The Greenhills Road runs between Walkinstown and Tallaght. This road is more associated with traffic and industry than being a natural phenomena and a geological feature of interest. It is in fact a striking example of an esker and as part of an ancient highway named The Eiscir Riada. Greenhills itself is named after the sand hills that formed the esker; these have since been mainly quarried away.

An esker is a long, snakelike ridge of sediment deposited by a stream that ran under, over, or within a glacier. 

Eskers were formed in the last Ice Age, around 10,000 years ago around the time of the Glacial maximum when the glacier was slow and sluggish. The Greenhills Esker is formed within an area of Lower Carboniferous limestone, but the esker itself is Quaternary in age.

Eskers generally contain a wide variety of materials, ranging from fine lacustrine silts and clays, sorted silts, sands, gravels and large boulders. Their sediment can be well stratified or layered and their particles can have travelled up to 15km. The sands and gravels within the feature are comprised chiefly of limestone clasts.

When glaciers melt, they produces meltwater that can flow on top, within, and underneath the glacier through channels. This meltwater moves large quantities of sediment from the glacier. The sediments deposited by glacial meltwater are called outwash. The running water that transports them sorts and layers them. Variations in meltwater influence the discharge of sediment Eskers can be up to 150 m high and as wide as 500 m wide and can extend from a few hundred meters to over 15 kilometers in length.

The esker feature is important in that it records faithfully the ice movement across this area of Dublin which is along its orientation. An ice marginal fan is a hill left high and dry when the ice retreats away from a locality. Associated sands and gravels along the Greenhills Road, northeast of the esker, are probably part of an associated ice marginal fan .

Warren and Ashley (1995) proposed that there are 4 different types of eskers (as cited in Tubridy and associates 2006). Debate continues as to which type of meltwater streams formed this esker. Eskers are usually formed from the deposits of meltwater streams, sometimes by partial or total blocking of channels.

1. An englacial or tunnel filled esker is formed when meltwater flows in a tunnel inside a glacier. This gives rise to continuous ridge eskers, either singular or multiple. Slower running streams were more likely to deposit sediment and this was laid down within the tunnel. Increasing amounts of sediment lying in the tunnel and the combined water pressure could raise height of the tunnel to allow the water to continue flowing. Eskers can form as a series of ridges, rather than being continuous due to the amounts of sediment being laid down when meltwater flow was slower. Once the glacier melted the sediment in the tunnel was lowered to the ground and laid down parallel to the underground river.

   Subglacial eskers are formed by water running below a glacier. They are not only driven by gravity like regular rivers, but also by the hydrostatic pressure of the overlying ice. The confinement of the surrounding ice can in some cases allow the river to flow uphill or over obstacles resulting in undulating long profiles.

   
   
2. Eskers can also be formed in ice channel fills This happens when sediment is deposited in open, ice walled channels (with no roof) between glacier bodies.
   
   
3. Alternatively there can be segmented tunnel fills, formed during pulsed glacier retreat.
   
   
4. Beaded eskers create lines of successive sand and gravel hills which were deposited during pulsed glacier retreat. This occurred under water ponded at the edge of the ice, and they are oriented parallel to ice flow. They may also be due to seasonal fluctuations in melt water, with more ice melting in Summer, resulting in greater deposits. Additionally eskers may be uneven due to the slumping of overlaying gravel after the ice melted. This can also result in irregular or discontinuous features, such as a string of small hills. These hills are called beads
   

Esker Formation Source: Geologic-org

You can see a characteristic feature of Eskers on the Greenhills Road. The esker stands as a ridge, with the land falling off to both sides of its long axis. Locals refer to the road (Kilakee Drive) descending from the Greenhills road into the nearby housing estate called Green Park as “The Dip”. Similarly at the reference point listed as Old Tymon Lane (see Waypoint) if you look towards the opposite side of the road to "The Dip", you will see how the road adjacent to the petrol station (leading to Ballymount Industrial Estate and the M50 ) descends steeply from the Greenhills Road.

While much of the esker itself has been removed by historical quarrying of its constituent sands and gravels, fragments of the feature remain intact. Where present the esker ridge is a striking feature, standing proud of the flat landscape of till (boulder clay) upon which it was deposited. Intact portions adjacent to Tymon Lane and adjacent to the M50 are especially impressive.

The Greenhills road was diverted in the 1960s and changed so that it ran closer to the Cuckoo’s Nest. Prior to this alteration the lane ran over the highest point of Greenhills. Sand and gravel had been removed leaving a dangerous near vertical drop on both sides of the road. A new section was made that ran level with the floor of the sandpit. Additionally sand from the esker was used in the construction of pathways during the development of the Tymon park.

The siting of Tymon Castle, now in ruins and nowadays the site of Tymon Park’s Ranger Station, was purposely on top of one of the esker’s highest hills, depicted beautifully in an eighteenth century painting by Gabriel Beranger. In fact it seems as if eskers are a type of endangered species.

The Greenhills road and Tymon Lane are only a part of The Eiscir Riada,  a ridge or continuous line of gravel hills whose ridge extends to Esker of Lucan, then south of the Liffey, near Celbridge, across that river near Clane onwards to Donadea,  and onwards as far as Galway.

The Esker Riada provided a route through the bogs of the Irish midlands, since ancient times, forming a highway joining the east and west of Ireland.  Its ancient Gaelic name is ‘An tSlí Mhór’, meaning ‘The Great Way’. In contrast with the surrounding bog lands, the glacial sands typical of the eskers provided well drained and relatively good quality land.

Pre-Christian travellers on the Esker Riada would have been coming and going to places like Tara, and the Rath of Feerwore (where the Turoe Stone originally stood). Earlier still, they would have used it for trips to Newgrange - which was built around 3,100 BC.  In fact the main N6 Dublin to Galway road still closely following the path of the Eiscir Riada

Golf courses on esker ridges have been most popular with their excellent natural drainage and uneven surface features and are found at Lahinch, Portmarnock or Ballybunion.

The English word Esker is derived from the Irish word “eiscir”.

To log this earthcache as a find:

1. Why do you think Eskers make good roadways?

2. What type of Esker do you think this is and why ?

3. How many meters do you think the Greenhills road is above the housing estate at the bottom of the Dip ?

4. Note the direction of the esker, in which direction (eg. N,S,E or W) do think its glacier would have receded?

5. Optional: Take a photograph at the Dip and include a GPS in it.

Once you have emailed you answers do not wait to log your find online; please send me your answers via my profile page and I will contact you if there is any problem. Online logs without any email will be deleted if the email is not received within a reasonable period of time

A very BIG thanks to Dr. Sarah Gatley and Dr. Robbie Meehan, Geologists at the Head Geological Heritage & Planning Programme, Geological Survey of Ireland Haddington Road, Dublin 4 who kindly shared their expertise with me and were so helpful!!!

****Conratulations to TucKids on being the FTF*****

Buses : 27, 77n

Sources:

Meehan, R., Parkes, M., Gallagher, V. and Hennessy, R.(2015) 'The Geological Heritage of South County Dublin', Geological Heritage Audit of South Dublin (as yet unpublished but due out soon)

Healy, P. (2004) All Roads lead to Tallaght, Local Studies Section, South Dublin Libraries

Tubridy, M and associates, (2006) Study to establish the extent, location of eskers and associated habitats in Co. Westmeath: Phase 2, An Action of the Westmeath Heritage Plan

www.sdcc.ie/sites/default/files/tymon-park.pdf

Geissel, Hermann (2004) : A Road on the Long Ridge, In Search of the Ancient Highway on the Esker Riada

http://en.wikipedia.org/wiki/Esker_Riada

http://www.geo-logic.org/Glacial%20Geology/Esker%20Deposits.htm

http://www.britannica.com/EBchecked/topic/19250/esker