Jurassic
On the south bank of the Tees, near Warrenby, a change occurs in the rocks below ground that leaves no trace on the surface. It is a transition from almost lifeless Triassic mudstones, to beds packed with the fossils of creatures that lived in the most notorious period in Earth’s History – the Jurassic! Stretching along the coast from Redcar to Filey can be found a sequence of rocks that have been highly acclaimed by generations of geologists:
“In no part of England is the relation of the surface topography to the nature of the underlying rocks more instructively displayed than in this district; nor can the succession of a considerable part of the Jurassic series of formation be anywhere more advantageously examined than along the coast-sections…”
[Archibald Geikie, Director of the Geological Survey. (1888)]
Jurassic deposits locally total around 560 metres of strata, and have been broadly classed into Lower, Middle, and Upper divisions. This strata was deposited between 204 and 158 million years ago in environments that ranged from deep sea populated by a variety of distinctive marine creatures, to well-vegetated river delta upon which dinosaurs once roamed.
Geologists long-ago realised that similar rocks in diverse areas could be correlated by examining their fossil content. One of the earliest to realise this was Louis Hunton (1814-1838), son of a Loftus alum-worker who studied remains of long extinct sea creatures in Jurassic rocks at Hummersea. As far as correlation of different rock units is concerned, the most useful fossils turned out to be the coiled shells of many species of ammonites,. Ammonites are an extinct creature related to modern day squid and octopuses. Modern geologists can identify over sixty ammonite zones, which finely subdivide the various strata and make relationships between them much easier to understand.
Lower Jurassic
Lower Jurassic rocks were deposited in a variety of marine environments ranging from shallow sea floor to deep sea. The shallow sea floor probably underwent brief periods of emergence and erosion. The deep sea had abundant plankton in the upper waters but an oxygen-depleted floor.
Redcar Mudstone Formation
After the noxious conditions of the Triassic incursions, the sea deepened, became fully oxygenated, and a new era of sedimentation commenced with the Redcar Mudstone Formation. The rock outcrops on the coast between Redcar and Staithes, and is perhaps most notable for the great number of fossil oysters (‘Devil’s Toe-Nails’) that wash up on the beaches nearby. At its base are cycles of soft mudstones capped by thin, hard limestones often packed with well-preserved fossils and best seen on the scar at Redcar.
Staithes Formation
During deposition of the preceding Redcar Mudstone, waters became ever shallower causing a change in the type of sediment laid down on the sea floor. The Staithes Formation comprises fine-grained sandstones, thin mudstones, and bands of iron rich nodules. Finely laminated sandstones, up to 0.8 metres thick, are frequently followed by units in which the bedding has been completely wiped out when increased wave action during storm surges churned up the sea floor. The Staithes Formation is possibly the most fossiliferous rock to be found locally, with extensive shell beds, belemnites, trace fossils left by many creatures that burrowed within the sediments, and much more. Sedimentary structures such as ancient ripple marks are also commonly seen amongst these rocks.
Cleveland Ironstone Formation
This is famous for the part it played in the growth of Teesside. The beds belonging to this formation formed in conditions of varying sea level. They are made up of layers of grey silty mudstones and, five distinct seams of ironstone, the latter deposited during shallower episodes, in a sea rich with life. Fossils are common and well preserved in the ironstones. The seams increase in both thickness and grade upwards culminating in the Main Seam which possesses an iron content of thirty-three percent and maximum thickness of 4.8 metres. Cleveland ironstone has an oolitic texture and is blue-grey or olive-green in colour when fresh, weathering to a rusty red-brown.
Whitby Mudstone Formation
After deposition of the Cleveland Ironstone, the sea reached depths not experienced across the area since Permian times. The change was gradual and there is little difference between the Grey Shales at the mudstone base, to those of the preceding formation. Following the Grey Shales is the Jet Rock, comprising beds of dark finely laminated shale containing pyrite. The pyrite smells strongly of mineral oil when freshly broken. Seasonal falls of dead plankton from the upper waters are responsible for the presence of oil. Seams of jet are found where waterlogged tree trunks became buried within the oxygen-depleted sea floor mud. The sequence ends with a thin limestone known as the Top Jet Dogger.
Shallower seas led to greater oxygenation of the waters and deposition of beds known as alum shale. Many large marine reptiles (plesiosaurs, ichthyosaurs, and crocodiles) have been recovered from these beds some of which are displayed locally. The Whitby Mudstone Formation usually finishes with the Cement Shales. These differ little in appearance from the alum shales, but contain numerous large limestone nodules once processed for hydraulic cement.
Middle Jurassic
The late Lower Jurassic experienced the deepest seas since the Permian Period. Transition to the Middle Jurassic saw extensive uplift of the local strata which allowed a river delta to encroach upon the area from the north and west. The sea twice covered the delta and associated marsh, resulting in cycles of deltaic and marine sedimentation, known collectively as the Ravenscar Group.
Dogger Formation
At the base of the Middle Jurassic, the area was gently folded into a series of low domes and basins, producing a mosaic of depositional environments. Parts of the former sea floor emerged above the water to be weathered and eroded. The sediment became generally coarser due to the closer proximity of land, and was inhabited by a diverse fauna of invertebrates and other marine creatures. Accordingly, the Dogger Formation displays a number of different rock types, (sandstone, mudstone, siltstone, sandstone, and ironstone), that vary across the area. These are often packed with fossilised burrows. The formation rarely attains a thickness in excess of a few metres and in places is absent altogether.
Saltwick Formation
As uplift progressed, so the deposits change to thickly bedded yellow sandstones, grey siltstones, and some minor mudstones. This change occurred as a great river delta advanced across the former sea floor. Within the Saltwick Formation can be found beds of fossil wood, some of which form seams of jet, indicating that the delta was well vegetated. In fact, the earliest fossil of a flowering plant (Weltrechsia whitbiensis) was recovered from beds in this formation near Whitby. Perhaps the most sought after, and least found fossils, are those of dinosaur footprints. These tell us that land-dwelling reptiles of many species once browsed and hunted on the delta-marsh.
Eller Beck Formation
This is a thin sequence deposited when the sea briefly covered the delta from the southeast. It laid down a basal bed of mudstone with marine fossils, which is replaced higher up (in places) by a thin or nodular ironstone, and lastly a sandstone. The presence of Ooids within some of the iron nodules indicate a shallow tropical environment.
Cloughton Formation
Renewed uplift of the crust once again banished the sea and the area continued to be under a deltaic regime. The Cloughton Formation is much more variable than the Saltwick, though it retains a very similar set of fossils. The imprints of delicate ferns and horsetails can be found amongst the beds of sandstone, siltstone, and mudstone. It also bears the imprints of reptilian feet at various levels.
Scarborough Formation
A second incursion of the sea deposited sandy limestone packed with marine fossils. This is generally a blue-grey rock containing the fossils of belemnites, shellfish, scattered wood fragments, and sea-lily stems. The formation attains a thickness of around 5 metres locally, though is seldom well exposed at the surface.
Scalby Formation
A return to deltaic conditions is marked by the Scalby Formation. The lower 10 metres consists of high-grade sandstone known as the Moor Grit with a quartz content as high as 98.15%. It contains few fossils except for the occasional wood fragment. Above it lays the Long Nab Member marking a return to the delta-marsh conditions of the Saltwick and Cloughton Formations. Further south are extensive plant beds, but in the Tees Valley, it comprises flaggy sandstones with intermittent mudstones deposited during periods of flood.
Upper Jurassic
The deltaic regime ended with deposition of the Scalby Formation at the top of the Middle Jurassic. Marine deposits above it are assigned to the Upper Jurassic and are rare in the Tees Valley. Only on the higher parts of the moors in East Cleveland can these rocks be found. The lowest beds belong to the Cornbrash, in other localities it exists as a limestone, but here it is brown sandstone with the imprints of numerous shellfish. Attaining a thickness of only about 5 metres, the Cornbrash is quickly succeeded by the Osgodby Formation. Up to 30 meters of this rock can be found in the south of the region, locally though it only reaches around 5 metres. Fossils of many kinds are abundant in the beds of blue-green sandstone that are only rarely exposed.