IQUA Field Trip to West Cork - 17th October 2020

Location numbers and feature markers refer to those appearing on the interactive map on the map page (opens in a new tab). Some of the photographs accessible through the map are displayed here within the report.

The Field Trip started at location 1, the level crossing at the western end of Clashnacrona gorge.

The first area to be visited on this field trip was the initial location of research, Three Lakes, a 2.5 km stretch of three lakes and surrounding bog which lie east-west halfway between Dunmanway and Drimoleague (feature 2). This is a hydrosere, an environment where plant growth encroaching on the sides of lakes causes successive creation of floating mat, marsh, and bog as the lakes slowly fill in with organic remains.

From the eastern end of Three Lakes the course of the old railway line can be followed back into Clashnacrona gorge (feature 1). It is clear that even prior to the construction of the railway in the early 1860's the outlet from the lakes draining through the gorge ran through a channel in the rocks. These were blasted and levelled to create a bed for the railway and a deeper channel for the water. The old land surface can be seen 2 to 3 m above, on top of the exposed rock faces – the original course, and height, of the drainage, when this was a meltwater channel, can only be guessed at; but the 1st edition OS map (c. 1840) shows that the course was not so different from the present. The presence of bedrock here in the gorge, at several metres above the lake and bog level, is probably the result of the narrowing of the valley to this point. This possibly lifted the ice as it moved east and the gouging effect that scoured out the basin in which Three Lakes now lies was reduced, leaving the bedrock to form a rock lip that subsequently held back drainage. This is possibly similar to the mode of formation of Lough Hyne where the rock lip is located at the rapids.

The road from the old level crossing - the still extant gates which are strangely of metal box section, not wood as is to be expected - and up to where the eastern end of the Three Lakes valley just by the outlet can be viewed. An extensive low mound was seen to represent a fan of glacial debris, which, having slumped down from the hillside and spread out onto the valley floor, possibly blocked the drainage - and then washed through as some level was overtopped and the water released. The local farmer passed, stopped, and added to the story; and also solved the query of the level-crossing gates, which he had made as replicas of the wooden originals.

There are problems with mapping such areas due to both the plant cover, which confuses the signals from satellite sensors, and the small size of some features, such as this outflow channel. In this case a map of the field trip area was assembled using public domain data from the Geological Survey as well as satellite data for digital surface models. But topography remains a problem, with Ordnance Survey Ireland only defining contours at 10 m intervals, which misses many small features. Due to the varying accuracy levels of the satellite data, Japanese data was used and 1 m, 2 m, and 10 m contours produced. These are shown on the map.

Moving into the area by the middle lake, location 2, the slopes of Killaveenoge could be viewed to the south. There is a good possibility of lateral moraines lying along the hillside, now covered in forestry and scrub. These lateral moraines would have been left by ice that was filling the valley, long mounds of debris left on the valley sides as the ice moved past, and particularly as the ice level dropped during deglaciation. Also visible on the slopes is extensive evidence of sediment movement and slumping between minor drainage channels. Such slumping would be common in the early post glacial landscape when high volumes of water, both rain and meltwater from snow and ice, enhanced the flow of sediment downhill. There was little or no vegetation at that time to hold the sediment in place.

The mound surrounded by bog can be compared to the form of a beaded esker, a snake like deposit of sediment left along the course of a meltwater stream under the ice. However, it is not known whether this is a rock or sediment mound, and this mound does not have the length that an esker would have.

A six metre core has been extracted from the bog near to the old railway line and away from the lake - 5.8m of organic sediment followed by a rapid transition to pale grey silt or gyttja. The bottom part of the core – still in the process of being analysed – is very high in diatom frustules, with many sponge spicules, suggesting the open water of a post glacial lake, probably coinciding with the more rapid rate of deposition of this lower part of the core, until that part of the lake was infilled by vegetation and became bog. This suggests that up until between about 6000 and 4000 BC the whole valley was occupied by one lake. More information will result as analysis of the samples continues.

Radiocarbon ages for the organic parts of the core are 1417 +/- 25 years at 1.0 m depth; 8868 +/- 43 years at 2.9 m; 10026+/-49 at 5.8 m; giving dates of deposition in calibrated years as 597-659 AD, 8223-7830 BC, and 9808-9359 BC respectively. These are whole peat dates, so because of the inclusion of intruding roots and fungal strands (hyphae) these dates probably represent younger ages than peat-fraction dating would give. It is clear that the lakes and bogs have existed in this valley for over 11,000 years, probably considerably more. These three radiocarbon dates were generously funded by IQUA through the Bill Watts Chrono Award 2018.

Gouge coring at the edge of the middle lake, nearer the valley bottom, also showed 5.9m of organic sediment underlain by pale grey silt. The question arose as to whether further organic sediment may lie under a layer of silt; and how much silt is lying in the base of the valley. The current base of the core at 6 m depth dates to approximately the time of the end of the Younger Dryas. This was a period of a return to ice age conditions after intial warming and deglaciation. The Younger Dryas lasted for about a thousand years, so it is possible that the current lake sediment at the base of the 6 m core dates from the Younger Dryas, and that this lies on top of older organic deposits from the first post glacial warm period (the Nahanagan), which in turn would be underlain by late glacial lake sediment.

Clearly further coring and dating is desirable, and further coring with a piston corer to obtain a sample from the centre of the middle lake would be the most likely to give most information. Obtaining a core through the silt and down to bedrock would be ideal. Any lower layers of organic sediment, if present, could of course then be dated.

It was commented that this mound bears a similarity to the Tullahedy mound, near Nenagh in Tipperary, albeit rather smaller. This suggests that like Tullahedy it may have been an atractive and secure site for an early settlement – Tullahedy was settled during the Neolithic period around 3600 to 2900 BC, but with indications of earlier Mesolithic and later Bronze Age use. Though very much smaller, te mound here at Three Lakes clearly represents what could have been a secure and easily defended site, in a valley bottom surrounded by bog. Field walking when the pasture is ploughed and reseeded might turn up some finds. A depression in the side of the mound, possibly caused by some small quarrying of sediment, is also reminiscient of similar depressions elsewhere (including at Tullahedy which was also partly quarried) which have been interpreted as caused by the melting of buried ice blocks. With the sediment in the valley bottom being more than 6 m in depth, the mound clearly is much larger than what is currently visible, extending to greater depths and probably more extensive.

Moving on to the westward end of the top lake (location 3) and the watershed between the three lakes with eastward flowing drainage, feature 3, can be seen and the Kilnahera bog, feature 4, that drains to the river Rua flowing west. The watershed is only about 10 metres above the lake level and 200 metres from the top lake. A good view was obtained from here of the main inflowing stream from the northern hillslopes into the middle lake. Research at the Three Lakes site was instigated whilst studying with the Archaeology Department of UCC, as an investigation into relationships between the pollen record, and activity related to the surrounding ringforts, of which there are several on the valley sides. Although having now extended the research to incorporate the full time-scale covered by the sediment at the site recovered through coring, research and analysis of the anthropogenic aspects at this site is also ongoing.

The next site to be visited was down the Ruagagh valley, past the tiny Loughaneleigh (feature 5) with several glacial sediment mounds to be seen in the valley bottom (feature 6). The river Ruagagh flows westwards alongside the road from Three Lakes to Drimoleague, parallel to and north of the Rua. About 2.5 km from Drimoleague there is a glacial mound that is currently being dug for gravel – location 4 and feature 7.

Crossing the field leading to the extraction pit is some exposed soil and upper level subsoil in the side of the track. Solifluction (the downslope movement of saturated sediments over a frozen subsoil) and slumping (similar to solifluction, but more rapid and chaotic), features from postglacial movement of the sediment under the influence of greater volumes of water than at present, were seen. This included some ice wedge traces which were apparent within the variety of fine sand and sorted gravel layers. Iron staining was very visible. The landowner had mentioned that some drainage channels are very high in iron which has caused problems with free flow.

The main pit has a high back wall of about 8 m which is largely discoloured through weathering and organic growth, and has been well used by sand martins. This was immediately and clearly recognised as a large block of deep lake bed deposits of fine sand. The size of the particles demonstrates that this is from a low energy environment where water movement was minimal and relatively peaceful. Soft sediment deformation was apparent, but whether as a result of transport of the sediment block, or movement due to melting, or shunting by moving ice bodies, is unclear. The question remains as to how lake bed sediment got here. Two possibilities were considered – either this is in situ sediment from an ice dammed lake in the valley bottom; or there is the possibility of the block of sediment, probably frozen, having been carried by ice to this location from elsewhere. In either case further investigation of other mounds in the valley may help to resolve this.

The top of the mound is at about 90 m, and the watershed to the east, just west of Three Lakes, is at about 115 m. If ice occupied and blocked the lower reaches of the valley it is possible that meltwater accumulated here, possibly overtopping the watershed and overflowing eastwards. The ice dammed lake would eventually have been released when the ice melted.

The gravel extraction is from the 'tail' of the mound pointing down valley, and the sediment was interpreted as a fluvial deposit of a high energy swiftly flowing channel, or maybe delta. Layering and sorting was apparent in these gravels and sands. Again, minor glaciotectonic deformation of the soft sediments was visible, with some minor faulting and folding of sediment layers. It is interesting to consider that the lakebed deposit representing a low energy environment, should be found in the valley floor, right next to the gravels and sands of the high energy deposit. It may be that after release of the ice damned lake, meltwater streams, possibly originating on the hills each side of the valley, eroded channels through the lake bed sediments, resulting in layered sands and gravels lying alongside deep lake sediment. This could be clarified if the area where the two sediment types meet was visible, but this is currently covered by mounds of extracted sediment.

It was agreed that this is a site of great interest, potentially holding information as to events during the late glacial and early post glacial periods in this valley. Such localised and intimate events are rarely discovered or researched. Further investigation, if possible, could be of great use.

  • Dating information from any organic deposits in either the lake bed or the gravels would be very valuable. Such organic deposits may take the form of black or dark brown patches or layers, branches or lumps of wood, or lenses of mud like sediment.
  • Samples of the lake bed deposits could be analysed for pollen, spores, diatoms and other microfossils.
  • The relationship between the two sediment types is of interest to determine whether this is an erosion surface.

More mounds further down the Ruagagh valley were seen both from here and from the road skirting the north side of the valley. These have been a focus of a 1 metre by 1 metre photogrammetric survey which is displayed below.

The road along the north side of the valley leads to Barr na Carraige, location 5, where there is a magnificent view of the high ground to the north, which the ice at the height of the glaciation overtopped, and from where the ice probably came.The trip continued on round the head of the Clodagh valley and came down on the east side of the Clodagh, where there are some good views of valley bottom glacial deposits, small mounds which demonstrate by their alignment that despite the general north to south movement of ice, locally it often followed the topography – feature 10 best viewed from location 6.

Further locations and features on the map were not covered by the field trip.

Many thanks to both Robert Beamish, landowner at Three Lakes, and to Wesley and David O'Driscol, landowners at the gravel pit, for so readily allowing access to their land and being both interested and understanding.

Further investigation into glacial sediment mounds in the Ruagagh valley

Following the visit to the gravel pit at Aghnafolla Bridge (location 4 and feature 7) further exploration of mounds in the Ruagagh valley, both to the east and to the west, is being undertaken. The first step in these investigations was the purchase of digital elevation model files from BlueSky. This company have flown over most of Ireland undertaking photogrammetric surveying.

Click here to go to the map of the Ruagagh Valley.