2. Carboniferous

Carboniferous strata underlay about 75% of the area. This is the best known system in the Northeast of England (if not in Britain), due to the widespread occurrence of the economically important coals and vein ores. Cyclic sedimentation followed throughout the Carboniferous, laying down limestone, mudstone, sandstone, seatearth, ganister (high silica clay - very hard and fine-grained), and coal (except during the Tournaisian, which was completely marine and contains no coal). Marine conditions  became shorter with each cycle, causing upward thinning of the limestones. In the Coal Measures, there is almost no evidence of marine conditions. Carboniferous strata reach thicknesses of over 3000 ft. (~ 1000 m). Herein are: Millstone Grit  - 900 - 1400 ft., Lower Coal Measures - 600 ft., Middle Coal Measures - 1000 ft. I have found no reference yet to the Stephanium. In the South, the strata are overlain unconformably by Permian rocks. The Coal Measures have been proven off the coast by deep offshore bores, and large reserves are still available either below the North Sea, or below Permian strata. It is, however, unlikely that these will be worked in the near future, for economic reasons. They are exposed in the West, striking NE - SW and dipping East at 1.35° - 1.4°. In the Coal Measures a cyclic succession of coals, sandstones and dark grey mudstones is present, moving up to lighter siltstones. Under the Lower Coal Measures, we have limestones of the Carboniferous Limestone. The Carboniferous Limestone and Millstone Grit both contain limestones and sandstones, although the type area is further to the South. The coals of the Durham coalfileds are known to have been worked by the Romans, and are the worlds first (known ?) commercially worked coalfield.

The sandstones are quartz with feldspar:

• orthoclase,


• microcline (potassium feldspar from highly potassic granites),


• oligoclase (a plagioclase).

They also contain:

• zircon,


• garnet,


• rutile,


• tourmaline,


• apatite,


• magnetite,


• sapphire and, in some places, so much


• muscovite (from granitic and metamorphic rocks further North) that the sandstones become fissile.

During the Carboniferous many dykes (dikes) and sills were emplaced, as far as the Middle Coal Measures. These rocks include quartz-dolerites, a medium grained basic rock, and tholeiites, an over-saturated basalt containing glassy free silica, but consist in the main of olivine basalts. These include the Kelso lavas, outside of the immediate area. In the area being considered, magmatites are restricted to the above mentioned dykes and sills. The rocks are very hard and have been used as roadstone. The best known of these dykes and sills is the Whin Sill, which crops out in Northumberland and Cumberland, and in Teesdale. It has also been proven in numerous bores, and has an area of > 5000 km². The average thickness is thought to be around 25 - 30 m, with a maximum recorded of about 73 m. At first considered to be a contemporaneous lava flow within the Carboniferous Limestone, it is now known to be intrusive and in places changes position and direction dramatically along fault planes, including distinct layers, sometimes separated by hundreds of meters. The Whin Sill was the subject of pioneer isotopic age determination by Arthur Holmes.

3. Permian

The Permian and the Triassic periods are often collectively known as the Permo-Trias(sic) in the United Kingdom, because of difficulties defining the boundary between the two in most of the country. It is often taken to be at that point where red sandstones overlay red clays (mudstones) and silts. These, however, have been proven to be diachronous with the sandstones. In parts of Britain there is also ongoing discussion as to the definition of the transition from Carboniferous to Permian. During the early Permian (beginning in late Carboniferous), the area was uplifted and eroded, as a result of the development of the Hercynian orogeny. There followed a period of continental deposition, with deep arid weathering processes.

The Permian deposits measure up to 1400 ft. (~ 425 m). They consist mainly of:

• Basal Permian sands and breccias, the so-called "Yellow Sands", a sandstone of up to 160 ft. (~ 49 m)


• Lower Magnesian Limestone


• Middle Magnesian Limestone


• Upper Magnesian Limestone


• Upper Permian Marl (contains Marl Slate = Kupferschiefer (from the German "copper schist"), with a high content of sygenetic and diagenetic sulphides and pyrite)

North of the Hartlepool Fault, everything above the Upper Magnesian Limestone is gone, although there are breccia of Upper Permian Marl origin in the Middle and Upper Magnesian Limestone. The Permian deposits lie unconformably on an eroded Carboniferous plain, which slopes gently to the East. The basal sands ("Weissliegende") and breccias (Zechstein conglomerate) contain many ventifacts ("Dreikanter") and show desert patination. These sandstones range from aeolian to subaqueous in facies. The area lay in the shallow, coastal waters of the Zechstein Sea and developed cyclic formations, as is the case in most of (North-West) Europe. Because of the proximity to the coast, many cycles are not complete. They consist mainly of carbonates, which are often highly dolomitised. Many of the cycles were later removed by solution. Strongly represented in the Hartlepool area are:

• reef dolomites


• lagoonal dolomites


• concretionary and evaporative limestones


• evaporites such as:


• Anhydrite


• Gypsum


• Halite


• and other salts

The estimated average temperature during the period was approximately 23°C (~ 45 F). Plant and animal fossils are present at the base of each cycle, slowly disappearing towards the top of the cycle. It is supposed that moisture was carried from the Zechstein Sea to the immediate coast area; as the sea gradually evaporated less moisture was available, and hence the reduction in flora and fauna towards the top of each cycle.

The Permian period is important for minerals in the North-East of England and a fair variety is to be found.

X = recorded
C = common

¹ - Called Hesleden Dene No. 2 bore in descriptions and analyses given by Dunham and others (1948) and Guppy and Sabine (1956, pp. 65, 72-3)

4. Triassic

The Triassic period is represented in the area by the Bunter Sandstone and the Keuper. The marine "Muschelkalk", which is well represented in North-West Europe, is not present in Britain. The area continued to be arid-continental. Some sources claim that the Muschelkalk was deposited here and there, but was later removed by erosion. It is therefore not usually included in the British nomenclature.

The Bunter Sandstone occurs between the West Hartlepool fault and the South of the area, consisting of the red sandstones which give this epoch its name (Bunter Sandstone comes from the German for "coloured sandstone", where it is now known as “Buntsandstein”). Only two exposures are known, both of which are on the Hartlepool coast. The Bunter Sandstone reaches thicknesses of up to approximately 700 ft. (~200 m). It consists of red and grey fine-grained sandstones, and mud and siltstones. The latter show current bedding, ripple marks and sun cracks, showing that a shallow water, semi-continental depositional environment was prevalent.

The Keuper Marl also reaches a thickness of approximately 700 ft. and consists of mud and siltstones.

5. Pleistocene and Recent (Holocene)

The whole area has been morphologically formed and reformed by glacial erosion and deposition. A whole series of glacials and interglacials moved across the North of England, including many minor, rapid moving, short-lived periods, often from local high ground. These centres were usually around the Lake District and the South of Scotland. The deposits left behind are up to 300 ft (~91 m) deep, especially in buried valleys, but usually only reach around 10 - 15 ft. On the whole, they consist of:

• Boulder clay


• Laminated clay


• Breccia


• Sand


• Gravel

The topographical features which mark a once-glaciated landscape are, amongst others:

• Hanging valleys


• Raised beaches


• Moraines (of several types)


• Raised river terraces

All of which are present in the Hartlepool area. Also present, off the coast, are submerged "forests". In Hartlepool docks, these reach as much as 17 ft. in thickness and just off the coast they are one mile wide and three miles long. They contain many bones, deer antler and stone tools. 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. Many of the buried valleys cut the present coast well below sea-level and have been proven for several miles off the coast.

The oldest glacial deposits are to be found mainly in fissures in the Magnesian Limestone. They are filled with grey, gritty clay and erratics from Scandinavia; no British rocks have been found in these deposits. The clays also contain animal bones and arctic shells not now found in Britain or its waters. Caves are also known in the Magnesian Limestone, most of which are filled with Pleistocene deposits. Human bones have also been found, however, together with Iron Age tools.

The main Holocene (Recent) deposit is Boulder Clay, which consists of:

• Upper Boulder Clay


• Middle Sands


• Lower Boulder Clay

6. Economic Geology

The most important components of the economic geology of the Hartlepool area are coal, rock salt, gypsum and anhydrite. Limestone and dolomite also play a role, as well as brick and fireclay, various stones and rocks for building and aggregates. Minerals and ores only play a minor role. I shall now deal with these in their rough order of importance.

6.1 Coal

As has already been mentioned, coal has played an important part in the development of the region and has been worked at least since Roman times. The seams in Durham vary considerably in thickness and quality, the best seams being those in the 250 m above and including the Brockwell Seam. These include the Busty, Harvey, Bensham and High Main Seams. The highest rank coal is produced in the West of Durham, supplying an exceptionally good coking coal which is low in ash, sulphur and phosphorus. In the South of Durham (the area being considered here) the volatiles increase and the carbon content and calorific value decrease. Most coal production is now open cast. The larger part of pits have now closed and only one or two collieries are still operating, notably in coastal areas.

6.2 Rock salt

Production of rock salt is concentrated in the Greatham area, which borders directly on the South of Hartlepool. The salt is removed by pumping of artificially induced brine solution. The salts are in a bed of the Permian Middle Evaporite Group of up to 45 m thickness. This 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. Reserves are ample for several decades at least. Table and preserving salt production is in decline in Teesside.

6.3 Gypsum and anhydrite