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Secrets of the Black Lagoon
True oilshale is found in the area of Scotland roughly defined by drawing a line on the map joining Tarbrax, Blackness, Anstruther, and Straiton. Many of the rocks within this area were formed during the lower Carboniferous (Dinantian) period, about 340 million years ago. To the west, north and east of the area lie the slightly more recent upper Carboniferous rocks that include most of Scotland's coalfields. (See the coal oil section for details of coal minerals once used for oil production). The area of this shalefield is thought by geologists to represent the site of Lake Cadell, a shallow area of static water on the edge of a giant continent that existed then disappeared on several occasions over a period of several tens of million of years. The rocks of the shale field represent a complex succession of sandstones, limestones, shales, oilshales and a few seams of coal, indicating that the area lay at various times beneath saltwater, freshwater and a low-lying swamp. Whilst such series of sedimentary rocks are common elsewhere, there are few places in the world where rocks similar to Scottish oilshale occur. It is thought that very unusual conditions must have existed in Lake Cadell that encouraged prolonged blooms of algae, the remains of which have been gradually transformed into the black, carbonaceous matter found in oilshale.
At least eleven distinct seams of oilshale are found throughout the shalefield, indicating that the black lagoon of Lake Cadell must have existed on many occasions during the course of about 20 million years. Some seams, such as the Fells Shale, seldom exceed a metre in width, indicating that appropriate conditions in Lake Cadell existed for a fairly sort geological period. Other seams are much wider; the Pumpherston shales, for instance, consist of five parallel seams, interspersed with other rocks, totalling up to 15m in width, suggesting a more prolonged and complex geology.
Seams of oil shale vary in the amount of oil they yield; the richest shale produced as much as 40 gallons of oil per ton of shale, although shales yielding as little as 15 were exploited in the later days of the industry. While most oilshale was "plain", composed of microscopically thin parallel layers; areas of "curly" shale were often encountered in which the fine layers had been contorted while still soft layers of silt. Over the course of geological time, the strata of oilshale and accompanying sedimentary rocks have been fractured, bent and tilted through the action of faults and other earth movements. Volcanic action has forced its way through sedimentary strata and injected molten rocks into cracks between layers. Such heat often drove off oil to leave a shale of no commercial value.
Working the Oilshale
This complex geology made shale mining a highly unpredictable business. Faulting of the rocks usually restricted the extent of shale accessible from any one shaft, and seams of shale were often steeply inclined, adding to the difficulty in their working. It was never known when the quality of a seam of oilshale might suddenly deteriorate, or where burnt shale or "wants" (areas where the shale seam was completely missing) might be encountered.
Seams of shale were worked by mines; inclined adits which usually followed seams of shale into the ground from their outcrop at the surface, and by pits; vertical shafts sunk to meet underground seams. Some quarrying or opencast working took place in the earliest days of the industry and resumed in the 1940's following the introduction of draglines and other earthmoving machinery.
Underground, seams of shale were usually worked by the stoop and room method in which the shale was mined to form a rectangular network of tunnels (or rooms) leaving the stoops (pillars of unworked shale) to support the roof. Once the network of rooms had been extended to all accessible areas of shale, the stoops were gradually removed, starting in the areas most distant to the mine entrance, and the roof allowed to collapse. Longwall working was used in certain areas of the shalefield, in which shale was worked along a long front and waste stone was used to build walls that supported the mine roof around access roads. Longwall working became more usual following introduction of shale-cutting machinery in the 1930's. Both methods of working could cause subsidence at the surface, but usually care was taken to avoid undermining buildings and services.
Understanding Mine Plans
Since 1872, there has been a legal requirement to deposit all mine abandonment plans with the relevant authority. These plans are now held by the British Geological Survey, while a range of other shale mine plans also survive in the Almond Valley collection. Despite these records, it is often difficult to identify particular mines or pits and the areas of shale worked from them. Not all of the many shafts and adits marked on mine plans and OS maps yielded much shale; many were exploratory workings that never warranted further development, others were "air pits" providing ventilation, or alternative route of access to underground workings. Many mine plans also show the location of boreholes sunk in advance to workings to determine the extent and quality of shale seams.
It remains difficult to identify all productive shale pits and mines. Some were known by several names, some were known only by a number, some changed identity when ownership changed or when a new shaft or adit was sunk. Young's Oil Co. were exceptional in consecutively numbering their mines and pits, with a sequence extending into the 40's. Often this number was prefaced by a geographical location. Other companies were much less systematic in naming their shale workings and it is often difficult, for instance, to differentiate between the many different "No. 2 pits".
Since 1862, it has been a legal requirement to provide more than one access into underground workings. It became usual to sink two adjacent shafts or two parallel adits. Men and fresh air would enter the mine by the downcast shaft, whilst shale and stale air would be drawn up the upcast shaft. There were however many variations to this arrangement. As workings were linked underground, there might be a number of mines or pits providing access into an area of workings. In some instances a "stone mine" might be driven through barren ground to access further areas of productive shale.
Many mines or shafts provided access to more than one seam of oil shale; some mine plans illustrate a single seam on each sheet, but more usually different seams, which might overlap, were colour coded on a single plan. In areas with complex geology and a long history of working, particularly complicated situations occurred. It is possible to stand at certain locations in West Lothian where three different seams of shale have been mined beneath your feet, each linked to a different mine or pit.