglian age overlying river gravel of proto-Thames (photo by John Carr) (RIGS 2003) The formations and types of sediments in Hertfordshire will be discussed as they pertain to development, and also as they pertain to restrictions of use (RIGS 2003). Besides the geohazards possible in any construction site, there are also hazards to the geology of a site from construction. All this must be taken into consideration before building can begin. Former construction through the ages has not taken this into account and there has been a degeneration of soils as well as destruction to the buildings by swallow holes and other phenomenon. For reasons of the brevity of this paper, these issues will focus on geohazards. Methods of research Numerous resources have been accessed to obtain the information for this paper. Many have been found on the internet and through Questia. The topography of an area, the geological history of an area, the stratographical framework of sediment deposits, the methods of discovery and analysis, and the potential geohazards of construction are all vast areas of research. This paper hopes to present overviews of this information for the benefit of anyone who may plan to investigate further. The Topography “The Melbourn Rock is sufficient to form a topography for this landscape,” says the Hertfordshire Geology Society (HGS 2005). But most of Hertfordshire is much more than just Melbourn Rock. Most of the county is wide rolling pastures and low hills, with the Thames running through, and few hills higher than 200 metres. A diversity of soil types come from weathering and other changes at the surface. A discussion of soil types follows. The differing combination of minerals and geological histories cause some parts of the county to be dark colored with beech trees and chalk grassland, while other areas have more arable land and oak/hazel woodlands. The northern areas of Stevenage and Ware are farming lands interspersed with woodlands and parks. Most of the early settlements were in the south or along the river valleys, so development is less in the upper sections of the county (enquire.gov 2006). Figure 2. From www.bgs.ac.uk/products/digitalmaps/digmapgb_solid.html Information gleaned from the quarries such as the Ware Road Chalk Quarry in Hertford, and the Ashwell Chalk Pits have afforded the best look at how the landscape was formed over the eras. Geologists and archeologists look to these quarries as important elements in their research since there are few natural exposures available. The Lithostratigraphy of Stevenage and Ware within Hertfordshire’s History As was mentioned already, Hertfordshire is not old geologically. Great Britain as a whole migrated north over the millenia, often underwater, until it reached its present position (West 2006). The stratum begins with a base of Gault clay, and impermeable hardgrounds that formed in the pre-Tertiary period no more than 100 million years ago (RIGS 2003). ‘Freestone’ or Totternhoe Stone within this layer was used for carving statues and doorframes of many churches. Different kinds of hardgrounds have been used to separate the Chalk into Upper, Middle and Lower Chalk (now referred to in two sections: Gray and White Chalk subgroups). The Melbourn is lower; the Chalk Rock is higher. Within all of the Chalk is flint, a hard silica, and marcasite, a dark brown mineral. The Chalk can reach a maximum height of 207 metres in some areas. Above this layer came a sub-palaeogene (below lower Tertiary formations) surface of sandy clay of two types, Upnor and Thanet. The Thanet is found only in the eastern part of Hertfordshire (RIGS 2003). Then, in another marine episode of the Tertiary period, a layer of sand, flint pebbles and clay was deposited and is referred to as the Woolwich/Reading Formation. Two types of stone are found in this: the Sarsen Stone and the Puddingstone, both unusual types of rock called silcrete. As erosion swept through this area, these types of stone developed “relic boulders” that can be seen in parts of the county. In the thinner sections of the Woolwich/Reading Formations, “clay-with-flints” can be found. It is also in this section that “swallow holes” develop (RIGS 2003). Once again the land became lower and another marine episode deposited 3-4 metres of the Harwich Formation, a greenish-gray silty clay, and as much as 100 metres of London Clay Formation, which is dark blue-gray. This layer contains rich fossil remains. These pre-Quaternary deposits are about 52 million years old (enquire.com 2005). The Pleistocene period did not have as much impact but the Quaternary period, 2.5 million years ago, began a lot of fast (relatively speaking) climate changes, such as cold stages and glaciation. Most of Hertfordshire was not invaded by glaciers but the northeast sector as far as St. Albans ended up with a deposit of “till”, a stony clay with chalk fragments, or Chalky Boulder Clay. One glacial event caused the Thames to be rerouted to its current position, leaving large areas with the gravel aggregate used in construction. Later glaciers left lake deposits at many places in the county, including Stevenage. One type of deposit called “tufa” is rich in mammal bones and molluscs (RIGS 2003). This current Halocene period may be considered interglacial. Halocene deposits have been the alluvial muds and peaty sediments which have given parts of the county excellent farm lands. The last direct impact on the geology of Hertfordshire, to date, is man’s industry that has reshaped much of the landscape. (See figure 3 on page 9) Ground (soil) conditions Different soil types include calcareous “rendzina” on the Chiltern scarp (West 2006) and in the east and northwest, loamy argillic brown earths and clay-rich acidic soils. Basically the county is divided by alkaline soils in the north and east, and acidic soils of the central area and the west. The line of demarcation cuts between Stevenage and Ware, defining the vegetation, agriculture and development. In the Stevenage area, the light chalky soils are lightly wooded, easily cultivated but not very fertile. On the slopes, cultivation was heavily done in the medieval period. Rich gravel deposits allow for better drained roadways (Enquire.org 2006). Toward Ware, the land’s cultivation is more difficult, is much more wooded and less settled because of tougher terrain. With the ecological significance toward preserving woodlands, especially on the heavy London clay, permissions for construction may be more difficult toward the Ware/Hertford areas. The protection of soils from degradation by human use has been mandated by the former Minister of Agriculture, Fisheries and Food (now DEFRA) in the code of good practice (Enquire.org 2006). Rivers such as the Ver have been exposed to over-abstractions (by water pumping stations) and drought conditions that lower the water tables considerably. But most are considered stable. Investigation methods Field investigations use geological and geophysical techniques, with in-situ measurements of the properties of materials and stresses in rock and soil. Because the Chalk can be destabilised by inadequate drainage, poorly designed ‘soakaways’ and construction, areas of potential collapse may be missed by conventional drilling (auguring) techniques and geophysical methods used to locate unstable areas is recommended (english-nature.com 2006). Other in-situ techniques for testing include Standard Penetration Tests, Cone Penetration Tests, Pressuremeter Tests, Flat Dilatometer Tests, Vane Shear Tests, and the Sharp Cone Test, which measures the “creep” properties of geological materials (Ladanyi 1992). Other tests would include Test Pits, dug by architects or civil engineers, to examine existing structures’ foundations, Ground Probing Radar, which is not always successful, and Trial Trenches that excavate to the top of deposits (Ladanyi 1992). Geotechnical analysis aids in the design of excavations, foundations, pavements, underground openings, embankments and ground support systems. Design Parameters / Geohazards Drainage patterns in Hertfordshire are significantly influenced by the geology of the area. In some parts of Hertfordshire, “swallow holes” that become sinkholes can cause some difficulties. Water End near Hatfield has a swallow hole where the Mimmshall Brook runs into the Chalk at the edge of a Woolwich/Reading Formation outcrop and then reappears several kilometres away in Ware (RIGS 2003). In other areas, as has been noted, the level of London Clay deters water drainage. Groundwater in aquifers cause some discontinuity in the bedrock, and in lower valleys is likely to be close to the surface. Erosion is historically a problem that needs investigation, particularly in areas where the Chalk and Woolwich/Reading Formations are closer to the surface soils. A small landfill area near Ware of about 2000m2 contains domestic waste and possible toxic containers. Little is known of its history (Enquire.org 2006). The waste is approximately 4 metres deep or a volume of 8000m3. The landfill lies above a layer of clay and flint. There is no noticeable water present and no noxious odors reported. Progressive ground movement must be considered when approaching new construction plans for the Stevenage areas in particular, due to its heavy mining industry, as well as natural occuring variations in rock layers (English Nature 2003). Swelling and shrinking clays, land slippage or collapse due to unstable deposits, and soft sediments are factors in this (Watson 1993). Another issue is radon emission from rocks, as a naturally occurring event which requires testing to avoid inhalation (Watson 1993). Geological mapping of sediments left by flooding, such as river alluvium, can help predict areas of Hertfordshire that may be impacted by flooding (BGS 2006). The British Geological Survey (BGS) is a service that offers this type of mapping. Assessments of the geological, geomorphological and hydrogeological features of the study area, as well as the capabilities have not shown any especially sensitive issues that would adversely affect a construction plan. The conclusion of this study would be that development toward Stevenage would meet with less difficulty than in areas toward Ware/Hertford. Removal of wooded sections of Ware/Hertford would be closely scrutinized by local and national parties. Drainage issues would more easily be dealt with in the Stevenage areas. References/List of figures Anderton, R., Bridges, P.H., Leeder, M.R. and Sellwood, B.W. (1979). A Dynamic Stratigraphy of the British Isles; A Study in Crustal Evolution. George Allen and Unwin, Boston, Sydney. 301pp. ISBN 0-04-551028-8 Pbk. BGS (2006) Data Layer Information. Retrieved 5/15/06 from www.bgs.ac.uk/scripts/gdi/layers/layerdesc.cfm?q_str=Bedrock Enquire (2006) General Landscape Features of Hertfordshire. Retrieved 5/15/06 from http://enquire.hertscc.gov.uk/landscsh/features.htm English Nature. (2003) Learning from the past to influence the future. Retrieved 5/15/06 from www.english-nature.org.uk/pubs/publication/PDF/502.pdf Geomatics World Articles. (2006) BGS’s GeoHazard Inventory. Retrieved 5/15/06 from www.pvpubs.com/read_article.asp?ID=1&article_id=11 Hertfordshire Geological Society. Newsletter 2004-5. Ashwell Chalk Pits. Retrieved 5/15/06 from http://www.hertsgeolsoc.ology.org.uk/NewsletterWinter2005_6.htm#Baldock Hertfordshire RIGS Group (2003) A Geological Conservation Strategy for Hertfordshire. Retrieved from http://www.hertsdirect.org/infobase/docs/pdfstore/geostrat.pdf. Ladanyi, B., and Sgaoula, J. (1992) Sharp cone testing of creep properties of frozen sand. Canadian Geotechnical Journal. 29: 757-764 Watson, B. (1993) The evaluation of urban stratigraphy – a question of luck or judgement? Retrieved from www.york.ac.uk/depts/arch/strat/pastpub/93edi.htm. Pp 43-48 West, Ian (2006) Geology of Great Britain – Introduction. School of Ocian and Earth Sciences, Southampton University, UK. Version 31.03.06 Retrieved from www.soton.ac.uk/~imw/Geology-Britain.htm Figures Figure 1 – map of Hertfordshire on cover sheet – retrieved from www.english-nature.org.uk/pubs/publication/PDF/502.pdf Figure 2 – topography map of Hertfordshire – retrieved from http://enquire.hertscc.gov.uk/landscsh/features.htm Figure 3 – graphic illustration/chart of stratigraphical formations Figure 3. Graphic describes stratographic formations in metres from north Hertfordshire county to south Hertfordshire county with the Thames river running through. Levels are not to be construed to be exact measurements but graphic representations of probable event consequences. Maps – from “Britain Beneath Our Feet” http://www.bgs.ac.uk/britainbeneath/base_bedage.html Surface Geology Age Sand & Gravel Pink Cainozoic Yellow Clay, sand, silt Tan Cretaceous Pale Green Pebbly silty clay Light Blue Jurassic Olive Green Peat Khaki Mudstone Lilac Sandstone Yellow Limestone Aqua Sandstone, mudstone Purple Metamorphic rock not shown Igneous not shown Bedrock Geology Mudstone Purple Sandstone Yellow Limestone Aqua Sandstone and mudstone Not Shown Metamorphic rock Not Shown Igneous Not Shown Read More