Geology of the OUGS Yorkshire Branch
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The Geology of Yorkshire by Peter Vallely- OUGS Yorkshire Branch In Yorkshire we are very fortunate to have rocks from nearly all the geological periods between the Ordovician and the present day. In particular, the geology and scenery of the rocks from the Carboniferous, Jurassic and Cretaceous periods are amongst the finest in Britain. Why go anywhere else? Who cares about a bit of rain, when you have plenty of good old Yorkshire Grit to talk about! (Higgar Tor, near Sheffield). The Ordovician Period Yorkshire’s oldest rocks are the Ingleton Group. These are slates and greywackes of Ordovician age which were formed on the continental slope of the ancient micro- continent of Avalonia. These rocks are found only in a small area around Ingleton and to an even lesser extent around Horton in Ribblesdale. They are overlain unconformably in some places by rocks of the Windermere Supergroup of Late Ordovician- Silurian age, and in other places by Carboniferous Limestone (as at Thornton Force). Close up of the famous waterfall of Thornton Force near Ingleton showing horizontally-bedded Carboniferous limestones overlying near-vertical Ordovician slates. Note the thin conglomerate band at the contact between the two rock types. The Silurian Period Rocks of the Windermere Supergroup (late Ordovician – Silurian age) are present only in a small area of the Yorkshire Dales near the Craven Fault. As their name suggests they are a continuation of the rocks found in the SE of the Lake District near Windermere. At the end of the Silurian, Avalonia collided with a continent to the north (Laurentia) in the final phase of the Caledonian Orogeny, and the Ordovician/Silurian rocks became tilted and folded. The newly-formed mountains were heavily eroded during the Devonian period. The Yorkshire region was above sea level for most of the time and in an arid zone south of the An example of a “Norber Erratic” near Austwick. During the equator, and there is little evidence of Quaternary Ice Age, a glacier bizarrely carried blocks of Devonian deposits. Silurian Greywackes uphill and placed them on younger Carboniferous limestones which have eroded faster. Lower Carboniferous – Great Scar Limestones At the start of the Lower Carboniferous (Mississippian), the crust below the Yorkshire area became stretched and split into a series of blocks and basins. The Askrigg Block (which broadly corresponds with the Yorkshire Dales National Park) remained relatively buoyant as it was underlain by the Wensleydale granite. Tropical shallow-marine conditions prevailed over a long period allowing very thick limestones to be built up – the Great Scar Limestones. Today these form many impressive white cliffs in the southern Yorkshire Dales such as Malham, Gordale and Kilnsey. The limestone country also displays many good examples of karst Hull Pot near Horton in Ribblesdale. This enormous “pot hole” features such as caves and potholes. is 100m long, 20m deep and 20m wide. Lower Carboniferous – Yoredale Group Towards the end of the Lower Carboniferous (Mississippian), sea levels on the Askrigg block began to fluctuate in a fairly regular manner. Each new transgression and gradual shallowing produced a cycle of sedimentation beginning with marine limestones, followed by mudstones, siltstones, sandstones, seatearths and finally coal. These are known as Yoredale cycles (Yoredale being an old name for Wensleydale) and there are a total of eleven cycles (or cyclothems) in the Yoredale Group. However, not all the cyclothems are fully developed and they cannot all be seen at a single location. Upper Swaledale near Muker. Each band of limestone cliffs in These Yoredale Group produces classic the hillside is the base of a Yoredale cycle. stepped topography on hillsides especially in the northern Yorkshire Dales. Lower Carboniferous – Craven Basin In marked contrast to the Askrigg Block, the area to the south of the Craven Fault developed into a deep basin (the Craven Basin) during the early Carboniferous. The earliest exposed rocks were limestones (Chatburn Limestones), but the basin deepened sporadically and mudstones became more dominant, with some in-situ limestones. Later on, turbidite limestones (which had slumped down submarine slopes) became common. Towards the end of the Lower Carboniferous (Mississippian) a great thickness of dark shales were deposited (the Bowland Shales) in moderately deep water. The photo shows a large slump in the Rain Gill Limestones (in Although the Craven Basin is less spectacular than the sinuous bed below and behind the lady’s left hand). This is the Yorkshire Dales, the geology is quite varied and evidence that these limestones were gravity-fed into the basin there are many areas where good fossils can be (Bonstone Brook, near Newton, Forest of Bowland) found. Upper Carboniferous – Millstone Grits At the beginning of the Upper Carboniferous (Pennsylvanian), sea levels became shallower throughout the Yorkshire area, and sedimentation occurred in large rivers and deltas. Occasional marine incursions produced thin marine shale bands containing goniatites which are useful for correlation and dating. The Millstone Grit Series (Namurian Stage) surprisingly consists mainly of shales, which are rarely exposed. The Millstone Grit is named after several hard and thick bands of gritstones within the sequence that produce prominent edges (=crags) and tors. They were originally used for making millstones. The gritstones often show cross-bedding at Some of the many examples of Millstone Grit ‘tors’ at Brimham various scales and interesting weathering Rocks. Cross-bedding is apparent at mid-height on the big rock features. tower in the centre-right of the photo. Upper Carboniferous – Coal Measures The Coal Measures (Westphalian Stage) overlie the Namurian Millstone Grits. They were deposited in similar deltaic conditions to the Millstone grits, but marine incursions were fairly rare and widespread swamps developed. The equatorial conditions gave rise to high rates of plant growth followed by rapid burial which formed the coal deposits that were so important to Yorkshire’s economy. The Coal Measures also demonstrate cyclic sedimentation. The idealized sequence being (from bottom to top): coal– mudstone-siltstone-sandstone-seatearth-coal. Small sandstone and siltstone crags are quite common in the Coal Measures but old quarries often give a better idea of the variety of rock types that are present. Clay Clough Quarry, Denholm Edge near Haworth. The owners are excavating the fire clay beneath the coal at the base of the quarry. Permian At the end of the Carboniferous Period, the Rheic Ocean closed to the south of Britain, causing a mountain building episode in central Europe – the Variscan Orogeny. Most of Yorkshire was uplifted as a result, but the rocks were not deformed apart from in a few localised areas. In the early Permian, conditions in Yorkshire were very arid and there was much erosion and limited sedimentation. Later in the Permian, rifting occurred in the North Sea area and shallow tropical sea (The Zechstein Sea) entered the east of the Yorkshire region. This sea frequently dried up and refilled forming thick evaporite deposits (such as gypsum, potash and rock salt) which were the basis of the Teesside chemical industry. The Carboniferous –Permian unconformity at Abbey Crag, Knaresborough. The well-cemented greyish Namurian gritstone (Carboniferous) with an irregular top is overlain by the orange weaker and sandier Cadeby Formation. Triassic Arid conditions continued into the Triassic. fluvial sandstones (Sherwood Sandstones) were deposited east of the Pennines and they now underlie the Vale of York area, However, they tend to be covered by alluvium and there are few good exposures. Just outside Yorkshire, they do form impressive sandstone cliffs e.g. at Nottingham Castle and Alderley Edge (near Manchester). Later in the Triassic, tropical shallow seas developed which were similar to the Permian Zechstein Sea and they deposited silty mudstones and evaporites (Mercia Mudstones). Thinly-bedded upper Triassic marls of the Mercia Mudstone Group at Melwood Quarry, Epworth, North Lincolnshire. Note the alternation of oxidised (red) and reduced (grey/green) layers. Lower Jurassic A substantial marine transgression occurred near the end of the Triassic and continued throughout the Lower Jurassic. There are extensive deposits of Lower Jurassic calcareous mudstones (the Lias Group) on the North Yorkshire coast between Redcar and Whitby. The mudstones contain abundant ammonite, belemnite and bivalve fossils. There are important ironstone beds within these mudstones – such as the Frodingham Ironstone which led to the development of the iron and steel industry near Scunthorpe (North Lincolnshire) A view of Whitby looking south-east. On the clifftop is St Mary’s Church and its graveyard where Dracula used to lurk - the ruins of Whitby Abbey are behind it. The reddish sandstones towards the top of the cliff are mid- Jurassic and they overlie highly fossiliferous Lower Jurassic, which can be inspected by walking round the toe of the promontory at low-mid tide. Middle and Upper Jurassic In the Middle Jurassic, there was some uplift of land and south-flowing deltas developed in the NE Yorkshire area. In some ways the sequences are similar to those of the Yoredale Series, with limestones, mudstones, sandstones and minor coals. Towards the end of the mid-Jurassic, the sea encroached more and more, and marine sediments became dominant again Uplift then occurred before