A definition of Geology could be "The science relating to the study of the Earth's crust". "
The Geology of Eyam" is an extremely brief study of the earth's crust in and around this
beautiful Derbyshire Peak District village.
If the oldest known rocks in the world are around 3,500 million years old, the oldest
rocks in the Eyam area that a walker would come across are around a 'mere' 350 million
years old. Only a very deep borehole would get to the really old rocks.
Geologists have divided the approximately 3,500 million years the earth has been in existence into periods. The oldest period is from 600 million years ago to 3,500 million years, and is called the Pre-Cambrian era. The youngest period is called the Pleistocene, which covers the last million years or so. Most of the rocks in the Eyam area date from the Carboniferous period around 350 to 290 million years ago.
Our hypothetical walker would quickly notice that the older buildings in the village are either made predominantly of a grey-white coloured stone, called limestone, or much more commonly, because it would have been easier for stonemasons to work, a dark brown stone called sandstone. This is due to the fact that Eyam Dale and west towards the neighbouring village of Foolow, the rock formations are almost always limestone, with the sandstone east towards Grindleford, and north towards Bretton. There is also much shale, but it is too soft for use in building.
In an age when stone had to be transported by animal drawn cart, the buildings of everyone except the very richest noblemen would be made of whatever stone was available locally. As well as being used for building, similar sandstones to the ones in Eyam, in other parts of Derbyshire, were used to make millstones, and are often referred to as Millstone Grit as a consequence.
Rocks are classified into three types: igneous, sedimentary, and metamorphic. Igneous rocks come from molten material brought up from below the earth's crust, which has solidified.
Igneous rocks, like all rocks, weather and are susceptible to erosion. This means that the rock is broken down by a combination of mechanical and chemical processes.
A principal constituent of many igneous rocks is quartz. Sand grains are mainly pieces of quartz no more than 2 millimetres in diameter, and no less that one sixteenth of a millimetre in diameter. Wind and water will take the grains from the igneous rocks formations that are breaking down to an area of deposition. These are usually large areas that become covered in sediment. Successive new layers of sediment squeeze out water from the lower ones, and this leads to compaction. Sometimes there is cementation between the grains, although the sandstones in the Eyam area do not show this as is evident by the fact that individual grains can be scraped off.
Sandstones formed from the mechanical weathering and erosion of earlier rocks join the cycle of existing rock which is broken down, transported, deposited in new sediment, and turned into new sandstone.
When igneous rocks weather they release a number of minerals, including soda, potash, and lime, which become a carbonate solution. Warm, tropical waters, such as those which covered Derbyshire in early Carboniferous times, were often particularly lime-rich. Invertebrate animals easily fixed the carbonate for their shells. Limestone is made mainly from the shells of the dead animals and fine fragments of the same, all cemented together.
Metamorphic rocks are existing igneous or sedimentary rocks which have been transformed due to being subjected to temperatures of anything between 200 and 700 degrees centigrade. These weather and erode, the same as any other rock.
The limestone found in Eyam is in very thick layers, with brachiopod and coral bands found between more nondescript layers. The limestone at the bottom of Eyam Dale and along Middleton Dale is not quite the same limestone as in and around the village of Eyam itself, although both were formed some time during the early part of the Carboniferous Period in what were then shallow tropical seas.
In the later Carboniferous Period there was an Ice Age. The sedimentation in the tropical area that Derbyshire found itself still in became strongly influenced by sea level changes as Polar Ice caps waxed and waned. The so-called Millstone Grit formation came into being as a result of a general lowering of the sea level. The sandstones and shales of Eyam were formed in deltas closer inshore, whilst the Eyam limestones had earlier been formed when the sea level had been higher.
Sedimentation will have continued to take place well after the Carboniferous Period, but some time before the Pleistocene there was a period of igneous activity which resulted in a large amount of uplift all along the Pennines, including the Eyam area. As the magma pushed upwards on the limestone, the latter bent and cracked, forming what has been described as the Derbyshire Dome. Molten material intruded into the cracks and formed the mineral veins that have provided the lead ores that have been mined here since Roman times.
The weathering and erosion that took place on the by now exposed later rocks meant that they were all broken down, leaving the much older Carboniferous rocks where our hypothetical walker can see them today.
Eyam Edge is principally made of sandstone that is approximately 77% quartz, and was formed by sedimentation in a large delta during the Carboniferous Period. Just below Eyam Edge there is much shale and relatively little sandstone. Shale is formed in more or less the same way as sandstone, although the grains are much finer and the layers are much thinner. The reason why Eyam Edge sticks out is that shale weathers and erodes much more quickly than sandstone, leaving the sandstone jutting out where we can see it today.