From Earth Science On-Site
Black Rock, Derbyshire
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KS4 Earth Science Background
The geological and geomorphological story of the NSC area is based on the interpretation of evidence from the rocks and from the landscape, some of which may not be visible in this immediate area. Here is the story and the evidence for it.
The rocks deposited in the area of the National Stone Centre are limestones of various kinds, overlain to the east by shales and sandstones. The limestones are made of calcite (calcium carbonate) in the form of fossil shells, calcite mud and other fragments cemented together. The species of fossils indicate that these rocks were deposited in the Carboniferous Period, about 340 million years ago. These fossils are mainly the remains of corals, crinoids, algae and brachiopods, which, by analogy with living forms, are interpreted as living in a shallow, clear, warm marine environments. Some of these fossils built wave-resistant mounds, called reefs, whilst others are bedded limestones, which were deposited in quieter water behind the reefs.
This Earth Science On-Site exercise uses the Principle of Uniformitarianism, which states that the biological, physical and chemical processes we see today, operated in much the same way in the past, and can be used to interpret evidence in rocks. The interpretation of this limestone area is based on the evidence in the rocks, and from your understanding of current reef environments. This allows us (with some reservations) to use the simplified picture of a modern reef to interpret the evidence of the ancient one in the limestones. (For additional information see document Reef Info Sheet).
||FIG|NSC4reefsection.gif|500|1|A section across a modern reef}}
Interpreting the Evidence
Although there are good reasons for trusting the Principle of Uniformitarianism in general when interpreting ancient environments, this exercise should also encourage pupils to think carefully about the limits of that trust. Reasons for caution include; ancient reefs had a different structure from modern ones; the corals in ancient reefs are different from modern corals; some modern coral reefs do occur in cold water as far north as Norway; geological processes have altered the evidence in the rocks after they were deposited. (chemical changes during cementation; uplift; tilting; faulting; weathering; erosion)
The Limestones of the National Stone Centre
Using the evidence from the rocks, and the simple model of a reef, a simplified interpretation of these limestone beds is as shown in Figure 2.
The Reef Quarry is almost circular and quarried to a depth of 10 metres. Although sometimes called “reefs” these structures are better referred to as “mud mounds” because they seem to be wave resistant mounds of calcium carbonate mud and lack the coral framework of modern reefs. The surrounding beds have many brachiopods in life position, ( best seen in the blocks along the footpaths) as well as many large crinoid-stem fossils that have not been transported far, if at all. These beds were deposited at a slight angle against the reef and so do not indicate the original horizontal.
To the north, in North East Quarry, are the bedded limestones interpreted as the back-reef “lagoon”, which were deposited horizontally by gentle wave action. These waves washed crinoid and other shelly material around before final deposition. These beds are exposed in North East Quarry, the floor of which represents the ancient lagoon floor.
To the south lie the fore-reef deposits, now largely quarried away, but which show the steep dips of the “underwater scree slopes” in the walls of South East Quarry. This implies that the deep water, and main wave direction, lay to the south, with more gentle waves washing into the lagoon.
Black Rock, is part of a ridge of more resistant Carboniferous coarse sandstone (gritstone) lying on top of less resistant shales, which in turn lie on top of the limestones. The limestone, and the fossils it contains, (bottom-living corals, brachiopods and crinoids) indicate deposition in a warm shallow sea area, with very little mud or sand being washed into it. The shales contain fossil goniatites, (very different marine fossils from corals, which swam like modern day nautilus, rather than living on the sea bottom). These shale beds indicate the onset of marine conditions affected by greatly increased amounts of deposited mud. The coarser (and therefore heavier) fragments of sand and pebbles in the younger gritstone indicate stronger currents. Not exposed here, but just above the sandstone (gritstone) are coal seams, indicating that the sea area had become silted up by deltaic mud, sand and grit deposits allowing land plants to grow, eventually forming the coal.
The depositional “story” in this area is one of a warm, clear, shallow “limestone” sea, with a large “muddy and sandy” delta building out into it, eventually forming a swampy, delta-top land area with vegetation. In the context of Plate Tectonics this is interpreted as consistent with evidence that this part of England was at the time of the limestone deposition, just south of the equator, in a latitude where modern reefs are still commonly formed. As it was moved northwards by earth forces this area crossed the equatorial region where heavy rainfall fed large rivers, forming deltas of sand and mud, as well as allowing vegetation growth on a scale large enough to form coal seams. The tectonic processes that moved this part of the Earth’s crust were also responsible for the folding, faulting mineralisation and uplift of these rocks.
These beds have been tilted to the east by about 10 degrees, and broken by many faults. These faults, and other fractures, particularly in the limestone, later became the site of mineral veins containing galena (PbS, lead sulphide) and blende (ZnS, zinc sulphide). The other minerals, known by the old miners as “gangue” (or waste) minerals, are calcite (CaCO3; barite BaSO4</sib>; and fluorspar CaF2) The evidence of radio-isotope dating suggests there were three periods of mineralisation beginning in the early Permian (270 million years ago), a second around 235 million years ago and the final one in the late Triassic period, about 180 million years ago. The shales, being less brittle than the limestone, and also less permeable, acted as a “cap” to the mineralising fluids, trapping most of the rich mineral veins in the limestone just below. This is one of the reason that the Cromford mine is so deep. The miners were trying to reach buried lead ore veins in the limestone below.
Uplift of the area has left these rocks up to a height of about 300m above sea level at Black Rock. They would have been affected by the last glaciation, but there is little evidence of that visible today. Present day weathering and erosion have left the more resistant grits forming the hills, and the less resistant, and less permeable shales, forming poorly drained valleys. In places, now largely obscured by the woodland, landslides and slips of the gritstone, overlying the impermeable shales have occurred where rainwater, percolating through the gritstone and collecting at the top of the shale has allowed slippage. The soils on the more easily weathered, less permeable, shales tend to be quite thick and damp, whilst the soils on the limestones are drier and much thinner. This is because the limestones contain few impurities and are otherwise chemically weathered and eroded away by carbonation and solution in acid rainwater. Weathering of the gritstone attacks the cement holding the grains together, releasing the quartz grains to form better drained, sandier soils.
The gritstone forms the high ground of a natural escarpment, the line of which is broken by a fault trending almost east-west across the area, down throwing the Black Rock outcrop to the north. (See sketch section, below). As a result this areas shows the effect of three very different rock types on the vegetation, landscape, drainage, and economic activity, in a very small area.
Mining and Quarrying
The area has a long history of mining and quarrying, and much of this is reflected in the landscape of the area, although you have to look quite hard for some of it.
The Cromford Moor mine represents part of a history of lead mining going back to the Romans, the evidence for which is a pig of lead discovered last century dedicated to the emperor Hadrian. Mining at Cromford Mine had begun by 1615 and ended in 1850. At times there were around 100 men and women working this site, where the shaft penetrated 128 metres through the shale into the mineralised limestone below. Uses for lead include: lead-acid batteries; roof flashing; lead piping (in the past) lead crystal; X-ray shielding; sheaths for underwater cables and as a chemical stabiliser in PVC plastics and petrol (in the past).
Now it is only recognisable by the spoil heap, with the shallow depression at the top being very close to the site of the shaft. Much of the lumps of limestone material on the spoil heap dates from the 1920s when the mine was re-opened as a source of white calcite, which is used, as lime, in industrial processes such as soap, paper, paint and bleach manufacture.
Up the slope to the east is the old Barrel Edge quarry which was a source of stone and millstones. This too is abandoned, and now overgrown by bracken and birch trees.
Dene Quarry was famously started by Don Harris in 1942. It became notable for its Fossil “marble” (to an architect, any stone that can be polished is a “marble”. It is actually a fossil rich limestone, not a metamorphic rock), which was used in the Bank of England, many universities and cathedrals as well as Heathrow Airport and the Royal Festival Hall. Output rose steeply in the mid 1960s due to demand from the construction of the M1. Now the output is around 1.5 million tonnes per year, mainly for roadstone and aggregate, but also for many other uses. There are reserves here for many years, and the issues for the continued extraction centre on the impact on landscape and the communities in the area.
Earth Science Principles
In this area it is possible to demonstrate the following Earth Science principles.
National Curriculum Links
In this area it is possible to: