Extracts from

The Boyne Valley in the Ice Age,

by Robert T. Meehan and William P. Warren

published by Meath County Council and the Geological Survey of Ireland

Slane to Ardmulchan

Take the road out of Slane towards Navan. Immediately past the castle on the north side of the road is Carrickdexter Hill, a dome of basalt mined extensively for road surfacing. Basalt is a dark, fine grained igneous rock, rich in iron and magnesium but with little or no quartz. Igneous rock is rock solidified from magma. It is an extrusive igneous rock emplaced at, rather than beneath, the Earth’s surface. On the east side of the hill a small stream has etched a channel and is underlain by a blanket of gravels, indicating that a wide river flowed through here during deglacial times. Gravel is a  sediment with particle size over 2 mm. Sediment is any material that has been obtained from earlier rocks by denudation, and subsequently redeposited. The flat area which borders this stream was completely taken up by the river which deposited the gravels.  The present stream has been incised since the Ice Age.

The Boyne River now parallels the road on the left and the area between the river and the road is underlain by till with some isolated, craggy rock outcrops. Limestone outcrops also occur in places along this road. Till is sediment deposited by or from glacier ice; unsorted and unstratified, and gererally tightly packed.

Turn left at Wicker’s Cross Roads and cross the Boyne River, and left again immediately after Broadboyne Bridge. There is a gravel pit situated around the bend on the right hand side of the road. The pit is cut into the extensive gravel terrace which flanks the Boyne River on its southern side. The deposits consist of horizontal beds of gravel and sand, the gravels are generally cobble to pebble size. The stones are rounded due to the action of water. The sorted nature of these deposits also emphasises the importance of water during deglaciation in evolving the modern Boyne landscape. The gravel consists of limestone mostly, but many clasts of greenish Lower Palaeozoic sandstone are also present. A clast is any particle above 2 mm across derived from pre existing rock.

The road from the pit has been built in the centre of a small meltwater channel which is cut into limestone bedrock. A meltwater channel is a deeply cut drainage channel which is a result of meltwater erosion during ice wastage. The rock can be seen at the road side. The channel  flanks become more hummocky coming up to Stackallen Bridge where two isolated hummocks are present at Bridewell Bridge. Hummocks are small irregular shaped hills, generally occurring in clusters. These are kames composed of coarse boulder and cobble gravel. A kame is a mound, knob or ridge of stratified sand and  gravel deposited by a subglacial stream. Stratified means the presence of layers or beds in a  sediment. A bed is a single unit of sediment, distinct from those on either side.

Between Dollardstown and Ardmulchan the road traverses a flat plain mostly underlain by till. Turn into Ardmulchan from the road and park at the viewing point close to the church.

Ardmulchan

The church and adjoining bell tower at this picturesque site date from medieval times. Nearby Dunmoe Castle, though in ruins, is worthy of inspection, especially for its ( still visible ) vaulted ceilings on the lower storeys.

The site at Ardmulchan offers an excellent view of the components of a relict meltwater system. The sides of the valley are particularly striking with the south side having been eroded into a steep slope. The northern side also has various levels of erosion associated with meltwater. East of the church a small meltwater channel has been incised into the valley side, now housing a stream. This channel actually divides in two; the hillock in the centre having been deposited as an “island”  when the channel housed a river during deglaciation. Deglaciation is the period of time during which there is a net melting of ice, rather than a net accumulation and spreading out of ice ie the during which climate is returning to an interglacial one. We are now in interglacial time.

Navan to Bective

Take the Trim Road, the R161, out of Navan. It crosses a flat till plane as far as Balreask where Teach na Teamhrach is. This flat area underlain by till was deposited beneath the ice which crossed  around  20,000 years ago. Till is sediment deposited by or from glacier ice; unsorted and unstratified and generally tightly packed. Passing Balsoon Demesne the road crosses a meltwater channel which has been incised into the underlying till by meltwater during deglaciation. Just past this meltwater channel, 7 Km south of Navan, turn left towards Bective Abbey. The signs are for Bective and Kilmessan.

Bective Abbey

Bective Abbey is a substantial remains of the second Cistercian monastery founded in Ireland in 1147. The remains are chiefly of a smaller 15^th century abbey built on the site of the 12^th and 13^th century Gothic complex.

The Boyne channel is quite wide at this point and has only one terrace on its northern side. The abbey is built on this terrace which provided an excellent level site, close to, but above the level of the river and its fertile flood plain and therefore dry and safe from floods when the river overflows its banks. The terrace is composed of glacofluvial gravels which were laid down during deglaciation by the meltwater flowing through this part of the Boyne System from the Midlands. Glaciofluvial means of or pertaining to rivers made up of meltwater from glaciers or ice sheets. The modern floodplain is relatively wide close to the Abbey and is generally sandy in texture, reflecting in its well drained appearance. On the southern side of the river a steep escarpment has formed where the river has cut into the bedrock which is quite close to the surface at this point. An escarpment is a steep slope or cliff on one side of a hill or ridge with sides of varying slope angle. In fact, on the southern side of Bective Bridge, 200 metres from the road a steep cliff has been cut into a well bedded limestone outcrop. Behind this outcrop much of the area around Balsoon and Balgeeth is dominated by thick glaciofluvial gravels and sands which were deposited at the margin of the ice as it retreated across the area during deglaciation.

Hill of Tara

The Hill of Tara was the Seat of the High Kings of Ireland and its source to many of the Great Legends. The site has been an important one since the Neolithic (Stone Age) when a passage tomb was built. It had its heyday in the third century A.D. following the reign of Cormac Mac Airt.

The hill is cored by limestone bedrock which was streamlined by ice during the ice age. The bedrock once outcropped on the southern side of the hill but has since been removed by small scale quarrying, the scars of which can still be seen. The view from the hill is quite spectacular, showing the entire Boyne Valley and a very large part of the east midlands. The region dominated by the Hill of Tara covers some of the richest farmland in Ireland. It is an area underlain by a thick covering of till and glacial sands and gravels. The high hills of Slieve na Calliagh and Lloyd Hill can be seen to the northwest, the Mourne Mountains to the northeast, the Wicklow Mountains to the southeast and Slieve Bloom( in the distance) to the southwest.

Navan,  Garlow Cross, Skyrne, Kentstown, Navan

The road from Navan to the M3 Motorway was a wide , alluvium floored tributary stream to the Boyne. Alluvium is sediments deposited by fluvial (river) processes. This channel conducted water towards the Boyne during the last deglaciation and is now flanked by sands and gravels which were deposited at that time. Turning left at Garlow Cross and looking towards Tara it can be seen that the high hill is enveloped by a lower, flat topped ridge which forms the southern flank of the channel we have just crossed. This is actually a moraine which was deposited by ice pushing up against the bedrock of the Hill of Tara. A moraine is a mound or ridge of unsorted and unstratified glacial debris, deposited commonly at the ice margin.

Follow the sign for Skryne. Taking the road to the right at the fork for Skyrne Hill, some pronounced rounded hillocks separated by channels are present on either side of the road. These hillocks are composed of gravel and were deposited by meltwater during deglaciation. Their form was accentuated by meltwater flowing between them, causing the channels and the associated steep sides to the features. The road rises towards Skyrne Hill with bedrock cropping out at the top of the hill. A short esker ridge can be seen at the base of the hill between here and the Hill of Tara. An esker is a long narrow sinuous ridge of sand and gravel deposited by a subglacial stream and left behind after the ice melted.

Skyrne Hill

Skyrne was an early Christian monastery, and is called after the Shrine of Saint Colmcille’s relics. The 15^th century holy well is dedicated to St. Colmcille. The moat or motte of De Phirpo, the first baron of Skyrne, can be seen from the hill, as well as the castle that replaced it.

Skyrne Hill, composed of shaley Carboniferous limestone, emerges through the flat and well drained glaciofluvial sands and gravels that extend east from the foot of Tara Hill. Glaciofluvial refers to rivers made up of meltwater from glaciers or ice sheets. Sand is  a sediment of particle size between 2 mm and .06 mm. Gravel is a sediment with particle size over 2.0 mm. These gravels comprise the outwash plain which was deposited in front of the ice as it stood between the two hills during deglaciation. The plain is dissected by a number of channels which eventually link up with the channels east of Garlow Cross. On the northern side of the base of the hill two elongated hummocks are present. These are two beads of an esker which were deposited (earlier than the outwash gravels) under the ice which filled the area between the two hills. It is difficult to see rock outcrop on the top of Skyrne Hill today as much of the area has been landscaped.

Skyrne to Kentstown to Navan

From Skyrne take the road northwards through Skyrne cross roads to Cusackstown where the road to the left should be followed. Here, at Cusackstown, the road follows the base of a network of meltwater channels. Drive on to the next cross roads and turn right into Kentstown along the northern edge of the Nanny Valley. This deep valley was again cut by meltwaterr during deglaciation and carried much of the Boyne water while the ice sheet stood just north of here. The road from Kentstown back to Navan follows a till plain with Brownstown Hill, which is rock cored, dominating the landscape on the northern side of the road. Till is sediment deposited by or from glacial ice; unsorted and unstratified, and generally tightly packed.

Navan to Athboy

Take the old Athboy Road past Navan Hospital. Before turning left, crossing the railway bridge, a high hill looms on your right. This is Navan Moat an early Norman fortified site. The moat, or motte, is built on a moraine which was deposited at the ice front when it stabilised at Navan for a short period during the last deglacial period. The Normans were adept at modifying hills such as this to provide a palisaded hilltop defensive site pending the construction of a castle. The hill itself is composed of layers of till and sorted gravels. Till is sediment deposited by or from glacier ice; unsorted and unstratified, and generally tightly packed. Gravel is a sediment with a particle size over 2 mm. The flat area west of the moat towards Athboy which extends for several kilometres west of Navan is a flat till plain. Tara Mines/Boliden can been seen to the north on the Bohermeen Road. Here the limestone bedrock beneath the till is intensely faulted and folded and has resulted in the mineralisation of lead and zinc along many of the major joints. The limestone is overlain by several metres of the till which comprises the till plain. The till gives rise to thick topsoils which provide almost perfect conditions for pastoral farming. Past Halltown Cross Roads where there is signpost for Dunderry on the left, the flat area is underlain by marl deposits which were deposited into a temporary glacial lake. Marl is a soil consisting of clay and carbonate of lime, a valuable fertiliser.

Tullaghanstown Bog

The bog starts where the road sign warns of subsiding road surfaces. The bog covers an area of about 5 square kilometres and in some places is over 10 m deep. It is an excellent example of a Midland Raised Bog, taking the form of a peat filled hollow or depression in which the surface of the peat rises from the margins to the centre forming a dome, thus it is “raised”. Peat formation began almost immediately following the last glaciation about 10,000 years ago. Tullaaghanstown Bog formed here in a wide flat basin which had held a glacial lake as the ice sheets melted and which was floored by glaciolacustrine clays. These clays pertain to lakes made up of meltwater from glaciers or ice sheets. In the early postglacial period there would have been a shallow lake here. As the vegetation (aquatic plants in the lake water and semi aquatics near the shore) died and accumulated in the basin, it began to form peat. This decreased water depth and resulted in the gradual encroachment of vegetation into the lake and a fen was established. As the margins of the fen became invaded by reedswamp plants and bulrushes, and as other submerged plants established themselves in shallow waters, the accumulating vegetable debris eventually consolidated into fen peat, which continued its build up as time went on. As the peat further accumulated and the bog rose above the surrounding area above the influence of the mineral soil the vegetation changed and it became a raised bog with a very different vegetative cover dominated by sphagnum moss. This moss has a remarkable capacity for capturing and storing rain water and is generally considered as the primary building block for the growth of peat bogs. Peat in an undisturbed bog is composed more than 90% of water. The rest is dead vegetative matter. Thus a bog contains less solid matter and more water than milk, yet it is possible to walk on it. On close inspection branches, twigs, leaves and moss can be clearly seen in the peat and it is these that tell us how the bog formed. Many of the fields along the road just past the main peat area were once part of the bog also, but at some of these locations the peat has been largely removed for fuel over the last two centuries.

Past the bog and as far as the Hill of Ward the road crosses a till plain. Around Rathmore, where the church and school are, there is an old church on the left. The land is very well drained as the underlying till is especially gravelly, and water can percolate through it with ease.

Turn left in Athboy for the Hill of Ward. The Hill of Ward is cored by limestone bedrock which is very close to the surface at its summit. The bedrock has been eroded and shaped into a dome shaped hill by the movement of ice from northwest to southeast during the last glaciations. At the northwest side of the crossroads close to the summit there exists an old, disused quarry cut into the limestone. The quarry exposes folded beds of limestone which have been sheared at the top by ice, truncating the folds. On the sides of the hill a thin covering of lodgement till has been deposited on the bedrock. The view from the top of the hill shows the plain of central Meath with sporadic high hills, such as Tara to the east, protruding above the till plain. To the west the bogs of Westmeath can be seen in the distance.

Nobber to Navan

Nobber is the birthplace of the last of the Irish Bards, Turlough O’Carolan, who was born in 1670. He traversed the northern half of Ireland as a travelling bard, and wrote more than 200 pieces of music. An Irish music festival in held in his honour at Nobber each summer.

One of the best views of the drumlins in the area occurs just north of Nobber village, where the road bends around to the right. West of the village, close to the River Dee, some hummocks can been seen. Hummocks are small irregular shaped hills, generally occurring in clusters. These are moraines. A moraine is a mound or ridge of unsorted and unstratified glacial debris, deposited commonly at the ice margin. Glacial debris is material being transported by a glacier in contact with glacier ice. The ice margin is the edge of an ice sheet or glacier. The first five kilometres of the road  between Nobber Village and Castletown is quite like a roller coaster as it rises up and down over the humpbacked drumlins.

Driving south from Nobber drumlins dominate the landscape. Passing Cross Guns Pub and Mullens Cross Roads, the drumlins die out and are replaced by smaller, more haphazard, 5m to 6m high hummocks which are comprised of sorted gravel. These are exposed in the road cutting at Darby’s Cross Roads. The gravels here record the huge amounts of meltwater that existed in this area flowing from the drumlin region during deglaciation. Turn left at Wilkinstown and then right towards Demailstown. A deep valley is visible on the left hand side after 2 km or so. Stop at the T junction with the minor road going towards Kilberry.

Demailstown is only three miles from Teltown, site of one of the four royal palaces of Ireland in ancient times. It was also the location of the Tailtean Games, an annual gathering of sport similar to the Olympics of ancient Greece. The games were held on the first of August from the15^th century B.C. to as late as the 12^th century A.D.

The view east at Demailstown across the valley is quite interesting and records the complex development history of the valley. The highest ridges are bedrock cored with a thin covering of till in some places; furze bushes often indicate areas where rock is outcropping. On the north side of the bedrock ridge (Mullagha Hill ) directly opposite a flat topped feature is seen, with houses built across its crest. This flat topped feature is a delta and is composed of bedded sands and gravels. A delta is a fan shaped plain of alluvial sediments at river mouth upon entry into the sea or lake which is crossed by many distributaries, often extending beyond the general trend of the coastline or lake shore. Alluvial sediments are sediments deposited by fluvial ( river ) processes. At the end of deglaciation ice had so reduced in thickness that only a tongue of ice covered the valley floor, with its front retreating towards the north. The ice stood still at the northern side of the valley and a large lake formed from the meltwater, filling the valley up to between the 80 and 90 metre contour. Meltwater rivers flowing off the ice at the northern side of the valley deposited sorted sands and gravels as a delta: then as the ice retreated further north the meltwater filling the valley drained away northwards via the meltwater channel at Clynch Bridge. The delta was then left standing as it is today, a relict feature of sands and gravels. The floor of the valley at Demailstown is covered in poorly drained glaciolacustrine silts and clays which were deposited while the lake was in existence. Glaciolacustrine pertains to lakes made up of meltwater from glaciers or ice sheets. These lakes may or may not be dammed on one or more sides by the ice itself.

Nearby , just inside the field on the west side of Corballis Cross Roads, there is an old quarry. The siltstone rock cropping out of the quarry has been striated along a west east plane. Measuring the striae provides valuable information about ice flow over the area during the last glaciation. Striae are ice scratches on rock surfaces caused by debris carried by moving ice.

Driving between Demailstown and Simonstown

Between Kilberry and Navan the ground is covered with a veneer of till, hence the well developed, fertile valleys with thick topsoils. Crossing the top of Proudstown Hill the bedrock can be seen cropping out at the roadside. The outwash plain at Simonstown is best seen at the base of Proudstown Hill. Outwash pertains to rivers made up of meltwater from glaciers or ice sheets. Looking west, the flat surface of the plain can been to terminate in a series of small hummocks on the western horizon. This is where the ice margin stood at the time of deposition of the outwash gravels. The extensive Kilsarn Gravel Pit can be seen and can accessed  by turning right at the lane beside the Navan Tyre Centre. The gravels in the Kilsarn Pit are organised in extensive horizontal layers which are sheets of sand and gravels which were deposited on top of each other by the meltwater rivers.

Navan after the Ice Age

In Geological Time we are in the Holocene Epoch. This is from 10,000 years age to today. 10,000 years is not so long ago when you consider that Newgrange is 6,000 years ago. 20,000 years ago Navan was completely covered in ice.The Holocene in Ireland is the post glacial period. The last glaciation or ice age occurred between 73,000 years ago and 10,000 years ago. This had a huge influence on both the landscape and underlying geology of Ireland and Navan.  Since 10,000 years ago the action of modern rivers and the infilling of lakes, along with the formation of peat bogs and development of beaches and other coastal features, have been the main natural processes affecting both our landscape and geology.