The Cleveland Ironstone Formation of North-East Yorkshire constitutes the topmost division of the Middle Lias. Palaeontologically it lies within the spinatum and upper margaritatus zones, occupying the whole of the Domerian with the exception of the stokesi subzone, which belongs to the underlying Cleveland Sand Formation. (For a discussion of this stratigraphic nomenclature see pages 15-16. Within the area covered by this investigation the Formation includes between 30 and 100 feet of shale and silty shale with siderite mudstone nodules, and subordinate oolitic iron ores. To this area the name Cleveland will be applied through these pages. Regarding the boundaries of this rather poorly defined region there is some disagreement (Rastall. 1949, Hemingway 1966; pp. 8-9) and it is therefore proposed to use the name loosely in order to include the whole of North-East Yorkshire where outcrop evidence of the Ironstone Formation is available The most northerly outcrops occur on the north-westerly facing escarpment of the Cleveland Hills. At the foot of the escarpment Lower Lias Shales are found rising steeply to the Cleveland Sand Formation. The less resistant strata of the Ironstone Formation often weather back to form a small bench, by which they may be located along various parts of the escarpment. The Upper Lias Shales above are usually poorly exposed, but the Jet Rock is a prominent horizon, followed by the tips of the jet miners, which serve as a useful marker. The summit of the escarpment is provided by the sandstones of the Middle Jurassic. Southwards from the hills around Guisborough a gentle south-easterly dip carries the beds down to sea level at Staithes and Kettleness. At Whitby the top of the Middle Lias lies some 200 feet below sea level but is returned to the surface at Hawsker Bottoms by the Robin Hood's Bay Dome. The dissection of the Eskdale and Cleveland domes by the River Esk and its tributaries once more reveals the Ironstone Formation in the area known as Blackmoor. On the north-western escarpment the beds reach a maximum elevation of 1,000 feet under Bottom Head, on the crest of the Cleveland Dome, and then fall south-westwards until they become obscured under the drift in the neighbourhood of Osmotherley. Outcrop information is therefore available in four main areas. 1. On the coast at Staithes, Kettleness and Hawsker. 2. Along the partly drift covered escarpment between Saltburn and Kildale including the outlying Eston, Upleatham and Hob Hills. 3. From the escarpment of the Cleveland Hills between Kildale and Osmotherley and from Bilsdale, Raisdale and Scugdale. 4. In the dales of Blackmoor (Baysdale, Westerdale, Danby Dale, the Fryup Dales, Glaisdale, Eskdale, Iburndale, Rosedale, Farndale and Bransdale), (fig. 1). Additional evidence is drawn from the records of shaft sections and exploratory boreholes within the mining area, and from an examination of the workings at North Skelton Mine prior to its closure in February, 1964. The geological mapping and location of exposures by the Geological Survey (1880-83) is very reliable, so that no mapping was necessary during this work. However all the available sections were re-examined, remeasured and correlated to provide the basis for the stratigraphic section of the thesis and the framework for the mineralogical and petrological description of the different sedimentary facies, the aim of the work being to determine the environmental setting and diagenetic history of the Cleveland Ironstone Formation and to compare and contrast it with other deposits of similar type.