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The Vacuum Issue 11 spacer Issue 11
Under Belfast
by Ian Mitchell
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The expansion and development of Belfast has been physically controlled by its site. The city has been described by Robert Johnstone, when viewed from the backdrop of hills, as "...splurged below you, a spill of buildings gathered in by great arms, looking outwards to the narrow sea, but leaking around the coasts and back to the interior." The city is squeezed into the valley of the River Lagan, particularly at the northeast end where the former tidal flats are replaced by huge areas of reclaimed land now used for industrial estates. Towards the southwest, about Lisburn and Moira, the Lagan Valley loses its identity as the bordering hills lose altitude. This is particularly evident on the north side of the valley as the basalt and chalk cliffs of Black Hill and Black Mountain, looming above west Belfast, merge imperceptibly into endless low land around Lough Neagh. To the north of the city the bleak moorlands of Black Mountain and Divis reach a height of 479m OD but fall steadily to the southwest. South of the valley the land rises onto the irregular low scarp of the Castlereagh Hills and rarely exceeds a height of 150m.
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The familiar topographical profile of the Belfast area and the Lagan Valley is a direct reflection of the hardness of the rocks that underlie the valley and which form the hills visible from the city. The Castlereagh Hills are formed of the oldest rocks of Ordovician and Silurian age (460-430 million years) consisting of beds of very hard greywacke sandstone separated by thin layers of, what is essentially, slate. These rocks also occur beneath the Lagan Valley and the line of mountains forming the Antrim hills but are deeply buried and are concealed by much younger rocks. The greywacke is exploited for road aggregate in numerous quarries in the area of north Co. Down. At Cultra, on the south shore of Belfast Lough, the Ordovician greywacke are overlain by a thin succession of Carboniferous (350 million years) rocks. Although only seen at surface on the Cultra beach these rocks extend beneath at least part of the Greater Belfast area and have been discovered in a borehole at the Belfast Harbour Airport. On the south side of the Lagan Valley the greywacke sandstone forming the Castlereagh Hills is concealed by Permian (275-250 million years) and Triassic (250-205 million years) rocks. These are relatively soft rocks with a distinctive brick red colour and are composed mainly of sandstone and mudstone. The Permian rocks are exposed once again on the beach at Cultra but are best known from many boreholes, including those drilled in Belfast at Connswater, Castlereagh and at Ormeau. Triassic sandstone floors the bottom of the Lagan Valley but is only rarely seen at surface, for example in the cliff line on the Shore Road at the Grove Park. For some time now the water-bearing capacity of the red Triassic sandstone has provided an important source of drinking water for the inhabitants of Belfast. This is obtained from numerous boreholes in the Lagan Valley as far southwest as Lurgan with the water being used both for public supply and for industial purposes. On the north side of the Lagan Valley the Triassic sandstone is succeeded by reddish Triassic mudstone which does not provide a water supply. Instead, in the Carrickfergus area, the mudstones, which may be as much as 1000m thick, contain salt deposits in a bed 40m thick which is mined underground at Kilroot.
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The youngest rocks in the Belfast and Lagan Valley area form the escarpment of the Antrim Plateau on Divis, Black Mountain and White Mountain. At the base of the cliffs is the white Cretaceous (80-65 million years) chalk which is overlain by black basalt lava flows of Palaeogene (60-55 million years) age. Both rock types are relatively hard compared to the older red sandstone and mudstone and, with their more or less horizontal attitude, readily form cliffs and steep sides to the mountains fringing north and west Belfast. The basalt lava in particular has been, and is still being, extensively quarried for road aggregate in many large quarries on those mountains.
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In the Belfast area there are no rocks representing the 55 million years of Earth's history between the end of the volcanic eruptions that produced the basalt lava flows and the present. However, what we do find is that in the last 100,000 years or so, the latter part of the Quaternary period, the repeated advance of ice sheets, up to 1km thick, has left a variety of deposits that fill the Lagan Valley and conceal most of the rocks described above. Boulder clay, the most widespread deposit, represents the ground moraine of the ice sheet. In the rest of Co. Down the boulder clay has been moulded into a huge number of small individual drumlins which form the world famous 'basket-of-eggs' topography. On the hills bordering the Lagan Valley the thickness of the boulder clay is variable and most of the rock exposed is found where the glacial deposit is thin or non-existent. In the Lagan Valley the boulder clay is often covered by later glacial deposits. These include sand and gravel which owes its origin to water flowing from the melting ice mass and occurs either as long, narrow, sinuous ridges, such as the Malone (esker) Ridge, or as extensive stratified deposits which formed in temporary lakes that flooded the valley. Red laminated clay, which occurs in the Belfast area and was formerly used in the manufacture of bricks, was also deposited in the late to post-glacial lakes. Lastly the Holocene (10,000 years to present) deposits that occur under Belfast were mainly deposited on the flood plain of the River Lagan. However, in addition to that alluvium much of central Belfast is underlain instead by a deposit of soft grey mud, silt and fine sand with numerous sea shells, in particular oysters. This was deposited at a time of elevated sea level in the estuary of Belfast Lough and is graphically known as "sleech". It is over 15m thick in the docks area of the city and 3-10m in the commercial and inner city areas. This deposit presents engineering problems because of its very low bearing strength and large buildings are generally founded on a forest of piles to transfer the weight down to stronger material. A classic example of the effect of subsidence due to failure of inadequate foundations and to the presence of the sleech is the leaning Albert Clock Tower in Belfast [J342 745] . Remedial action has stabilised the tower and prevented an increase in the angle of lean which reached a maximum of 1º, equivalent to 0.77m of movement from the vertical (Doran Consulting, pers. comm, 2003).
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Northern Ireland is located on one of the most stable areas of continental crust in the world. The Belfast area therefore lies well beyond the range of the world's major earthquake zones and with the end of volcanic activity, some 55 million years ago, the geological environment of the city, in contrast to the political and social framework, is remarkably tranquil. However, complacency is never acceptable to a geologist when development of the city infrastructure depends so heavily on geology and geologists for sound advice on what lies under Belfast.
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Under Belfast
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