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For archaeologists the subject of geology can be simultaneously daunting and fascinating. It is important for us to have a basic understanding of geological processes if we are to make meaningful interpretations of the relationship between landscape and human activity.
Adrian Kidd provided the Upper Wharfedale Heritage Group with a presentation explaining the fundamentals of the geology of the Yorkshire Dales, with a particular reference to the Craven area, and the limestone quarry at Threshfield, the subject of one of our most recent field projects.
A lack of fossil evidence prevents geologists confirming the age of the oldest rocks in the Dales, which may be Pre-Cambrian. The most definite dating therefore takes us back to the Ordvician period. The Askrigg Block and Craven Basin contain the oldest datable layers of rock – Ordvician and Wensleydale granite.
From here Adrian gave us a whirlwind tour of the successive geological periods and the layers of different types of rock that were laid down across the Dales – from the Sedbergh Conglomerates of the Ordvician to the coal, millstone, and limestone of the Carboniferous succession, and how these appear in the landscape we see today. He explained how tectonics played a part in the geological formations of the British Isles, as we moved up through the Equator, when the Askrigg Block and Craven were formed, into the cooler climes of the northern hemisphere. The limestones of Craven were formed when the temperature of the shallow, tropical seas covering this area was 30 degrees C. Two main types of Limestone were laid down in our region – the type that was mixed with mudstone can be seen in such places at Kilnsey Crag; while the “Cove” type and “Goredale” type, which are laid down in distinct bedding planes, can be seen around Malham. This geology is, of course, directly linked to the development since prehistoric times of the quarrying industry in the Dales.
There are 36 major limestone pavements in the Dales, comprising 50% of the total found across England. The caves are another very distinctive feature of this landscape, and this has enormous implications for archaeology. A survey of 300 limestone caves in the Dales by Sheffield University in 2006 revealed that human remains were found in 16% of them (compared to 25% of the caves in the Peak District).
In addition to the Limestone, six major depositions of Millstone Grit lie in the region, mostly to the east and south of the Craven Fault.
The impact of the earth movements which followed in the late Carboniferous and early Permian periods showed how much of the distinctive nature of the Dales was created – the folding of the Earth’s surface creating the mountains and hills. The relatively stable Askrigg block was far less affected than the Craven Basin.
Mineralisation followed along the resulting fault lines and it is from this that we see the development of the mineral veins which were to be exploited later by people, leaving behind them a rich industrial heritage.
The Quaternary period (from 1.8 million years ago to the present) was a time of erosion and deposition and glacial activity, which further affected the nature of our local landscape. Wharfedale, for example, is a glacial eroded valley; Conistone Dib is a meltwater channel; and Halton Gill clearly shows where landslides took place at the end of the last Ice Age.
There is a surprising amount of geological research which still needs to be done in the Dales, and this will be partially remedied by the Your Dales Rock project run by the North Yorkshire Geodiversity Partnership.
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