Three Lakes - Palaeoecology of a Lowland Lake and bog

The Interactive map

This interactive map is built in layers which can individually be switched on and off. The map can be zoomed in and out by use of the + and – buttons in the top left, or by the use of a mouse wheel.

The measure icon (under the '+' and '-') will activate when clicked. Left click to start a line of measuring, double click to stop that line of measuring; and click again on the icon to turn off measuring. Lengths are displayed in metric units.

The legend for all layers is found by moving the mouse pointer over the icon in the top right. The map is displayed within its own frame and can be moved by dragging with the mouse left button.

Clicking anywhere on the map will raise a popup box that describes the layers active at that point.

When the map first opens the hillshade layer, and some other selected layers, are switched on. Other available layers include -

  • Townlands – a lot of townland borders follow streams and rivers, so if 'rivers' are switched on some townland boundaries (black lines) will be covered by blue lines.
  • 10 m contours - these accentuate the relief and are best. These have been generated and so their accuracy is dependent upon the data they are based on - in this case the 30 m grid of the ALOS satellite. 2 m contours have been included on this map. Some editing of the generated contours may have been necessary.
  • Ringforts are displayed as red dots - they may be Raths, Cashels, or unclassified.
  • Deglaciation landforms, Glacial features, Quaternary Sediments and Bedrock Geology are all available and complement each other but can also mask each other.
  • Roads are available, principally as a means for identifying the locations on the map.
  • Sampling points are indicated by either white dots for coring locations, or green dots for moss polster sampling locations. Currently there are four coring points indicated, and three moss sampling points.

A high resolution digital surface model (DSM) will be obtained from BlueSky and displayed on this map. This company has flown aerial surveys over the whole of Ireland. The accuracy of the elevation data is the best that is currently available.

The difference in accuracy between the satellite data and the aerial survey data will mean that there is a discrepancy between the 50 m and 10 m contours of the whole map and the 2m and 1m contours of the DSM.

The lower accuracy of the satellite data does mean some generated contours are in error - the topography and contours in the Clashnacrona gorge to the east of the lake depression will be improved by the use of a DSM. It is important to bear in mind the satellite images were taken from 700 km altitude.

The Area of the Map

This map shows an area of about 6.5 km by 4.7 km of the landscape surrounding the area of Three Lakes. These lakes and surrounding bog are probably of early post glacial origin, but this has yet to be confirmed. This will be done by ascertaining the full depth of the sediment and obtaining radiocarbon dates for some samples of organic matter, most crucially from the bottom of the deposit. A core has been taken from the bog, a second core is due to be taken in the Spring of 2021 from the centre of the middle lake. These cores will be sampled, processed and analysed. In addition several samples of surface moss have been taken. All sampling locations are shown on the map.

Of particular interest is the outlet from the bog / lake depression that leaves from the north east and continues flowing north east to Dunmanway (off the map). The overall narrowing of the lake basin towards the north east where outflow enters the Clashnacrona gorge is quite clear from the map if 'DEM' is switched on in the legend. Clashnacrona gorge is designated by GSI as a glacial meltwater channel, suggesting it was cut by high volumes of water at ice melt times in the early post glacial period. Prior to this the lake basin was likely caused by ice gouging as the ice moved north east along the valley. As the narrowing of the gorge was encountered the ice probably lifted somewhat and the gouging effect lessened, resulting in a rock lip at the gorge. This can be seen today if the old railway track is followed - at a level more or less equal with the lake water level, bedrock can be seen. The railway engineers clearly had to remove rock for the railway bed, and also cleared out the stream channel to ensure it stayed below the track.

At the opposite end of the basin, to the south west beyond the south western end of the western lake, the basin reaches the watershed. This is where the basin draining north east meets the basin and valley draining south west. This is just 250 metres SSW of the lake basin and about ten to fifteen metres higher. From this watershed in Kilnahera East townland a bog basin lies to the west, which then drains south west as the Rua river, eventually meeting the Ruagagh river just south west of Drimoleague. A second watershed into a parallel drainage basin lies to the north, again about 200 metres from the Three Lakes basin, between the western lake and the small Loughaneleigh that lies alongside the road. The Ruagagh river flows south west from this lough down to Drimoleague. The road follows this valley, as did the railway that is now removed.

The area of the Three Lakes lake basin is defined by GSI as a glacially derived 'mountain streamlined ridge' (blue), and this relates to the 'subglacial lineations' seen as red dashed lines on the map, that is bedrock on the upper hillslopes scarred by ice sheet passage. Overlying this are the quaternary sediment deposits of peat (brown) infilling the main part of the basin. Peat also occupies the Kilnahera bog basin to the south west of the watershed and further down the Rua valley. The Ruagagh valley is shown as having significant deposits of 'gravels derived from Devonian sandstones' (orange) deposited along the northern slopes of the valley.

These same areas of gravels are also designated as deglaciation landforms, 'hummocky sand and gravel' (pink). These are deposits of glacial or fluvoglacial coarse grained sediment deposited as a result of the melting of the ice and the consequent release and then washing out and deposition of sands and gravels that the ice had scraped up and carried in its journey across the landscape. These can now be seen as hummocks in the valleys and across the lower hills, and on further inspection often cannot be linked to any clear process of formation by river erosion. This is one of the defining features of this post glacial landscape - seemingly randomly placed mounds of sediment, that are now grass covered, across valley bottoms and onto the lower slopes of hillsides.

The lakes lying in one basin that is now largely peat - blanket bog - suggests to us that the ice gouged depression, originally possibly containing some coarse sediment mounds left as the ice melted, became an area of restricted drainage. So we can expect non organic coarse sediment, possibly a glacial till, at the base of this basin, overlain by a layer of non-organic silt washed in and deposited out of the retained water, settling at the bottom of the murky lake, in a landscape of gravel mounds, boulders and alluvial channels. As the climate improved and plants and other organisms returned to the area, diatoms, sponges and algae occupied the lake waters and plants became established on the shores and in the shallow margins of the lake. Plant life developed further and became more prolific as the climate improved. As the plants at the edges of the lake died and fell down into the water, they formed a layer of ever increasing and widening organic deposit. In the water oxygen was restricted and deposition slowed, and so peat formed. Within the waters of the lake organic sediment built up to form a lakebed deposit or mud. The margins of the lake became peaty marsh and then boggy land and the lake (or lakes) reduced in size.

By taking a core down through these sediments, the bog peat overlying the lake mud, and down into the silt underneath, we can examine the succession of sediments, and the microfossils that are preserved in those sediments. Organic remains can be radiocarbon dated and we can then recreate the environments with some degree of certainty.

It is also noteworthy that the moss polster sampling points are west of Clashnacrona - two north of middle lake and one north west of the western lake. Current woodland growth through the Clashnacrona Gorge includes Beech (Fagus), Hazel (Corylus) and Oak (Quercus). With prevailing winds at the current time being generally from the west, the existence, or absence, of pollen (particularly Corylus) from these trees in the moss samples can provide useful base information on pollen distribution in this area. For example, if at depth high levels of Corylus pollen are found, and yet Corylus pollen is not found in the present day moss samples, that can suggest that prevailing weather distributes the pollen from trees in the gorge eastwards, and the pollen thus suggests the presence of Corylus to the West of the gorge in the past.

How this map was made

The map has been generated, using a program called QGIS2Web, from an Open Source GIS called QGIS. This software is constantly being improved and, like all open source products, is highly efficient and free. The layers have all originated from data made freely available to the public domain. There are some inconsistencies in the map, arising partly through the different sources the data came from not being in alignment either with accuracy, completeness or definition.

Read more about how these maps are made and where the data comes from.

Click here to centre the map in the screen

Click here to centre the map in the screen