Summary of site:
An enormously thick regional ice sheet with an axis running east/west across the Lough Neagh Basin is a central concept of all interpretations of events in the last major phase of the ice age (the Midlandian) in Northern Ireland. Evidence of its existence, size and the pattern of its wastage are therefore essential to our understanding of late Quaternary events on the eastern side of the province. The deposits near Moneymore and Annahavil with smaller sites in the south provide important evidence along a western fringe extending north to south over a distance of 30 km.
The Moneymore deposits, taken together, provide clear evidence of a continuous complex of deltaic sediments accumulating in an ice marginal lake extending from Lecumpher in the north to Moneymore and Turnaface in the south. Over an area of 4 square kilometres, roughly along the valley of the Ballymully River, there are four important exposures. At Carmean there are two sediment units; a lower, 5 to 8 m thick, of steeply inclined, cross-bedded gravel with parallel laminations and ripple marked sequences and an upper, clearly separated, between 1 and 2.5 m thick, of horizontally bedded pebbly gravel with large sand lenses. The top surface of these horizontal beds is at the 60 m contour.
At Boheroy Cottage, in the side of a steep mound, gravels were recorded becoming progressively finer towards the top surface which is at 57 m above modern sea level. These deposits also become finer to the north east. The gravels are faulted on the north side close to a north west/south east trending meltwater channel that cuts through them. Pebbles in the gravel are predominantly of rocks from the Tyrone Igneous Complex, 25 km to the west.
West of Moneymore is a series of small mounds at the eastern end of a spread of meltwater channels. Again there are two divisions, a lower, up to 3 m thick of horizontal, bedded gravel, separated by a sharply defined bedding plane from the upper 1.5 m of gravels and sands in small, cross-bedded units. Flow direction in both was roughly to the east.
Further west at Turnaface 9 m of sediments are exposed. The lowest unit is a sandy diamict with some larger rock fragments, topped by 6 m of cross-bedded sands and gravels. The top of the deposit is on the 60 m contour and the flow direction during formation was to the north. A nearby abandoned sand pit shows 10 m of deposits in two units. The lower shows alternations of cross-bedded sands, some with ripples, and gravels, inclined at over 15 degrees east of north. The second unit is 70 cm thick, made up of thin beds of cross-bedded sand and silt deposited from a current flowing east of north. Larger particles are mainly of sandstone with some igneous rocks matching a source in the Tyrone Igneous Complex.
6 km south of Cookstown, at Derryragan, is the northern tip of a delta that extends south and west towards Carland, a distance of around 4 km. The surface is gently rolling and there are several east/west channels cutting through it. Its eastern edge has a north/south trend and forms a steep face with steep-sided, linear, beaded ridges to the east. There are three good exposures, at Annahavil Hill, Derryragan and Bloomhill.
At Annahavil Hill, towards the western margin of the delta, there are two large units of sediment. The lower consists of thick, cross-bedded sands and pebbly sands. Further north these grade into very thick sand units with occasional larger rocks and further north still clay laminations start to appear in channels in the sands with ripple-marked cross-bedding. The flow direction indicated by the cross-bedding was a little east of north. The second unit, up to 3 m thick, consists of laminated sand, silt and clay with broken fragments of lignite but at its northern extremity are several silty-sand filled, bowl shaped hollows up to 3 m deep. At Derryragan at the junction between the Annahavil deposits and a flat area to the north, there is an exposure of around 10 m of flat-bedded, rippled sands with small-scale cross-bedding and flat-bedded gravels. Structures in the sands indicate a current flow during deposition to the north of east. At Bloomhill there is an abandoned sand pit in the side of a mound revealing 10 m of cross-bedded and ripple-marked sands with some silt. This deposit is interesting because it contains lumps of lignite up to 4 cm long. Again current flow was east of north.
There are also a few minor sites providing evidence, such as that east of Ardgivna, where silts and clays are draped over diamicts. The top of this section is 71 m above modern sea level and current directions here were to the west. Another is at Tullyaran on a mound reaching 100 m above sea level where coarse-grained gravels are covered by laminated silt drapes. Finally, there are poor exposures north of Coalisland where terraces at 61 and 30 m above sea level are composed of fine sand, some cross-bedded.
All this evidence supports the melting of a large, static ice mass roughly centred on modern Lough Neagh providing meltwater and sediment for a linear lake along its western margin. At an early stage the lake level was 70 m above present day sea level (from deposits at Cranny) while further south there is evidence for a 60 m level associated with ice blocks embedded in thrust faulted sediments which have a southerly origin. The Turnaface, Moneymore and Boheroy Cottage evidence shows that sediments and meltwater were also derived from the west (supported by current directions and pebbles of Tyrone Igneous Complex rocks) as ice melted westwards into the Tyrone uplands. Closer analysis suggests that several small deltas coalesced in the area to create the feature with its top surface at or around the 60 m contour. As time passed and impounding ice melted and sediment barriers were breached, the lake drained, leaving a local plain on what had been the delta top and lake bed. Meltwater draining from the west then carved courses across the unconsolidated sediments en route to Lough Neagh.
The Annahavil delta formed differently showing early sediments of a deltaic nature that were later overwhelmed by finer-grained lake sands and silts. The evidence supports the concept of a meltwater lake impounded to the 110 m contour level in front of a wasting ice sheet. There were steady water flows from a number of sources below the ice, which was to the east and the sediments they carried were later re-activated as they slid down the delta front. The ice margin was calving icebergs into this lake as it retreated. Esker ridges (gravel to boulder filled channels within the ice) crossing the modern landscape mark the courses that were the sources of water and sediment and give some idea of the drainage patterns. The steep, east margin of the deposit at Drumard was created by sediment banking against the ice margin which retained its steep profile after the ice had melted away into the lowlands.
The minor sites suggest that there were many small lakes trapped between the extensive spread of drumlins smothering the southern lowlands and which were generally barriers obstructing a regional drainage pattern.
These deltas and associated deposits are not such prominent features in the modern landscape, as so many of the upland deltas are, but they provide vital evidence of events between roughly 14,000 and 10.000 years ago when one of the most important glacial influences in the shaping of eastern Ulster was in its final throes of decline. They have further importance as a reference for other glacial lake deposits further west. They also show structures created by a late re-advance of ice in the form of faulting and sediment buckling. It is also an area where glacial erratics are readily traced to source, betraying ice and meltwater flows; to the Tyrone Igneous Complex in the Sperrins to the west and the Oligocene lignites around Lough Neagh to the east.