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Cleveland Dyke from Wind Hill quarry (NZ 7597 0643), near Wind Hill Farm, North Yorkshire. The dyke is one of the Tertiary swarm emanating from the isle of Mull, Scotland, no doubt caused by crustal tension during the opening of the Atlantic which eventually separated North America from North Western Europe. A is a photograph of a thin section in plane polarised light. Opaque minerals of iron oxides and a cloudy cryptocrystalline mesostasis (e.g. bottom near centre) are apparent as well as a high relief mineral at the near centre. |
B is the same thin section under cross polarised light. The Cleveland Dyke is classed as a basaltic andesite. It is a porphyritic rock with small (1-2mm) phenocrysts of plagioclase feldspar. The groundmass is seen to consist of laths of plagioclase (grey-black interference colours and the pyroxene, augite (blue, purple & yellow interference colours). The near centre shows what appears to be an olivine. It measures 1mm in length NW-SE. Further analysis, however, e.g. its almost uniaxial interference figure, indicates that it is a type of augite known as pigeonite. The literature states that this dyke is olivine free. |
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The photo on the right shows part of the dyke wall, on the right, in contact with Lower Jurassic sediments (Cleveland Ironstone Formation), on the left. Presumably part of the dyke was left there to act as a retaining wall. |
The photo on the left is a polished section of Redcar Mudstone Formation taken from within a few feet of the dyke contact. Normally it would be a dark gray shale or mudstone. However it has been thermally altered resulting in the "spotting" as well as induration. There does not appear to have been any detailed published analysis of the spots, but it is thought that they could be the result of coagulation of the hydrocarbon content within the mudstone.
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