Grinllwm Slates - Capel Curig and Betws-y-Coed. Description of 1:25 000 sheet SH 75
|From: Howells, M. F., Francis, E. H., Leveridge, B. E. and Evans, C. D. R. 1978 Capel Curig and Betws-y-Coed. Description of 1:25 000 sheet SH 75 Classical areas of British geology, Institute of Geological Sciences. (London: Her Majesty's Stationery Office.)
Chapter 5 Grinllwm Slates
The Grinllwm Slates are exposed only in the faulted outlier around Gwydir High Park, in the north-eastern corner of the district. There they conformably overlie the Llanrhychwyn Slates, though farther north, in the Trefriw district, they are separated from that formation by basaltic tuffs and associated mudstones (Davies, 1936). They form a prominent scarp on the west side of the Conway Valley south of Llanrwst, where they are estimated to be 220 m thick.
The beds are cleaved grey silty mudstones with irregular thin bands and lenses of sandstone. The bands, generally less than 4 cm thick, are ripple-marked and cross-laminated, medium- to coarse-grained and commonly show soft iron-stained weathering products. There are also rare ironstone nodules. Trilobite fragments have been obtained at one locality [SH 7963 5992]. Along the Llyn-y-Parc Fault the beds are brecciated and mineralised.