Summary of site:
The reconstruction of landscapes at the close of the final glaciation of the last ice age is always difficult and the complex of deposits south and west of Derrylin prove to be no exception.
The main deposit takes the form of a bench on the eastern flank of Slieve Rushen, extending from Gortmullan in the south almost to Callowhill in the north. It is a little over 4km long and varies in width from 0.5km to 1.1km. Its eastern edge is steep and strongly defines the feature. Three divisions are seen within the sediments: a lower series with vertical beds of sand, pebbles and cobble gravel; a middle series made up of an interfering series and stacked channels also filled with sand, pebbles and cobble gravel; and a final 1.5m bed of massive cobbles, sitting on top of the planed surface of the middle series. In general the deposits are coarsest against the steep eastern slope and the entire bench is crossed by shear fractures with associated asymmetric folds with steep northern limbs.
In the south of the area around Gortmullan there are three strongly defined, almost straight ridges, the biggest being 30m high by 70m wide, the smallest 4m high and 15m wide. They are oriented north west to south east and the biggest extends over 1km south of the border into the Republic of Ireland.
A third feature of the area is a series of mounds, ridges and kettleholes concentrated between Derrylea and Callowhill, extending north east and terminating immediately east of Derrylin.
Other features in the area, particularly the extensive Upper Lough Erne drumlin field to the east, are essential to an understanding of this landscape. The main body of sediment, banked against Slieve Rushen, flanks the drumlin field and does not encroach over it, suggesting that the origins of the two are related.
The steep eastern edge of the bench was almost certainly in contact with the ice to the east and the high volume and heavy flow of meltwater draining from the ice margin carried the glacial debris into a fluctuating glacial lake dammed against Slieve Rushen. The deposits initially appear to have formed rapidly growing deltas that merged to build a continuous mass braced against the ice margin. In this setting the coarsest material settled first with finer grades being swept further west. This surface was then so heavily channelised that the original structure is now hard to identify. The ice margin must have fluctuated, with climatic variations affecting flow rates, distorting the sediments in the immediate contact zone and exerting considerable compressive force along the entire margin of the bench. This would have caused the shearing and folding of the entire bench and the geometry of these structures indicates a flow of ice from the south and south west, confirmed by the lineation of the drumlin field.
The prominent linear ridges at Gortmullan were probably channels in the ice, feeding meltwater with its rock load into the marginal lake. Such channels tend to choke on their own load as new, freer flowing drainage courses open up. When the ice finally melts, the channel fills sink on to the land surface as linear mounds (eskers).
The mounds, ridges and kettleholes from Derrylea to Derrylin are typical of ice margin deposits and are probably all that remains of moraines, eskers and embedded ice masses (that melted to form the kettleholes).
The drumlin field was probably already in existence below the ice before the Derrylin glacial lake sediments were deposited and the northerly flow evidence fits well with the general picture of regional glaciation with ice sheets flowing east from the main Lough Erne Basin joined by this flow from the south; the combined sheets then pressing on east into the Lough Neagh area. This flow regime offers an interpretation of Knockninny Hill, north of Derrylin, as a gigantic roche moutonne (an upstanding mass or rock moulded by the passage of ice).
The interpretation of these deposits reveals a late glacial landscape dominated by a massive northerly flowing ice sheet, melting rapidly and trapping its meltwater against the eastern slopes of Slieve Rushen. This lake quickly filled with washed glacial debris. Improving climatic conditions reduced the rate of ice flow and gradually the ice sheet lost its integrity leaving stranded ice fields. The marginal lake drained during this process, leaving its deposits in the eastern lea of Slieve Rushen and, as the lowland ice decayed, the massive drumlin field was exposed, laying bare the foundations of the modern landscape.
There is considerable scientific interest in this area and effort should be made to ensure that development does not damage its salient features.