Formerly classified as part of the Middle Lias, these strata were deposited over a period of about 2 million years, as soft sediment on the floor of a shallow arm of the ancient Tethys Ocean, between c.185 million and 183 million years before present.
The formation lies conformably on the prolifically-fossiliferous shallow marine sandstone, siltstone and mudrock of the Staithes Formation.
There are six named ironstone seams which are, in order of deposition, the Osmotherley, Avicula, Raisdale, Two-foot, Pecten and Main Seams.

At its type locality, on the coast around Staithes, North Yorkshire, the formation attains a thickness of 25.3 metres and comprises five 'coarsening-upward' cycles of marine shale and siltstone each capped by a seam of ironstone of varying thickness, composition and iron-content.
The shales and siltstones are grey in colour, and exhibit occasional bands of calcareous or sideritic nodules, shell-beds and cross-bedding.
The uppermost reaches of some of the shale units may be laminated and have been dubbed tempestites.
Immediately beneath the Raisdale Seam at Staithes can be seen laminated beds with basal gutters up to 1 foot 8 inches (0.5 m) wide and 16 feet (5 m) long which were scoured out during high energy storm events.
The gutters have been subsequently infilled with fine sand and silt which shows signs of pyritisation.
These features tend to be oriented in an east-west direction and are fairly persistent laterally, occurring almost 12 miles (19 km) to the south-east at Hawsker Bottoms.

Definitive zonal work has been carried out on the coast by Howarth (1953),  Howard (1985) and extended inland by Chowns (1968).
Their efforts reveal that these strata span two faunal zones prompting subdivision into corresponding informal units.
The lowest 62 feet (19 m) are predominantly shaley and include the Osmotherly, Avicula, Raisdale and Two-foot Seams, the best-developed being the Avicula Seam attaining c.0.5 metre at Jet Wyke.
They reside within the upper part of the Amaltheus margaritatus faunal zone and are collectively referred to as the Penny Nab Member, after a headland c.150 metres east of Staithes Harbour.

An unconformity marks transition to the overlying Pleuroceras spinatum zone which covers the more ferruginous upper 6 metres of the formation.
It is referred to as the Kettleness Member, and includes the Pecten and Main Seams, the latter c.1.8 metres thick and split into a 0.7 metre Top Block and 0.8 metre Bottom Block by 0.3 metre of medial shale at Old Nab.

The most important seams, from an economic perspective, are the Main and Pecten Seams which attain their greatest development along the northern edge of the ore-field, near Eston.
There the Main Seam, 3.66 metres thick, rests directly on the Pecten Seam (1.23 metres) which includes an impressive shell-bed.
The seams are relatively persistent in an east-west direction, but shale-partings intervene and thicken to the south at the expense of the ironstone.
In East Cleveland the strata pass through a structural syncline known as the Skelton Syncline where the Main Seam descends to around 410 feet (125 m) below sea level around North Skelton.

The whole formation thins and becomes less-ferruginous to the south as the Main Seam, itself much diminished, oversteps each of the underlying seams one-by-one.
Strata of this age are completely absent at the southerly limit of the Yorkshire Basin around Market Weighton.
Formerly classified as part of the Middle Lias, these strata were deposited over a period of about 2 million years, as soft sediment on the floor of a shallow arm of the ancient Tethys Ocean, between c.185 million and 183 million years before present. The formation lies conformably on the prolifically-fossiliferous shallow marine sandstone, siltstone and mudrock of the Staithes Formation. There are six named ironstone seams which are, in order of deposition, the Osmotherley, Avicula, Raisdale, Two-foot, Pecten and Main Seams.
At its type locality, on the coast around Staithes, North Yorkshire, the formation attains a thickness of 25.3 metres and comprises five 'coarsening-upward' cycles of marine shale and siltstone each capped by a seam of ironstone of varying thickness, composition and iron-content. The shales and siltstones are grey in colour, and exhibit occasional bands of calcareous or sideritic nodules, shell-beds and cross-bedding. The uppermost reaches of some of the shale units may be laminated and have been dubbed tempestites. Immediately beneath the Raisdale Seam at Staithes can be seen laminated beds with basal gutters up to 1 foot 8 inches (0.5 m) wide and 16 feet (5 m) long which were scoured out during high energy storm events. The gutters have been subsequently infilled with fine sand and silt which shows signs of pyritisation. These features tend to be oriented in an east-west direction and are fairly persistent laterally, occurring almost 12 miles (19 km) to the south-east at Hawsker Bottoms.
Definitive zonal work has been carried out on the coast by Howarth (1953),  Howard (1985) and extended inland by Chowns (1968). Their efforts reveal that these strata span two faunal zones prompting subdivision into corresponding informal units. The lowest 62 feet (19 m) are predominantly shaley and include the Osmotherly, Avicula, Raisdale and Two-foot Seams, the best-developed being the Avicula Seam attaining c.0.5 metre at Jet Wyke. They reside within the upper part of the Amaltheus margaritatus faunal zone and are collectively referred to as the Penny Nab Member, after a headland c.150 metres east of Staithes Harbour.
An unconformity marks transition to the overlying Pleuroceras spinatum zone which covers the more ferruginous upper 6 metres of the formation. It is referred to as the Kettleness Member, and includes the Pecten and Main Seams, the latter c.1.8 metres thick and split into a 0.7 metre Top Block and 0.8 metre Bottom Block by 0.3 metre of medial shale at Old Nab.
The most important seams, from an economic perspective, are the Main and Pecten Seams which attain their greatest development along the northern edge of the ore-field, near Eston. There the Main Seam, 3.66 metres thick, rests directly on the Pecten Seam (1.23 metres) which includes an impressive shell-bed. The seams are relatively persistent in an east-west direction, but shale-partings intervene and thicken to the south at the expense of the ironstone. In East Cleveland the strata pass through a structural syncline known as the Skelton Syncline where the Main Seam descends to around 410 feet (125 m) below sea level around North Skelton.
The whole formation thins and becomes less-ferruginous to the south as the Main Seam, itself much diminished, oversteps each of the underlying seams one-by-one. Strata of this age are completely absent at the southerly limit of the Yorkshire Basin around Market Weighton.
Cleveland Ironstone Formation