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 between Redcar and 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 (Lias), Middle, and Upper divisions. These strata were deposited between c.204 and 145 million years ago in environments which varied between deep sea, populated by a variety of distinctive marine creatures, to well-vegetated river delta upon which reptiles 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 and Boulby. 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 ammonite. Ammonites are now-extinct creatures related to modern day squid and octopuses (Cephalopods). 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 marine incursions, the sea deepened, became fully oxygenated, and a new era of sedimentation commenced with the Redcar Mudstone Formation. Lower Jurassic rocks crop-out along the coast between Redcar and Staithes, and are perhaps most notable for the great number of fossil oysters (‘Devil’s Toe-Nails’) abundant in rocks of this age and which wash up on the beaches in great numbers. At the base of these rocks are cycles of soft mudstone 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 almost completely obliterated by burrowing organisms, a process know as bioturbation. Episodic storm surges resulted in ultimate deposition of finely-laminated beds known as striped beds or tempestites.
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 within 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 usually 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 often contains ooliths. These are concentrically ringed, sub-spherical, sand-sized particles of chamosite, a silicate mineral. They were formed by oscillating sea floor currents. The ironstone is blue-grey in colour when fresh, weathering to a rusty red-brown.
Whitby Mudstone Formation
Following deposition of the Cleveland Ironstone the sea reached depths not experienced across the area since Permian times. This led to deposition of the Whitby Mudstone Formation which consists of, in ascending order, the Grey Shale, Mulgrave Shale and Alum Shale Members. The younger Members (i.e. the Peak Mudstone and Foxcliff Siltstone Members) are not present except in a few pockets of the North York Moors having either not been deposited or removed by erosion. The sea level change was gradual and there is little difference between the Grey Shale Member, at the mudstone base, to those of the preceding formation.
Succeeding the Grey Shale is the Mulgrave Shale Member which comprises, in ascending order, the much quoted informal units Jet Rock, Top Jet Dogger, Bituminous Shales and the Ovatum Band. A double band of pyrite skinned concretions often containing the rare ammonite Ovaticeras.
The Jet Rock consists of beds of dark finely-laminated shale containing pyrite. This shale smells strongly of mineral oil when freshly broken. Seasonal falls of dead plankton from the upper waters are responsible for the presence of the oil. Seams of jet can be found where waterlogged tree trunks became buried within the oxygen-depleted sea floor mud.
The Top Jet Dogger is a thin limestone. This is overlain by the Bituminous Shales which have similar characteristics of the Jet Rock. Shallower seas led to greater oxygenation of the waters and deposition of beds known as the Alum Shale Member, informally divided, in ascending order, into the Hard, Main Alum and Cement Shales. Many large marine reptiles (plesiosaurs, ichthyosaurs, and crocodiles) have been recovered from these beds some of which are displayed locally.
In the North York Moors area The Alum Shale Member 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
Transition to the Middle Jurassic saw extensive uplift of the local land surface which allowed a river delta to encroach upon the area from the north and west. The sea intermittently transgressed 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) which 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 returned to 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 ninety-eight percent. 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 Osgodby Formation at the top of the Middle Jurassic. Marine deposits above it are assigned to the Upper Jurassic and are non-existent in the Tees Valley. Only on the southern parts of the North York Moors (Tabular Hills) can these rocks be found. Here, the Upper Jurassic Formations, all marine, are in ascending order: Oxford Clay, Lower Calcareous Grit, Coralline Oolite, Upper Calcareous Grit and Kimmeridge Clay.