North Yorkshire
In general terms the geology of North Yorkshire comprises a range of sedimentary rocks that slope gently to the east so that the oldest rocks are present in the west of the County and the youngest in the east.
Limestone pavement is characteristic of the North Yorkshire landscape.
The Carboniferous Limestone and the overlying Upper Carboniferous Millstone Grit dominate the exposure in the west and give rise to the characteristic upland countryside of the Yorkshire Dales and the North Pennines. There is a distinctive difference in the two habitats that these rock types support, with the limestone giving rise to calcareous soils and the shales and sandstones of the Millstone Grit giving rise to acidic soils and large areas of upland grassland and bog.
To the east the Millstone Grit is overlain by the Permian Magnesian Limestone and Triassic mudstones and sandstones. This change in rock type is marked by a transition to the lower-lying Vale of York and the river valley of the Swale. North-east Yorkshire is dominated by the Hambleton Hills and the North Yorkshire Moors which rise abruptly to the east of Thirsk. This whole area comprises mudstones and sandstones of Jurassic age, with the mudstones generally forming the lower ground around the edge of the North Yorkshire Moors and Hambleton Hills. Jurassic sandstones form the main scarp slope around the edge of the Hills and give rise to the free-draining soils that support the heathland vegetation of the Moors. These marine and delta deposited rocks are superbly exposed on the North Yorkshire coast from Staithes to Filey and yield superb fossils including marine reptiles such as ichthyosaurs and plesiosaurs. The youngest part of the Jurassic, the Kimmeridge Clay, underlies the Vale of Pickering, but exposures are rare. This gives way to the marine sediments of the Speeton Clay and the succeeding pure, white limestone of the Chalk, which forms the Yorkshire Wolds in the south-east of the County.
Much of North Yorkshire was covered by ice during the last glacial of the Quaternary, or Ice Ages, and as a consequence the solid geology is largely covered by layers of glacially derived sediments. These form a skin of superficial deposits, or drift, which in places are so thick as to eradicate all visual clues as to the nature of the underlying solid geology. This drift has been modelled and shaped by fluvial, marine, aeolian and frost processes which combine to create distinctive features and landscapes.
Silurian
The oldest rocks found in the County are of Silurian age (443-417 million years ago) and outcrop just to the north of Settle in the southern part of the Yorkshire Dales. These comprise a series of marine mudstones, sandstones and silts which contain fossil graptolites, brachiopods and trilobites.
Carboniferous
Rocks of Carboniferous age (354-290 million years ago) outcrop in the western third of the County. The oldest of these comprise the limestones, sandstones and shales of the Carboniferous Limestone which give rise to the classic limestone scenery of the Yorkshire Dales. These rocks were deposited in a shallow marine-estuarine environment, and contain the fossils of marine organisms such as brachiopods, corals and sea-lilies.
The Carboniferous Limestone is overlain by the Millstone Grit Series. In this area of England, the differences between the Millstone Grit and the older Carboniferous Limestone are less marked than further south, with sandstone common in the Carboniferous Limestone and limestone present in the Millstone Grit. In North Yorkshire the Millstone Grit series consists of a series of limestones, marine shales and sandstones, and the ‘Millstone Grit’ itself, thick coarse-grained sandstones, siltstones and mudstones. These rocks were deposited in the late Carboniferous (approximately 300 million years ago) in a coastal environment where large river deltas were building out into the shallow marine waters. Continuing deposition over the millennia led to the further building out of the deltas and the formation of an extensive low-lying, swampy land area in which the succeeding Coal Measures were deposited. Only a small outcrop of Coal Measures rocks are present in the County and occur as a small patch around Ingleton.
Permian
To the east, the Carboniferous succession is overlain by rocks of Permian age (290-248 million years ago). At the base of the sequence are sandstones known as the ‘Yellow Sands’ which represent the remnants of Saharan-like desert sand dunes. The Yellow sands are overlain by cream to buff coloured limestones know as the Magnesian Limestone which form a low west-facing ridge running through the central part of the County from Wetherby northwards to Ripon and Scotch Corner. The Magnesian Limestone was deposited in a relatively shallow landlocked sea extending from north east England to Poland, known as the Zechstein Sea. The estimated average temperature during the period was approximately 23°C. Due to the landlocked nature of the sea and high temperatures, gypsum and anhydrite (collectively known as evaporites) were produced as the sea water gradually evaporated and today form extensive deposits beneath the floor of the North Sea. Within the Magnesian Limestone the remains of fossil reefs (often including numerous fossil algae, bivalves, gastropods and bryozoans) which grew on the edge of the Zechstein Sea can be found.
Triassic
The Triassic (248-205 million years old) is represented by red mudstones and sandstones that underlie much of the lower lying Vale of York and the valley of the River Swale. However, although these rocks have a relatively extensive area of outcrop, apart from at the coast, the solid rock geology rarely emerges from beneath a thick covering of glacial and post glacial deposits. The Triassic rocks of Britain were deposited under arid conditions over a large desert plain with numerous basins and mountain ranges. Lower Triassic sandstones represent the deposits of large braided rivers that crossed the desert plain. These are overlain by the Mercia Mudstone Group, which comprise mudstones, sandstones and thin horizons of gypsum and rock salt deposited over the desert plain in large, shallow basins.
Jurassic
Rocks of Jurassic age (195-140 million years ago) outcrop over the whole area of the Cleveland Hills, Howardian Hills and North Yorkshire Moors and form a narrow belt along the western margin of the Yorkshire Wolds. The end of the Triassic and beginning of the Jurassic is marked by a marine incursion and the establishment of a shallow sea. In this sea, the shales, clays, thin limestones and sandstones of the Lias were deposited. The rocks of the Lias are superbly exposed on the coast between Staithes and Ravenscar and yield many species of fossil ammonites and occasional marine reptiles such as ichthyosaurs and crocodiles. Following a period of gradual uplift of the land, the succeeding, Middle Jurassic sandstones and mudstones of the Ravenscar Group were deposited on a low-lying coastal plain crossed by large rivers. Within this part of the Jurassic succession a number of fossil plant horizons occur in the sandstones and shales and include fossils of types of cycads, ferns and conifers. Periods of marine inundation over the coastal plain are marked by beds of more calcareous-rich rock that contain the fossils of marine creatures such as bivalves and bryozoans. The sandstones, mudstones and limestones of the Ravenscar Group form the bulk of the outcrop over the North Yorkshire Moors and give rise to many of the landscape features of the North Yorkshire Moors.
The Upper Jurassic is represented in North Yorkshire by sediments deposited under marine conditions. The lower part of the sequence comprises the shallow-water calcareous sandstones and limestones of the Corallian, which contains many types of fossils including bivalves, sea-urchins, brachiopods and corals. This sequence of rocks forms the Tabular Hills with strong northward-facing scarp slopes between Scarborough and Helmsley and west of this the Hambleton Hills terminate in a west facing scarp rising to about 400m above the Vale of Mowbray. The Corallian is also excellently exposed on the coast between Filey Brigg and Gristhorpe Bay. Overlying the Corallian are the marine mudstones and thin limestones of the Kimmeridge Clay, which underlies the Vale of Pickering. Exposure of the Kimmeridge Clay is very limited but the highest part can be seen on the coast near Speeton.
Cretaceous
The end of the Jurassic Period and the beginning of the Cretaceous (142-65 million years ago) was marked by a global fall in sea-level and the retreat of the sea from the area now occupied by North Yorkshire. This led to the formation of land and a period of erosion began. After a considerable interval, the early Cretaceous sea invaded this land area from the east. The first sediments laid down in this sea are represented by the Lower Cretaceous Speeton Clay which occurs along the north-eastern edge of the Wolds, and is exposed on the coast at Speeton. These fossiliferous marine clays yield well preserved ammonites, belemnites and bivalves, which indicate deposition in a tropical, shallow sea. Overlying the Speeton Clay is the Red Chalk, a pink limestone and brick red marl which are rich in fossil bivalves, ammonites and other marine creatures. A major phase of sea-level rise and deepening of the Cretaceous sea marked the beginning of the Upper Cretaceous. In this warm, sub-tropical sea, the almost pure limestone of the Chalk was deposited. This rock forms the Yorkshire Wolds and outcrops in the south-east of the County between Thixendale and Hummanby.
Quaternary
Over the last two million years the climate of Britain has varied tremendously with periods of temperate climate interrupted by repeated advances and retreats of glaciers and ice sheets. Collectively these periods have become known as the Ice Age (we are still in one of the temperate phases) and the actions of the ice sheets have been instrumental in forming the landscape we see today.
Much of the bedrock geology of North Yorkshire is mantled by sediments deposited during the Ice Age by ice sheets moving over the area or from glaciers originating in the area. There is a vast array of glacial deposits, mainly of till (boulder clay) lying over the surface of the County, deposited from beneath ice sheets and glaciers. Till from the last glaciation, the Devensian, covers the lower-lying ground around the North Yorkshire Moors, which were ice-free during the Devensian, while there is evidence of an earlier glacial period provided by till of differing composition in and around the Vale of Pickering. The Yorkshire Dales have been glaciated on several occasions which has led to the formation of spectacular glacial valleys such as upper Wharfedale. There is evidence of a local ice cap or centre above Langstrothdale and at the top of Wharfedale, which fed the main valley glaciers flowing eastwards down Wharfedale, Wensleydale, and Swaledale into the Vale of York. Deposition of glacial material in the valley floors is thought to have led to the development of glacial lakes within the valley floors of Wharfedale (near Kettlewell) and Wensleydale. Large amounts of till were left in the area following retreat of the last ice-sheet some 13,000 years ago.
During the melting of the last ice sheets and glaciers, over some 15,000 years, large amounts of material were released and transported by the meltwaters to form large spreads of fluvio-glacial sand and gravel deposits, such as those that occupy a large part of the Vale of York. Blockage of both the western and eastern ends of the Vale of Pickering by ice led to the formation of a large lake in which clays, sands and gravels were deposited. This lake found an outlet to the south west, which cut through the rocks to form the deeply-incised Kirkham Gorge. The course of the River Derwent was thus naturally and permanently diverted to flow south to the Ouse basin. Glacial drift deposits from the retreating Vale of York ice sheet have also left their mark in a more undulating landform to the south and west of the area.
Geological Highlights:
- The Jurassic rocks in the Whitby area are well known for the Jet that they contain. Jet is a lustrous, intensely black mineral that is easily worked and takes a high polish, making it a valuable source for the production of jewellery. Jet was formed from the logs of conifer-like trees (probably related to the modern-day monkey puzzle tree) that were swept out to sea and became buried in the sea-floor sediments. After millions of years the wood became replaced by minerals. The workable Jet is found in the upper part of the Lias shales and in the Whitby area its exploitation became a prominent industry in the second half of the nineteenth century. It reached a peak when mourning wear became fashionable following the Death of Queen Victoria's consort Prince Albert. At its zenith in 1870 the jet industry employed over 1,500 people in Whitby in over 200 jet workshops.
- The Upper Lias rocks of the coast east of Whitby are the richest source of fossil marine reptiles of this age anywhere in Britain. Many plesiosaurs, ichthyosaurs, and marine crocodiles have been found including the type specimens of 10 species. The first fossil ‘alligator’ from Whitby was reported in 1759, and discoveries since then have proved extremely valuable in studies in marine reptiles. Most of the fossils have been recovered from the Alum Shale Formation which was formerly worked for the production of cement and alum in the Saltwick and Boulby area.
- The Carboniferous Limestone outcrop of the Yorkshire Dales gives rise to some of the finest limestone, or karst, scenery in Britain. Prominent features include large areas of limestone pavement at places such as Ingleton and Malham. This weathering feature of hard limestones arises through the gradual dissolution of the limestone by rainwater, which is slightly acidic. This process creates a number of diagnostic features commonly including massive, flat, tabular limestone blocks (clints) with intersecting vertical fissures (grikes). Caves and underground streamways are also a common feature and again result from the dissolution of the limestone by water. Many extensive cave systems occur in the Ingleborough-Pen-y-Ghent Hill area and include sites such as Hull Pot which has the largest cave entrance in Britain. Many of the caves were used by man and mammals during the Quaternary and the bones of animals of temperate climate phases such as hyaena, lion and straight-tusked elephant have been found along with colder phase animals such as reindeer and lemming.
- The magnificent deeply incised valley of Newtondale is today occupied by Pickering Beck, a small stream that could not have formed the large valley. The misfit between stream and valley is considered to be due to the carving of Newtondale by a much larger river that issued from a large glacial lake. The lake was formed by meltwater from the retreat of the last ice-sheets of the Devensian Glacial some 13,000 years ago by water draining from the Esk Valley. However, there is also some suggestion that the valley formed, at least in part, by seasonal meltwaters from a lobe of ice occupying the Esk Valley.
Local sites
The following localities represent, in part, the geology of this county. Each locality has a grid reference, a brief description of how to get there and a short summary of the geology you are likely to find. All the localities listed are openly accessible.
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