Slane to Ardmulcahan
The Boyne Valley in the Ice Age, Robert T. Meehan and William P Warren
Published by The Geological Survey of Ireland 1999
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. Till is sediment deposited by or from glacier ice; unsorted and unstatified, and generally tightly packed. 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.
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.
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.