The valley of the Ghann River downstream from the gap between Slieve Roe and Cruggandoo [J184 250] is characterised by Late Midlandian recessional moraines. The most distal moraines at the exit of the Ghann River valley consist of two dissected, low, subparallel arcs immediately to the west of Rostrevor. They trend northwest to southeast, transverse to the valley axis, and occur in a similar topographical position on the coastal fringe to the moraine fragments to the east of Rostrevor. A well-marked ridge at the entrance of the Ghann River valley [J170 198] trends northwest to southeast and continues westward as an arc across the Moygannon River valley. The 2.5km of the lower valley [J172 195 - J181 220] is characterised by a central, undulating longitudinal ridge (<10m high). The ridge is bounded to the west by a broad (<200m) channel which contains the underfit Ghann River, and to the east by a shallower dry channel. These are meltwater channels, which cut at right angles across the closely spaced frontal moraines of a glacier which retreated northward through the valley. High points on the ridge are aligned with rounded ridges and benches which can be traced upslope to the north along the valley sides. In the west of the upper valley northwest to southeast trending cross-valley benches turn sharply northward along the steep lower slopes of Slieve Roe. In the east of the valley, benches are broader along the gentle lower slopes of Slieve Roosley.
