The Boulder Bed succession at Glencolumbkille, County Donegal
Richard J. Howarth, Christopher R. J. Kilburn, BERNARD ELGEY LEAKE · UCL Discovery (University College London) · 1968
A detailed description is given of the sedimentary sequence at Glencolumbkille, County Donegal. This is an important area for the study of the Dalradian Boulder Bed because of the completeness of the succession and the excellency of the exposures. Graphitic schists are followed successively by limestone, striped schist, dolomite, dolomitic boulder bed, semipelitic boulder bed, increasingly psammitic boulder bed with interbedded quartzites, a distinctive thin calcareous dark schist, and finally a thick quartzite, that of Slieve Tooey. The Boulder Bed is described in special detail. Four important periods of movement are recognised in the area, of which the third and fourth are the most important as they produced the Glencolumbkille anticline and syncline. Four lesser phases of late movement followed. The metamorphism reached a peak during the declining stages of the second folding in the almandine amphibolite facies when the assemblage muscovite-biotite-garnet-quartz-oligoclase-ilmenite-graphite was stable in the pelites. The presence of dolomite marble is believed to indicate that the partial pressure of carbon dioxide was high. A late period of re-heating with static growth of biotite and hornblende in the upper greenschist facies was followed by a retrogressive, lower, greenschist facies metamorphism and the production of chlorite. The metadolerites are described.