Summary of site:
Although many of the key sites in the Belfast area of the Lagan Valley are now extensively built over, there are copious historic records, including large numbers of borehole logs (drilled and measured sections), that combined with a few surviving glacial deposits to give a picture of events in the late glacial period, the time of glacial melting and the final retreat of ice that followed.
The main features, east to west, are the Malone delta, the Drumbeg ridge, the Sandymount ridge, the Hillhall ridge, the Lisburn esker, the Causeway End esker, the Maze plain and the Aghalee delta. There is also raised beach notch on the western margin of the valley. Glacial striations in bedrock at Carryduff and Upper Ballinderry also provide vital evidence.
The Malone delta is an extensive spread of sands with some muds on the south eastern side of the valley between Ballydrain and Purdysburn extending eastwards to the fringes of the city centre, for example, at Bradbury Place.
The Drumbeg ridge extends across the valley for 1.2 km. It has a smooth crest reaching the 30 m contour and the southern slope is steep, suggesting that the sandy clay and clay laminations from which it is formed were originally banked against an ice margin.
The Sandymount ridge, a little over 2 km to the south west, is curved and, in plan, is concave to the north. Its maximum height is about 35 m above sea level and, like the Drumbeg ridge, has steeper southern slopes. Its eastern end curves towards the south western end of the Drumbeg ridge.
The Hillhall ridge is really a complex of sub-parallel segments and mounds. At their fullest height they reach the 40 m contour. A small lake immediately to the north is a kettle hole.
The Lisburn esker is now largely lost beneath housing developments but originally was 2.5 km long extending from Dunmurry in the north east to the Hilden area on the outskirts of Lisburn. This linear ridge probably formed in a meltwater conduit below the local ice sheet.
Another esker, at Causeway End, between Brookmount and the western edge of Lisburn is at the 60 m contour and has been extensively worked for sand and gravel. Its larger particles are a mixture of local and more distant rocks. Triassic sandstone (very local) predominates but chalk, basalt and dolerite (all local) form a good proportion and rocks from Tardree and the Tyrone Igneous Complex have also been identified, indicating the former reach of the ice sheets.
An extensive area of flat ground, the Maze plain, forms the axis of the Lagan valley from Blaris in the east to Soldierstown in the west, a distance of 10 km. It is clearly viewed to the north of the M1 motorway that skirts its southern margin. It is an extensive spread of sands and muds with occasional pebble beds and is more than 14 m thick. To both the north and south of the plain there are drumlins and other streamlined landforms, all with an east west orientation.
At the north west corner of the plain at Soldierstown there is the north west trending channel of Friars Glen, partly occupied by the Broadwater. At its north west end where it opens onto the lowlands of the Lough Neagh shore there is a delta deposit of sands and gravels with its top surface at 30 m above modern sea level. The gradient is towards the lough, descending from 39 m at Soldierstown to 20 m at Aghalee.
Two widely separated sites with clear glacial striations provide important evidence of the direction of flow of the major ice sheets during the last glaciation. At Upper Ballinderry, 4 km north of Soldierstown, there is a clear set indicating a west to east flow, while at Carryduff, 9km due south of Belfast city centre, striations in an abandoned quarry gave a direction slightly east of south.
The final features of note are a raised beach at the 50 m contour extending from Newtownabbey to White Mountain and a wave-cut notch 6 m above modern sea level.
From these deposits and landscapes an outline history of events from late glacial times to the present can be reconstructed.
The last major glacial period in Ireland is called the Midlandian and in the Belfast area in its late stages there were two major ice sheets. The first was the Scottish Ice, running down the north channel of the Irish Sea roughly along the present east coast of County Antrim probably with a short tongue intruding into, and blocking, the Lagan Valley somewhere in the Belfast Lough area. The second, the dominant force, was a huge ice cap with its axis trending east west across the Lough Neagh basin, radiating ice in all directions. The Upper Ballinderry glacial striations indicate an easterly flow into the lower Lagan valley while those at Carryduff record a southerly flow. This swing, through almost 90 degrees, was probably the result of Scottish ice that blocked the easterly flow and diverted it southwards over most of County Down.
With improving climatic conditions, the Scottish Ice sheet stagnated and withdrew progressively to the north while the Lough Neagh Ice retreated north and west into the Lough Neagh Basin leaving smaller ice centres stranded on high ground. During this melting process a remnant plug of Scottish Ice continued to block the lower Lagan valley in the Belfast Lough area while Irish ice still occupied the uplands to the south. As this upland ice decayed and meltwater poured into the Lagan valley from the south, a lake, Glacial Lake Lagan, flooded the lowland. The meltwater brought with it huge volumes of red sand derived from the local Triassic sandstone bedrock which accumulated at the mouth of the main feeder flows with finer material, suspended in the lake water for longer, carried further out into open water. In this way the Malone delta was formed against the southern shore, its top surface around 40 m above modern sea level at its maximum. The Causeway End and Lisburn eskers mark two of the meltwater channels, feeding sediment through the ice into the lake. Large blocks of calved ice were trapped and inundated by the rapidly deposited marginal sands of the delta and, when they eventually melted, formed deep subsidence cavities, now water-filled kettle holes. As melting continued further, the ice margin retreated westward, stabilising from time to time and it was in these periods of standstill when the flow rate was cancelled by matching rates of melting, that the Drumbeg, Sandymount and Hillhall ridges were formed as released sediment banked against the ice face (evidenced by the steep southerly faces).
With Belfast Lough blocked by ice to the east and the remains of the Lough Neagh ice axis still occupying the area to the west, the only route initially open for the overflow from Glacial Lake Lagan was to the south through Poyntzpass to the sea in the Carlingford Lough area. As the Lough Neagh ice retreated and dwindled westward it freed Friars Glen between Soldierstown and Aghalee and the lake occupying the Maze plain drained through it to enter an elevated Lough Neagh (15 m above its present level). At the channel mouth, where the current dissipated and lost its power to transport its load of sediment, the small delta at Aghalee was formed.
At some time following this the Scottish Ice relinquished its grip on Belfast Lough and Glacial Lough Lagan drained eastwards restoring access to the Irish Sea but this was not a simple process. The complication was almost certainly due to the depression of the Lough Neagh axis under its previous massive ice load. As the ice sheet melted and the land stuttered upwards towards a new, ice-free equilibrium (a process called isostatic adjustment), local drainage in what is now the western end of the lower Lagan valley would have progressively explored more easterly channels until all except the Ballinderry River and Aghalee Burn were captured within the gradient feeding the Lagan. The 6 m notch in the Lagan valley is probably the result of the final stages of the process, cut by a relatively high sea level before isostatic stability was finally achieved.