Purdy's Burn provides an almost continuous section over 2.4km through the Ordovician and Silurian rocks of the district. About 410m northwest of Purdysburn House [J343 680] almost vertical and slightly contorted purplish grey gritty shales and greywackes of the Gilnahirk Group (Upper Ordovician) strike NNW, a direction at right angles to the regional strike. About 91m upstream from these shales and greywackes, a reddish brown boulder clay rests on purplish grey greywackes with some hardened shales. These beds dip SSE at 45°. Upstream to Purdysburn Bridge there are several exposures of greywackes with occasional shales. The former are usually purplish in colour but grey in places; generally they are thickly bedded, even massive at one exposure, and vary from fine- to coarse-grained. Purplish grey, fine- to medium-grained greywackes, generally thickly bedded, are intermittently exposed for 365m southeast of Purdysburn Bridge. The direction of dip is a few degrees east of south and the inclination is 60°. Purplish medium-grained greywackes dipping north at 78° are exposed 383m southeast of Purdysburn Bridge.
Continuing upstream (southeast) there is almost continuous exposure for the next 365m. The predominant rock type is purplish greywacke but beds of a grey colour are also present. About 1.8m of bluish shale was noted 18m upstream from a weir [J346 675] and 549m southeast of Purdysburn Bridge, and red-stained hard shales were noted about 91m southeast of the same weir. Beside this weir the rocks are vertical but the dip decreases south-eastwards to a minimum of 25° at an exposure [J347 673] 732m southeast of Purdysburn Bridge. These exposures, along with those upstream from this point, are considered to be part of the Strangford Group (Lower Silurian). Reversals in direction of dip 607m WNW of Charity Bridge [J358 671] suggest the presence of a small syncline and the axis of another syncline crosses Purdy's Burn 500m WNW of the same bridge. In the steeply rising ground to the south of this stretch of Purdy's Burn there are many small scars and knolls of bare rock, mainly purplish grey coarse greywackes. To the west of Knockbracken Reservoir [J364 664] a northwards flowing tributary of Purdy's Burn has cut a deep gully in greywackes with rare bands of hardened shale. The greywackes are purplish grey and grey, fine- to coarse-grained and micaceous in places. A lamprophyre dyke of Caledonian age is exposed in Purdy's Burn a few metres west of Charity Bridge [J358 671] and in a knoll 274m to the south.
