On average the Cleveland Ironstone Formation comprises around 70% shale and 30% ironstone though the latter occurs in the form of the six named seams of variable thickness.
The ironstone is classed as being low-grade with an iron-content of up to 33%, and is deemed economically viable only above ~27%.

The primary iron-bearing minerals are the iron carbonate siderite (FeCO₃) and berthierine (formerly known as chamosite,(Fe²⁺
₂Al(Si,Al)O
₅(OH)
₄).
Ancillary components include calcite (as MgCO₃ and MnCO₃), pyrite (FeS₂), collophanite (Ca₃P₂O₈ H₂O ), silica (Si), clay minerals and derivatives such as octahedrite (brookite) (TiO₂), and dickite (Al₂Si₂O₅(OH)₄).
The seams are often oolitic, especially in the north of the district, but vary in constitution across the ore-field.
The ooids (on average) comprise ~33% siderite, ~33% berthierine, and a similar proportion of ancillary minerals and demonstrate deformation whilst the sediment was still plastic.

The origins of the ironstone seams have been the subject of much argument and counter-argument since their large scale exploitation commenced in the mid-1800s.
The most widely accepted explanation emerged in the 1920s following analysis by Hallimond (1925).
This suggests that the concentration of dissolved iron within the surrounding sea-water remained approximately the same during deposition of both shale and ironstone.
The physical differences between the two arises as the result of variations in sediment influx.
Rapid rates of sediment input caused deposition of the shales, whereas a diminished rate of input enabled the same amount of iron to be concentrated within a lower sediment load thereby producing the ironstone seams.
Evidence of bivalves in life position, Rhizocorallium trace fossils, cross-bedding, episodes of non-deposition and (not least) the oolitic texture all indicate that the ironstones were laid down in shallow water.

Fossils are ubiquitous, especially within the ironstone seams, to the extent that the Avicula and Pecten seams are named after their most abundant faunal inclusions.

NOTE: The only surviving extensive outcrop of the Main Seam occurs in woodland north of Skelton  (close to NZ 6554 1997) at around 50m O.D above Skelton Beck achieving a thickness of around 2.4m (8 feet).
It is underlain by the Black Hard, and three beds making up the Pecten Seam.
On average the Cleveland Ironstone Formation comprises around 70% shale and 30% ironstone though the latter occurs in the form of the six named seams of variable thickness. The ironstone is classed as being low-grade with an iron-content of up to 33%, and is deemed economically viable only above ~27%.
The primary iron-bearing minerals are the iron carbonate siderite (FeCO₃) and berthierine (formerly known as chamosite,(Fe²⁺
₂Al(Si,Al)O
₅(OH)
₄). Ancillary components include calcite (as MgCO₃ and MnCO₃), pyrite (FeS₂), collophanite (Ca₃P₂O₈ H₂O ), silica (Si), clay minerals and derivatives such as octahedrite (brookite) (TiO₂), and dickite (Al₂Si₂O₅(OH)₄). The seams are often oolitic, especially in the north of the district, but vary in constitution across the ore-field. The ooids (on average) comprise ~33% siderite, ~33% berthierine, and a similar proportion of ancillary minerals and demonstrate deformation whilst the sediment was still plastic.
The origins of the ironstone seams have been the subject of much argument and counter-argument since their large scale exploitation commenced in the mid-1800s. The most widely accepted explanation emerged in the 1920s following analysis by Hallimond (1925). This suggests that the concentration of dissolved iron within the surrounding sea-water remained approximately the same during deposition of both shale and ironstone. The physical differences between the two arises as the result of variations in sediment influx. Rapid rates of sediment input caused deposition of the shales, whereas a diminished rate of input enabled the same amount of iron to be concentrated within a lower sediment load thereby producing the ironstone seams. Evidence of bivalves in life position, Rhizocorallium trace fossils, cross-bedding, episodes of non-deposition and (not least) the oolitic texture all indicate that the ironstones were laid down in shallow water.
Fossils are ubiquitous, especially within the ironstone seams, to the extent that the Avicula and Pecten seams are named after their most abundant faunal inclusions.
NOTE: The only surviving extensive outcrop of the Main Seam occurs in woodland north of Skelton  (close to NZ 6554 1997) at around 50m O.D above Skelton Beck achieving a thickness of around 2.4m (8 feet). It is underlain by the Black Hard, and three beds making up the Pecten Seam.
Cleveland Ironstone Formation