The stream section and adjacent road cutting at Ballymiscaw exposes a section through the greywacke turbidites of the Gilnahirk Group. The Gilnahirk Group comprises the sequence of greywacke turbidites that lies northwest of the Castlereagh Fault. These rocks are similar in lithology and degree of deformation to the sequence assigned to the Ordovician northwest of the Orlock Bridge Fault further along strike to the northeast. Compared to rocks of the Strangford Group (Silurian), most of the turbidite units are thinner (0.15-0.5m thick), grading is less common and bottom structures less prominent. In places (e.g. J402 756), there are intercalations of coarse-grained sandstone beds up to 0.4m thick containing pebbles of vein quartz, pink granite and dark grey mudstone. These coarse-grained to pebbly beds are tentatively correlated with the pebbly Luke's Point Member in the Ballymacormick Block on the north County Down coast. Argillaceous interbeds include blue-black slaty mudstones, grey siltstones and pale yellowish micaceous bands which probably represent volcanic ash-falls.
The arenaceous turbidites commonly possess a fabric of domainal character which consists of platy micaceous minerals roughly aligned and wrapping lenses of only weakly aligned quartz and other rock fragments. Overall there appears to be a higher state of deformation in the Gilnahirk Group compared to the younger Silurian rocks in County Down and more of the matrix is very finely crystalline, perhaps as a result of the increased deformation and possibly increased metamorphic grade.
