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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.
Before setting off 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 (as you can see from the photograph, which shows the stone just after it was uncovered), but weathering and lichen growth have altered it over the years.
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.
Make your way up the side-road past the pub, and turn right down the track into the field. This track runs parallel to the river; turn down to the river when you reach the wood.
Before you cross the stile down into the wood look back and admire the view over Cadamstown village and right across Offaly. Most of county Offaly is underlain by rocks (mostly limestones) of Lower Carboniferous 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).
The steepness of the slopes above the river, and the danger of animals falling into the precipitous gorge here have meant that this area has always been fenced off, so that animal access has been very limited and the wood in consequence has retained much lf its original natural character. The atmosphere in this lovely old wood is especially wonderful in the early part of the year when most of the herbaceous plants on the woodland floor are in flower, with an abundance of such species as lesser celandine, wood anemone and bluebell (for more on the woods in spring see Knockbarron). You will find a provisional plant list below. Let us know if you can add to it.
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.
Overhanging rock can be dangerous: Keep to the track (or the bed of the river itself) and try to avoid walking along steep slopes. Apart from the element of danger, the plant communities on these unstable slopes are easily damaged. Do remember that rocks in the bed of the river can be extremely slippery, especially after rain.
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.
In the cliff upstream from the bridge there is a very fine 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. You will find a diagrammatic log of the rocks in the cliff in the diagram on the right. 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.
Along 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.
The pale terraced rock with the springs flowing down on the opposite bank is called tufa.
The stream flowing into the river at this point is has a very high concentration of lime – it is oversaturated in fact, so much so that when the water evaporates some of this dissolved lime falls out of solution and is precipitated, wrapping a limy cocoon around whatever debris the water is flowing over. The resulting deposit – which is in fact a young limestone – is known as calc or stream tufa.
Because the growth of tufa is so rapid it often preserves a detailed record of life in the stream or marsh where it forms, going back for centuries or millennia, in some cases all the way back to the end of the Ice Age ended.
The Silurian Rocks
Where they are mostly clearly exposed the Silurian rock are blue-grey rock which are clearly banded [slide]. This banding is due to grading in the rocks: each band is coarser- grained at its base, and becomes finer-grained towards the top. Notice too that the bands in the rock are not horizontal, but incline at a steep angle. Can you see the way this grading can enable the geologist to tell whether steeply-folded rocks such as these are in an overturned position or not? (this can be very important when you are trying to work out the overall geological structure of the area). Each band in the rock represents a single depositional event in the ancient deep ocean.
Where major rivers enter the sea today, their valleys continue out into the ocean as deep-sea valleys or canyons, and the vast amounts of sediment carried by the rivers to the sea accumulate at the head of these canyons. When this great mass of sand, silt and mud reaches a certain critical thickness, it becomes unstable, and can be set in motion down the gentle slope by, for example, an earthquake shock. The mass of sediment picks up speed because of its momentum, and can thus move far out to sea. In motion it behaves as a very dense fluid, and is known as a turbidity current. As the current slows down, the sediment it is carrying begins to fall to the bottom, the coarsest first, then the less coarse, and finally the finer-grained silt and mud are deposited furthest away from the source of the current. The resulting sediments are known as turbidites. Although the individual bands of sediment may have been laid down in a short period of time – a matter of days or even hours perhaps – the time interval between turbidity events may have been considerable: many years, perhaps hundreds of years.
When they were originally laid down 425 million years ago, the Silurian sediments were, of course, horizontal. Here in the Silver River the beds are almost vertical. At some time after their formation therefore, the sediments were thrust upwards. This folding of the rock took place during a great mountain-building episode known as the Caledonian Orogeny, when Slieve Bloom became part of a mountain range as immense as the Himalayas. (Orogeny is simply the geological word for mountain-building: this particular orogeny is called ‘Caledonian’ after the Scottish highlands, which also formed at this time – Caledonia is Latin for Scotland). It took place about 500-400 million years ago, though we should not think if it as a sudden, dramatic event, for it took tens of millions of years. As soon as these great mountains were formed, wind, rain, frost and sun began to wear them down, and by the end of the Devonian period of earth history 354 million years ago they had been reduced to almost level ground. At this time the whole area was part of a continent which lay at a latitude of around 15? south, so that it had a climate akin to parts of Africa today. Rivers flowed across this continent, depositing their gravel, sand and mud on top of the roots of the earlier mountains: and here in the Silver river we see this river sediment of the late Devonian – the Old Red Sandstone – resting uncomfortably, as a low angle, on the steeply-dipping Silurian rocks.
The banded Silurian rocks can be followed upstream for a long way. Interbedded with them are thick beds of a coarser rock, a kind of sandstone known a greywacke (which is originally a German word meaning simply ‘grey rock’).
When you have completed your exploration of the rocks, follow the path back through the wood and return to Cadamstown.
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. (The pub beside the car park used to be a stopping place for stage coaches).
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.’
If you have time it is worth making a detour to visit the ruins of St Lugna’s monastery at 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. Across the road from the monastic site is the now sadly derelict Letter House. Note the unusual architectural feature of the external chimneys.
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.
The foundations of an O’Carroll tower house can still be seen just to the west of the village (you will pass it on your way to Letterluna; it is in the field on the right hand side of the road). Some members of the Cadamstown branch of the family dispossessed in the later 17th century received grants of territory in Maryland USA in compensation. From this branch was descended Charles Carroll, the only Catholic signatory of the American Declaration of Independence in 1776, and the last man of the signatories to survive; he was known as ‘Charles the Signer.’
The rock succession can be traced further upwards through time along the bed of the river downstream of the village, until eventually (some distance below Ardara Bridge) you reach the lowest shales and sandstones of the overlying Lower Carboniferous period. The rocks in this very attractive stretch of the river are mainly sandstones. Fine-grained rocks are almost absent.
Ardara Bridge is a strikingly tall, narrow, single-arch masonry bridge possibly constructed in the late medieval period. It has now designated a protected structure.
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Type: Eco Walk Loop
Trailhead: Cadamstown Car Park- 5k from Kinnitty Co Offaly
Time : 2-2.5 Hours
Terrain: Mix of laneways, riverbank paths, forestry tracks and minor roads.
To Suit: Good level of fitness
Minimum Gear : Sturdy Walking Boots, Rain Wear, Fluids, Mobile Phone.