Previous SiteNext Site

Killard PointDown
No picture
Summary Full report
Site Type: Coastal section
Site Status: NNR
District:
Grid Reference: J36273486, J36133434, J35583373
Rocks
Rock Age: Quaternary (Late Glacial)
Interest
Other interest: glaciomarine apron, Glaciomarine deposit, drumlin, subglacial sediment, marginal marine

Summary of site:

This designated site records the events and processes that operated in this area from 15,000 to 13,000 years ago during the closing stages of the final glaciation to affect Ireland, the Midlandian. The site is of such importance that this short period has won a place in the scientific literature as the Killard Point Event.

From Ardtole, about a kilometre north of Ardglass, a moraine extends along the coastal strip to Killard Point about 7 km to the north east. Sections into the deposit at the back of Benderg and Ballyhornan Bays extend over 2 km at the north eastern end of the moraine and reveal a complex and variable sequence of sediments quite different from their inland equivalents. There are three different sedimentary associations of in both bays. The first, a diamict group, everywhere lies on a rock pavement planed and etched by the passage of ice across the local Silurian rock basement. Diamicts are a mixture of all particle sizes of rock debris from the finest clay particles to boulders. Here the succession starts with massive beds that grade into more stratified forms that fill hollows and are banked against the first. Thin beds of red mud full of marine micro-organisms occur within the diamicts as well as draping over them. Slumping and the effects of current winnowing can be seen within this group. The second association is of alternating sands and muds, at first filling channels in the top of the diamicts, then spreading over them in finely laminated sequences, finally grading into massive beds of medium-grained sand. Isolated cobbles and diamict pellets are a feature of these beds. The final association is dominated by gravels and cobble gravels forming a sharp division with the sands below. The gravels are fills for 14 channels seen in the observed sections, ranging from 10 to 80 m wide and 5 to 11 m deep. At the east end there is one large channel 250 m wide and 15 m deep. The gravels and cobbles are stacked in them and are graded and sorted in a variety of ways. Red muds occur at very regular intervals in the gravels and sometimes as massive beds. Occasionally they drape channel margins. The diamict association on the rock bed is not a basal glacial deposit. Its bedding, channelling, winnowing and slumping all indicate mass, plastic flow. This is a mass movement of turbulent and intermixing of sediment in standing water, under the influence of gravity. The red muds represent periods of relative stillness when suspended fine material had the opportunity to settle. The sands above the diamicts, forming an ˘apron÷ on top of them, appear to have been free-flowing components of sediments with different particle sizes settling in sequence; the finest forming the most distant thin beds (the laminations). The isolated cobbles were probably dropstones, pebbles released from glaciers and growlers floating in the waters above. The gravel-filled channels show a general flow patter to the south and south east with gravels pulsing through, constantly re-excavating the flanks to form new channels. Periods of quiet between pulses allowed the red muds to settle in the channels and periodically to drape the entire system. The gravels show all signs of turbidity flow, where the coarse sediment acts as a unified mass, plunging down-slope at speed. The general picture that emerges from the Killard section is of a rapidly flowing ice sheet entering a tidal sea. Inland the ice was shaping the debris at its base into the extensive drumlin swarms as it melted and swept, unconfined, into the sea to the south east. Massive quantities of meltwater beneath the ice flowed along the bed transporting all kinds of glacial debris that was discharged at varying rates below the glacier then calving icebergs into relatively deep, open water. The moraine is spaced about 1 km beyond the limits of the drumlin field as would be expected as the ice sheet was buoyed from bedrock by the elevated coastal waters. Sea level was then much higher due to the lag effect following the loading and depression of the land by massive thicknesses of earlier ice. This is the only site in the Irish Sea with a spread of sub-glacial sediments that discharged directly and extensively into the marine environment. The evidence it provides has been used to argue that the rapid ice flow in the area was due to its unconfined access to the sea which has an accelerating effect on rates of flow, called draw-down, enhanced by the elevated sea level in the area at the time and not due to general climatic conditions. It may also have been responsible, at least in part, for the final break-up of the glacier that closed the northern part of the Irish Sea. This spectacular example of deglacial sedimentation into a tidal marine environment already holds Area of Special Scientific Interest (ASSI) status with the protection that status affords. However, it needs to be managed to maintain the current coastal dynamics and all coastal works should be discouraged. Dumping over the cliff at Ballyhornan is a recurrent problem still seeking a solution.


Previous SiteNext Site

|Home|