Lower Creevagh ASSI

This site offers an opportunity to study sedimentological aspects of the Londonderry Formation along with structural relationships on the south-eastern limb of the Lough Foyle Syncline.

The rocks exposed at Lower Creevagh were originally sedimentary rocks, deposited as turbidite sequences in an open marine environment some 550 million years ago.  They were then deformed during the Caledonian Orogeny or mountain building event, of some 470 million years ago. This event folded, faulted and weakly metamorphosed the rocks, producing structures that help interpret the sequence of events. They are structurally related to rocks found in Scotland and north-west Ireland.

The rocks are part of the Londonderry Formation which is found at the core of the Lough Foyle Syncline, a downward fold, centred on Lough Foyle. The rocks themselves are psammites interbedded with phyllites and quartz mica schist. At Creevagh Hill, the rocks dip toward the north and northwest but have retained sedimentary structures e.g. graded bedding that allows the original ‘way up’ of the strata to be deduced. The dominant metamorphic fabric, formed during the deformation of the rocks, is a close spaced penetrative cleavage (S2) sub parallel to the bedding that dips more steeply to the north-west. This cleavage is itself cut by kink folds, small scale folds, formed at a time after the cleavage.