Map

The Silver River EcoWalk

The Silver River is a very special place from a geological viewpoint, because along its course you will find one of the finest rock exposures in the whole of Slieve Bloom. The Old Red Sandstone can be seen in a splendid, almost continuous section along the river both below and above the village. About 660m upriver from the village of Cadamstown the lowest beds of the Old Red Sandstone are beautifully exposed in a 12m high cliff section. Upstream from this cliff the contact between the Old Red Sandstone and the underlying Silurian rocks is exposed, and above this the Silurian rocks themselves are well exposed in the river bed. If you follow the river downstream from the village you will eventually reach the point where the Old Red Sandstone is overlain by the lowest shales of the succeeding Lower Carboniferous period of earth history.

The glens that radiate out from Slieve Bloom are post-glacial in age. The new streams first cut through the deep blanket of soft glacial till and then exploited lines of weakness in the underlying rock, removing loose blocks and tending to follow the prominent joints in the bedrock. An apparent anomaly in the course of the Silver River is the way it turns to flow eastwards briefly just below the village. This is because here it was briefly captured by an earlier meltwater drainage channel that ran east-west along the margin of the mountain; this now forms a conspicuous dry valley east and west of Cadamstown.

Directions to the Trailhead

The trailhead is located in the village in Cadamstown Village which is situated on the R421 between Clonaslee and Kinnitty.

Stop 1. The Village of Cadamstown

Your starting point is the car park in the village of Cadamstown. Dempsey’s Pub beside the car park used to be a stopping place for stage coaches. After you leave the car park take a look at the strangely- shaped grey boulder in the little enclosure with the picnic benches. This is claimed to be the inauguration stone of the O’Flanagans of Cinel Arga. It was discovered during land reclamation on the slopes above Cadamstown about 25 years ago, in an area rich in archaeological earthworks. The stone is limestone; its true colour is white in fact, but weathering and lichen growth have altered it over the years.

As you cross the bridge over the Silver River notice the ruins of the old mill across the road. According to local tradition the first mill at Cadamstown was built by an Ulsterman called McMorrow. During the Cromwellian Land Settlement it was granted to the Manifold family, who were the local millers and big employers until 1890. As well as the usual milling tasks, they used water power to churn butter and even secretly produced poteen. The present mill was built by Sina Manifold in 1831, reputedly with stones from the ruins of Letterluna Abbey.

In the river just beside the mill you can see a millstone that was left unfinished in its riverbed quarry. Up to the 19th century local sandstone was used to make millstones, but they were later replaced by imported French ‘burr stones’.

Stop 2. Letterluna

You second stop is at the ruins of St. Lugna’s monastery ay Letterluna. All that survives today are the remains of a medieval parish church, St. Lugna’s Holy Well (restored in 1995) and the outline of the monastic enclosure in the fields to the north and east of the church. This monastery must have been a busy place in the past when it lay near the Slí Dhála, one of the ancient highways across Ireland. Interestingly, the part of the Slí Dhála near Letterluna was called the Munster Road locally: this area was in Munster until 1605.

Across the road from the monastic site is the now sadly derelict Letter House. Note the unusual architectural feature of the external chimneys.

Stop 3. The Tufa Springs and unconformity

A long this stretch of the river the contact between the Old Red Sandstone and the older underlying Silurian Rocks is clearly exposed. The contact between the Silurian rocks and the Old Red Sandstone is described in geological language as an unconformity: this means that they were not laid down one on top of the other without a break. Instead, after the deposition of the Silurian rocks there followed a long period during which these sediments were folded and deformed, and later exposed to the wear and tear of a long period of weathering, before the Old Red Sandstone was laid down on top. You can appreciate this unconformable relationship clearly when you study the angles at which the originally horizontal layers or beds of the different rocks now lie.

Stop 4. The Sandstone Cliff

As you make your way along the riverbank there is a very fine cliff section through the Old Red Sandstone rocks that overlie the lowest beds of the series that can be seen in the river bed upstream of this point. The sequence begins with fine-grained red sandstones, which are followed by red siltstones and mudstones, and these in turn by the pebbly sandstones, which are followed by more red siltstones and mudstones, and these in turn by the pebbly sandstones which make up the cliff itself. Notice how erosion of the soft mudstones leads to undercutting of the overlying rocks – hence the many large blocks at the foot of the cliff.

The finer-grained red sediments are interpreted as flood-plain deposits: sediments laid down on the flood-plain of the ancient Devonian river in which these rocks originally formed, during times of flood. The sandstones are the deposits of the river channel itself. Downstream you can follow the rocks that you see here in the cliff for some distance. As you do so you will be encountering progressively younger rocks because of the gentle downstream dip of the sediments. You can see why this is so from an inspection of the section through the rocks shown in the diagram on the left.

You can find a record of the different rocks you will encounter in the diagram. They are essentially similar to those you have already seen in the cliff – coarse sandstones, sometimes containing pebbles of quartz, alternating with finer-grained red sandstones, siltstones and mudstones. The alternation is especially significant. It is due to meandering by the vast river in which the original sediments were laid down. Over a long period of time the river swept slowly across its flood plain, so that at any one point channel sediments (sandstones and conglomerates) and flood-plain sediments (siltstones, mudstones and fine grained sandstones) would be deposited at different times.

Stop 5. The Waterfall and River Gorge

Most of the area of the Slieve Bloom is underlain by a group of rocks known as the Old Red Sandstone at the very end of the Devonian period (about 354 million years old; the Devonian ran from 417–354 millions of years ago. These rocks were originally fluviatile sediments. They were laid down as gravels, sands and muds in a truly vast river system that existed on the late Devonian continent where you now stand. In Slieve Bloom the Old Red Sandstone is about 300m thick (it is much more in other parts of Ireland). Most of the rock in the formation is sandstone: originally sand deposited in the bed of that ancient river; but along with the sandstone there are finer-grained sediments; siltstones and mudstones, originally deposited on the flood-plain of the ancient river during times of flood. Occasionally conglomerates occur, rocks that began as gravels in the bed of the river.

The present Silver River gorge is post-glacial in age. The original river channels in Slieve Bloom were probably all destroyed by glacial movement during the Ice Age. As the glaciers of the Slieve Bloom range began to melt at the end of the Ice Age (around 12,000 years ago), the meltwaters first eroded the soft glacial deposits and then began to concentrate on the planes of weakness in the rock. In the early post-glacial period the river at this point flowed across the sloping platform above the waterfall. At the lowest end of the platform the powerful river was forced to change direction and turn sharply. This change in direction created an eddying movement which was particularly erosive in times of flood and the waterfall was formed in the zone of weakness by the circulating current.
 
 

Stop 6. Exiting the Wood

As you make your way down through the wood note the rich covering of common tamarisk moss (Thuidium tamariscinum) along the path down to the river. This moss is very common in the shady damp areas along the river. All through the year you will see the fungus Hymenochaete corrugata on the hazel trees. This dark-grey, brittle fungus which is almost always found on hazel (though occasionally on other broad-leaved trees) can rapidly produce mycelium which binds the branches together, allowing the fungus to move from one host tree to another, a survival tactic quite common in tropical rain forests but rare in temperate woodlands.

After you cross the stile on the exit from the wood look left and admire the view over Cadamstown village and right across Offaly. Most of country Offaly is underlain by rocks (mostly limestones) of Lower Carbniferous age. Only in the Slieve Bloom Mountains do we find older rock. And with the exception of Croghan Hill in the north-east corner of the country (which you can see on the horizon away to the north), the whole of Offaly is underlain by sedimentary rocks. Croghan Hill is what is left of an ancient (Carboniferous) volcano (for more on Croghan Hill see the Glenbarrow EcoWalk).

Downloads

The Silver River EcoWalk Downloadable Map