Natural England - Durham (including Darlington, Stockton-on-Tees, Middlesborough, Hartlepool and Redcar and Cleveland)

Durham (including Darlington, Stockton-on-Tees, Middlesborough, Hartlepool and Redcar and Cleveland)

The landscape of Durham and the lowland valley and estuary of the Teeside area is extremely diverse, from the limestone cliffs and foreshores of the North Sea coast in the east, through coalfields, lowlands and dales to the ridges and plateaus of the North Pennines in the west.

Fossil fish from the Permian Marl Slate of County Durham

Just as the history of the area is intimately linked to the geology, with centuries of exploitation by man, so too is the shape and form of the landscape dominated by variation in the nature of the differing rock types. The underlying geology is relatively simple and is dominated by rocks from four main periods of geological time. The rocks slope gently to the south-east, so that the oldest occur in the west and the youngest in the east. Rocks of Carboniferous age deposited in an area of shallow tropical seas and humid swamps outcrop in the west. These rocks are overlain in the east by younger rocks belonging to the Permian Period when a hot and arid climate prevailed and this area of Britain was dominated by desert dunes and a large shallow, salty sea. These give way to red sandstones and mudstones deposited under arid, desert conditions during the Triassic and which underlie much of the area south and west of Hartlepool.

Between Redcar and Loftus and west to the outskirts of Middlesborough, clays, limestones and sandstones of Lower Jurassic age form the northern edge of the North York Moors. These shallow marine rocks are famous for the fossil reptiles and other forms of marine life that they yield.

The whole area was affected by glacial erosion and deposition during the Ice Age of the past two million years. As a result thick deposits of till, or boulder clay, deposited by the ice sheets cover the solid geology.

Carboniferous

Rocks of Carboniferous age are divided into three characteristic units: the Carboniferous Limestone Series, the Millstone Grit Series and the Coal Measures. These Carboniferous rocks have been the subject of mineral exploration for many centuries and dominate the mining history of the area.

Rocks of the Carboniferous Limestone Series outcrop in small areas in the west (around St John's Chapel and Newbiggin) and south-west (around Barningham). These rocks are composed of thin beds of sandstone, mudstone and limestones that were deposited in a marine-estuarine environment. This Series is overlain by the younger rocks of the Millstone Grit Series, which occupy the remainder of the western side of County Durham. The Millstone Grit consists of limestones, marine shales and sandstones, and the 'Millstone Grit' itself, which comprises thick coarse-grained sandstones. The nature of the rocks within the Millstone Grit Series exert an important influence on the landscape. The alternate hard (sandstones) and soft (mudstones) beds has produced a stepped appearance to the dalesides and the flat-topped nature of some of the hill summits in the western half of County Durham. The rocks of the Millstone Grit Series were deposited in a shallow marine to estuarine environment, with the sandstones representing the sediments laid down by rivers forming part of a large complex of deltas.

This environment persisted into the subsequent Lower and Middle Coal Measures. The Coal Measures dominate the central northern section of County Durham, from around the margins of the Pennines in the west to the lowlands surrounding the River Wear in the east. The influence of the soft and thinly bedded mudstones and sandstone of the Coal Measures on the landscape is reflected by the gently sloping valleys of this part of Durham, with occasional steeper bluffs formed by harder sandstones. The coal seams within the Coal Measures were heavily exploited by man until the gradual decline of the coal and steel industries from the 1940's onwards.

At a number of locations around County Durham, intrusions of igneous rock are associated with the Carboniferous rocks of the area (e.g. the Whin Sill). These intrusions principally occur on the southern side of Teesdale and, because of their relative hardness, form a series of imposing features in the landscape. These include High Force, where the River Tees flows over the lip of Whin Sill to form England's largest waterfall.

Permian

To the east, the Coal Measures are overlain by rocks of Permian age. In north-east Durham, sandstones known as the 'Yellow Sands' represent the remnants of Saharan-like desert sand dunes. The Yellow sands are overlain by a thin mudstone known as the Marl Slate and an extensive sequence of cream to buff coloured limestones know as the Magnesian Limestone. The Magnesian Limestone dominates the east of the area between Seaham and Darlington forming a prominent escarpment and plateau tilted towards the North Sea coast and is also exposed at the coastline between Seaham and Hartlepool. These rocks were deposited in a relatively shallow landlocked sea, which extended from north east England to Poland, and has been named the Zechstein Sea.

The estimated average temperature during the period was approximately 23 degrees Centigrade. Due to the landlocked nature of the sea and high temperatures the sea water gradually evaporated leaving behind salt deposits (gypsum and anhydrite, collectively known as evaporites), that today are commercially worked to produce brine. 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. These are particularly well displayed in and around Sunderland and form prominent hills such as Tunstall and Humbledon.

The unique chemistry of the Magnesian Limestone (the limestone is high in Magnesium carbonate, when compared with other limestones) gives rise to soil conditions which support an interesting combination of plants, with grassland species typical of the Chalk of southern England occurring along with rare plants, such as thistle broomrape, which is not found away from the Magnesian limestone outcrop.

Triassic

Rocks of Triassic (248-205 million years old) age form the bedrock to the lower section and estuary of the River Tees. During the Lower Triassic all the continents had moved together to form the supercontinent of Pangaea (meaning 'all Earth') and in the area occupied by Britain, an arid, desert environment predominated. In the Hartlepool area, rocks from this period of continental conditions comprise red and grey fine-grained sandstones, and siltstones belonging to the Sherwood Sandstone Group. Only two exposures are known, both of which are on the Hartlepool coast, although up to 200m of rock have been proved in boreholes. Evidence from localities around England show that the Sherwood Sandstone was deposited on a large desert plain across which large braided rivers intermittently coursed.

The mudstones of the succeeding Triassic Mercia Mudstone Group probably represent wind-blown dust that settled in vast shallow salt-lakes and sun-baked mudflats on the extensive desert plain. These mudstones which are up to 200 metres thick, outcrop at the mouth of the Tees and underlie Middlesborough.

Jurassic

Towards the end of the Triassic and into the Lower Jurassic (205 to 142 million years ago) a shallow marine sea invaded much of the continental landmass, resulting in the flooding of the desert plain. In this shallow, tropical sea a series of mudstones and limestones were deposited. The lowermost Jurassic rocks belong to the Redcar Mudstone, part of the Lias Group and are exposed along the coast between Coatham and Saltburn. These are in turn overlain by a series of shales, limestones, thin sandstones and ironstones, belonging to the Middle Jurassic Ravenscar Group, which outcrop along the coast towards Staithes and inland along the northern flank of the North York Moors. These rocks are very fossiliferous and have yielded large numbers of fossil marine creatures including ammonites and several species of reptiles. The organic-rich shales were originally quarried for the production of Alum in the Boulby area.

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.

The majority of the underlying geology of the area (particularly in low-lying areas) is covered by a thick layer of sediment deposited during the Ice Age. Most of these deposits are tills (or boulder clay), with small areas of sands and gravels centred around existing rivers and small patches of clay deposited in glacial lakes.

The dramatic forces of repeated ice sheet advances and retreat has formed and reformed the landscape of Northern England. Following the retreat of the last ice sheet as the land has become gradually uplifted rivers have cut steep valleys through the more resistant Carboniferous sandstones and Permian Magnesian Limestone. The 'incised' meander of the River Wear at Durham City and the gorge-like Denes that cross cut the Magnesian Limestone outcrop down to the coast are classic examples of this relatively recent landscape evolution.

Immediately off the coast of Hartlepool are a series of sediments including a peat bed which represents the remnants of a forest. Fossil material from the peat including snails, deer antlers and human-made stone tools show that this area was land approximately 5,000 years ago. The forests grew at a time after the ice had melted away to the North, and the climate was warming, but before the sea-level had risen to its current position.

Geological Highlights:

  • The term 'geology' (latin, 'geologica') was coined by Richard de Bury, Bishop of Durham from 1333-1345.

  • County Durham’s history is dominated by mining with important historic links between the economy and geology. Exploitation of the coal seams within the Coal Measures probably occurred from early times, but intensified in the 18th century, as did the lead mining industry. The peak of the coal mining industry in County Durham was in the 1920’s, but gradually decreased in the late 20th Century, with the last deep mine being closed in 1994.

  • The Carboniferous Limestone outcrop in the west of the County Durham is heavily mineralised and forms part of the North Pennine Orefield. Exploited minerals include galena (lead), silver, fluorite, siderite (iron) and haematite (iron). By the 19th century the North Pennines was the leading iron ore field in the World. Reminders of the former coal mining and minerals industry can be seen throughout the landscape and include areas of reclaimed land, spoil heaps and disused quarries.

  • Fossil plants, notably ferns and horsetails, commonly occur in the Coal Measures. In some locations whole fossil tree trunks have been found. An example, probably of the fossil tree Sigillaria, which has been restored in an upright position, is present in Stanhope Parish Churchyard, Co. Durham. The horizontal roots of the tree are typical of growth in swamp conditions.

  • Some quarries in County Durham provide exposure through the Permian Marl Slate, which occurs at the base of the Permian succession. This bed of mudstone contains well preserved fossil fish and occasionally reptiles. Durham and Hartlepool provide the only extensive Magnesian Limestone coastline in England and is characterised by rocky steep cliffs, shore platform, stacks and arches.

  • Durham provides the only extensive coastline formed in the Magnesian Limestone. The limestone has been eroded to form a series of rocky, steep cliffs, foreshore platforms, stacks and arches between Seaham and Blackhall.

  • Thick deposits of Permian rock salt are commercially worked in the Greatham area. The salt is removed by forming a brine solution deep underground which is then pumped out. The brine is used in the chemical industries on the Tees, where it is an important raw material for the alkali trade and for chlorine and chlorine products.

  • Potash (Potassium chloride) is mined commercially at Boulby. The potash occurs within a Permian evaporite sequence in a seam ranging from 0-20m thick and occurs at depths between 1,200 and 1,500m below ground. The mine has been working since 1973 and the tunnels extend as far as 7km underneath the North Sea. It supplies some 55% of the UK’s potash consumption which is mainly used in fertiliser production.

  • The old Alum quarries at Boulby are well known for the fossil reptiles that have found from the Upper Lias shales that outcrop here. Two species of plesiosaurs and one ichthyosaur were first described from here and the site has also produced the only known pterosaur (flying reptile) from the Upper Lias of Britain. Although not at all common the fossils found here are unique and quite different to those found from similar aged rocks 20km to the south at Whitby.

  • The North Pennines AONB was the first area in Britain to be inscribed as a European Geopark in 2003. The AONB is noted for its world-class geological heritage, mainly of Carboniferous age. This geological interest, coupled with the efforts of the AONB staff and others for its conservation and interpretation, led to the European Geopark status being conferred on the area. It is part of a growing network of European Geoparks and membership of this network will lead to more events activities, trails and leaflets focusing on the geology of the area.

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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