A city built on coal

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The presence of extractive industries throughout the Bradford area was dictated by local geology. A basic understanding of Bradford’s rocks is not too difficult. The city is built on a series of Carboniferous period rocks named, from their most famous constituent, the Coal Measures. Shipley, and the Baildon Moor outlier, are at the north western edge of the great Yorkshire & East Midlands coal field. The solid geology of the Bradford region lies in that division known as the Lower Coal Measures (LCM). Around Bingley and Shipley these LCM strata are approximately 175m in total thickness. The LCM strata lie conformably on the deeper Rough Rock of the Millstone Grit series (MG), which is well exposed at Shipley Glen. Thick beds of coarse sandstone are very obvious in the MG, whereas the LCM are essentially a collection of marine and non-marine sedimentary rocks interrupted by coal seams. In total nine coal seams are recognised in the whole Bradford area. The cyclical sequence of events that generated them were repeated on several occasions. Not all phases are present in every sequence, and some sequences contain additional bands of sandstone. About 310 million years ago ‘Yorkshire’ was part of a large tropical river delta stretching into Europe, which was subject to the effects of periodically rising sea level. The rivers brought vast quantities of sand and grit from the erosion of granite mountains to the north-west. Periodically huge muddy lagoons formed, rather like the Florida Everglades today, and the death and decay of associated profuse tropical plant-life provided the organic material that was transformed into coal. It is not correct to call these plants trees, which were yet to evolve, rather they were giant horsetails and club mosses; their fossilised remains are still common in local sandstones. The water was full of fish and shell-fish; insects and amphibians were also present although their fossils are now less common.

Coal miners have long recognised that the LCM are considerably affected by faults (which they would have called throws). In fact so many faults are recorded in this area that it has been described as a ‘shatter belt’. This faulting is probably pre-Permian in date and may have ultimately resulting from crustal movements that brought all the continents together in a single super-continent called Pangaea. In this part of West Yorkshire there is no further direct evidence of geological history from the time of the formation of the Coal Measures until the Quaternary period. In the Pleistocene era of the Quaternary (the last 2.5 million years), the whole area was affected by glaciation. As a result of this the solid geology of the LCM is covered by varying thickness of ice deposited material known as till or boulder clay. Deep valleys such as Heaton Woods (and nearby Northcliffe Woods) were cut, or perhaps re-cut, at the end of the last (Devensian) glacial period by the passage of torrents of melt-water. The circumstances that led to these channels being formed is still subject to expert research but for our purposes it is sufficient to know that this process was responsible for the deep clefts now occupied by misfit streams in the two wooded valleys. These small streams could never have eroded the ravines now such a dramatic feature of both areas.

Miners gave the various coal seams they encountered descriptive names. In Heaton and at Thornton, Denholme, Wrose, Baildon and Shipley we are mainly concerned with the (Halifax) Hard Bed. Beneath the Hard Bed was the (Halifax) Soft Bed, occasionally called the Low Bed or Coking Coal. These were the two deepest commercially exploited seams in the whole LCM. The seam higher than the Hard Bed, the 36-Yard Coal, is less constant; it is missing at Baildon Moor for example. All the coal seams are overlain by narrow marine fossil bands, indicating incursions of the sea. These marine bands contain goniatites which are related to the later, and more familiar, ammonites for which Whitby is famous. The goniatites of the different bands are distinguishable by those with the necessary expertise, and if found in colliery spoil can be used to identify the associated coal seam.

Half the coal mined in Bradford was once coked and supplied iron smelting blast furnaces. Not all coal seams were considered thick enough to be mined at all but other associated minerals were also of economic value. Elland Flags sandstone, York Stone, was quarried for paving and roofing material. Shales could be ground up and used to supply machine-presses for the making of house bricks. Fireclay and ganister were the valuable seat-earths found under several coal seams; ganister (calliard) being virtually pure silica, whereas fireclay is a combination of silica and alumina. Both were employed to make refractory firebricks for lining furnaces, factory chimneys and domestic fireplaces. Fireclay was also used in the manufacture of chimney pots, sewer pipes and sanitary wares. The Hard Bed contains balls of iron pyrites and large calcareous nodules full of goniatite fossils. In places these were processed for iron sulphate, and lime. Very little of these industries survived the 1920s and the last coal mine in Bradford, supplying the kilns of the Thornton Fireclay Company, ceased working fifty years ago.


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