The Cumber delta consists of two discrete delta fragments, having been dissected along the course of the present day Glenrandel river. It is bordered on its northern and northeastern margins by the River Faughan. The main delta surface at Binn [C557 056] consists of a plateau of low relief (<5m) lying at 126m O.D. An area of low amplitude, hummocky terrain occurs at Tullintrain [C558 053] where small-scale hollows in the surface are occupied by raised bogs and lakes. A large-scale depression in the gravels [C550 053] contains post-glacial alluvium. The hummocky terrain is truncated abruptly on its southern margins by a relatively steep, 6m high slope running parallel to the Longland Road at this location. Further south and southeast, approximately 2km2 of gravels are banked against the rising slopes of Slieveboy Hill [C560 033] and extend southeastwards towards Clagan [C582 048]. The exposure at Robinson's quarry [C538 061] shows a sequence consisting of conformably bedded sands overlain unconformably by pebble and cobble gravels.
The sediments consist mainly of large-scale foreset sequences which are associated with deposition in ponded water up to 126m O.D. The well-preserved flat top of the plateau represents a topset gravel sequence which prograded over foresets as water levels shallowed. Palaeocurrent directions of the foreset beds to the north and northwest indicate that ice was retreating generally to the south and east along the western part of the Foreglen depression into the Sperrins. The general absence of morainic ridges indicates that the deltas were probably fed from subglacial sources emanating from ice-margins grounded in water. The absence of glaciotectonic activity suggests that the ice margins were halting for short periods with no major ice-marginal re-advances. The foresets exposed at Robinson's quarry form a typical glaciofluvial delta deposited near a subglacial efflux.
