Challenge Yourself

Stone, Water & Ice – Challenge Yourself: Suggested Answers: Here are some suggested answers for the Challenge Yourself sections in ‘Stone, Water & Ice – A geology trip through the Burren’

Page 29 – CHALLENGE YOURSELF 1: Using a glacial erratic with a good pedestal, estimate the depth of surrounding surface limestone that has been removed by dissolution every thousand years for the last 15,000 years.

Use a ruler to measure from the upper edge of the pedestal to the base, which is the top of the flat limestone surface surrounding the erratic. Measurements will vary due to the difficulty in identifying the base of the pedestal but typically are around 8-12cm. 12cm per 15,000 years =an average of 8mm per thousand years.

Page 35 – CHALLENGE YOURSELF 2: The name Fanore is derived from the Irish Fán Óirr meaning ‘golden slope’. The sand at Fanore beach is golden/orange colour, however the surrounding limestone is grey and the shells of the creatures that make up the sand are predominantly white. Can you explain why the sand is orange?

While the shells of most sea creatures are composed of white Calcium carbonate, some calcareous algae are red or pink and will contribute a slight reddish colour to sand. This doesn’t account for all the colour of the sand at Fanore however; a 2014 BT Young Scientist project (winner of the Geological Survey of Ireland Special Award) by students Tess Casasin Sheridan and Aoife Doherty of Mary Immaculate Secondary School, Lisdoonvarna, indicated that many of the shell fragments that comprise the sand at Fanore have been stained orange by iron oxide precipitating from water in the pore spaces of the sand on the beach. Iron oxide is the mineral that gives rusty metal its characteristic colour. The beach sands are rusty!

Page 39 – CHALLENGE YOURSELF 3: The sediments that comprise the limestone rock at Balliny are known to have been deposited below sea-level. What process has caused the limestone to be at its current elevation of 230m above sea level?

The limestone at Balliny is aver 300 million years old and was formed near the equator, thousands of kilometres away from the Burren. The process of plate tectonics moves large slabs of Earth’s crust which sometimes collide with each other and cause folding and uplift and the formation of mountains. The mountains that once existed at Balliny have since been eroded to the present elevation. A lot can happen in 300 million years!

Page 43 – CHALLENGE YOURSELF 4: Glacial boulder erratics from the most recent Ice ages are common in the Burren, however there is no evidence of glacial erratics in the limestone that was deposited during the ice age that occurred over 300 million years ago, why is that?

During the Ice age that occurred while the Burren rocks were forming over 300 million years ago, Ireland was near the equator. The ice sheets were centred on the South Pole and didn’t extend as far as the equator, thus no erratics would have been carried that far by the ice sheets. It is worth noting that icebergs carrying frozen sediment melt as they drift on ocean currents and can drop rocks onto the seafloor some considerable distance from the ice sheets. These are known as ‘dropstones’ but none have been found in the limestones of the Burren.

Page 47 – CHALLENGE YOURSELF 5: As you look south along the coast, the dark profile of the Cliffs of Moher can be seen in the distance; what type of rock would you expect to find beneath the Cliffs of Moher?

Limestone. The limestone of the Burren is tilted gently to the south and extends beneath the Cliffs of Moher. Looking back towards the Burren you can imagine the shales and sandstones of the Cliffs of Moher that once extended over the entire Burren, remnants of which remain at the top of Slieve Elva.

Page 51 – CHALLENGE YOURSELF 6: Rainwater dissolves calcium carbonate as it flows through fissures in the limestone. How then is it possible for stalactites and stalagmites made of calcium carbonate to form in caves?

Surface water such as rain has no Calcium carbonate and so will dissolve it from the limestone until it is saturated with carbonate. The ability of water to dissolve Calcium carbonate is dependent on temperature, amount of dissolved carbonate and the amount of CO2. When water percolates through caves some of the CO2 that was absorbed is released into the cave which forces the chemical reaction to reverse and calcium carbonate is precipitated: CaCO3 + H2O + CO2 ↔ Ca2+ + 2HCO32-

Page 54 – CHALLENGE YOURSELF 7: When shale is weathered it breaks down to produce clay minerals which give rise to heavy soils known as ‘GLEYS’. What type of soil would you expect to be formed from the weathering of limestone?

The limestone in the Burren is quite pure and composed almost entirely of Calcium carbonate. When the limestone is dissolved this calcium carbonate goes into solution as dissolved ions and is carried away by water, only the insoluble impurities have a chance to be left behind. An awful lot of limestone would have to be dissolved to produce materials to form soil, so weathering of the Burren limestone is not the main source of soil in the Burren. Most soils are thin and are the result of the accumulation and decomposition of organic matter. This soil type is called ‘Rendzina’. Weathering of the limestone continues beneath the soil which gives rise to a gravelly limestone beneath the soil.

Page 59 – CHALLENGE YOURSELF 8: The sedimentary layers of the Cliffs of Moher can be divided into two bundles of shallowing upwards sediments called CYCLOTHEMS. Each cyclothem has deeper water sediments at the base and shallows upwards. These shallow water sediments are then covered by the deeper water sediments at the base of the succeeding cyclothem. What type of event(s) could have caused these repeated changes in sea-level?

While the terminology of Sequence Stratigraphy has largely replaced cyclothems, it is still a useful term to describe the repeated sequences of sediments found in Carboniferous deltas. There are three main potential controls on sea-level;

  1. Rising/falling sea-level due to major ice sheets melting/forming.
  2. Large inputs of sediment supply and compaction can lead to a relative fall or rise in sea level as deltas build land outwards into the sea.
  3. Underlying larger scale subsidence or uplift of a sedimentary basin due to regional tectonic activity will have a significant influence on relative sea-level over time.

All three controls generally operate to varying degrees at the same time and can make unravelling the history of ancient deltas a little more complicated than the simple cyclothem model might suggest.

Page 67 – CHALLENGE YOURSELF 9: Glacial striations are almost exclusively found along the coast of the Burren, why is this?

Glacial striations are the result of rocks at the base of moving ice sheets scratching grooves into the surface of the limestone. When the ice sheets stopped moving, masses of glacial till were deposited, burying the grooves. When the glacial till is removed by erosion, the striations are exposed. Once exposed the limestone striations are subject to the same dissolution by water as all the rest of the Burren. For this reason only recently exposed surfaces show striations and this most commonly occurs along the coast where coastal erosion by storms exposes fresh ‘new’ limestone surfaces.

Page 71 – CHALLENGE YOURSELF 10: The entrance to Aillwee Cave is located on the side of a hill 120m above sea-level. The cave is a phreatic cave, which means it was formed by water flowing below the water table; in other words the cave was filled to the roof with water for a considerable part of its life. The current water table is about 100m below the cave. What can we infer from this about the ancient landscape of the Aillwee area when the cave was forming?

We can infer that the landscape was quite different. The area that is now Aillwee Cave was below the water table and it is likely that the valley that extends to Ballyvaughan hadn’t been fully formed yet. The area may have looked more like the present day Carran Turlough.

Page 74 – CHALLENGE YOURSELF 11: Much of the information about former vegetation in the Burren comes from analysis of ancient pollen. Where is the ancient pollen most likely to be found?

Pollen from plants gets blown by the wind and eventually settles on land. In most cases the pollen will be washed away by rain and lost, however when the pollen lands in lakes or ponds it can collect in the mud at the bottom and may be preserved. For this reason pollen is normally collected by taking core samples of lake floor muds. As lakes are scarce in the Burren most of our ancient pollen information comes from areas surrounding the Burren.

Page 79 – CHALLENGE YOURSELF 12: The limestone slabs that were used to construct Poulnabrone Dolmen were sourced locally by prying away relatively thin slabs along horizontal lines of weakness in the bedrock. How did these horizontal lines of weakness form?

During the last period of glaciations (the Ice age) huge, thick sheets of ice pressed down on the rock with enormous pressure, causing it to compress slightly, when the ice melted this pressure was relatively suddenly removed and the rock then rebounded and expanded very slightly. This expansion caused thin cracks to occur along bedding planes in the limestone where there were slight differences in the composition of the limestone or where there were different layers of fossils. Once the cracks formed water was able to get in and start dissolving the limestone. Sometimes when you walk on the limestone some large slabs will tilt slightly or sound hollow as the slabs have become increasingly loose over time.

Page 81 – CHALLENGE YOURSELF 13: The earliest evidence for humans living in the Burren is from Fanore where kitchen middens have been dated to over 6,000 years old. The oldest evidence of humans in Ireland has recently been found to be from 12,500 years ago from a site near Ennis in County Clare. While there are several Neanderthal sites in the UK, none have been found in Ireland. Can you suggest why not?

Neanderthals lived in Europe and into the UK from 250,000 to 40,000 years ago. In Ireland we have very little record of sediments from this time period as Ireland was covered by Ice sheets that obliterated most of the evidence. Neanderthal fossils are incredibly rare and it may be that they did come to Ireland but we simply haven’t found the evidence yet or that any bones just weren’t preserved.

Page 85 – CHALLENGE YOURSELF 14: The valley that extends from Gregans Geosite to Galway Bay was shaped most recently by a major ice sheet that extended from the NNE and only melted about 15,000 years ago. Though little evidence remains there were other ice sheets prior this as climate oscillated from freezing glacial periods of approximately 100.000 years to more temperate interglacial periods of 10-15,000 years. What could cause major climate cycles like this to occur for almost 2 million years?

A major factor in the development of glacial periods is the seasonal variation in the amount of sunlight that strikes the Earth’s surface; this is strongly influenced by the long term cyclical variations in the Earth’s orbit around the sun, and by variations and wobbles in the tilt of the Earth’s axis. These cycles are known as Milankovitch cycles after Milutin Milankovitch the Serbian astronomer and mathematician who was the first to calculate the time scale of the cycles.

Page 91 – CHALLENGE YOURSELF 15: While there are no fossil tortoises at Lisdoonvarna there are other vertebrate fossils which have phosphatic skeletons. These fossils contributed to the phosphate horizon that extends as far as Doolin where it was once mined. What type of creatures were they?

All vertebrates have a calcium phosphate skeleton. The vertebrates that were living in the ancient seas at that time were fish and Conodonts. After they died and were buried the phosphate leached out of their bones and teeth and accumulated in the surrounding mud.

Page 97 – CHALLENGE YOURSELF 16: If you have ever fallen in the Burren you will know that the Burren limestones are extremely hard and rigid, however the layers at Mullaghmore have been folded as if they were flexible, how is this possible?

As rocks get buried and pushed deeper into the Earth’s crust they get heated and put under intense pressure, this can result in changes in the structure and arrangement of crystals which make up the rock and over vast amounts of time the pressure can cause the rocks to deform slowly and bend and fold.

Page 101 – CHALLENGE YOURSELF 17: Most of the water flowing in the Burren is in underground conduits far away from human eyes, how is it possible to know in which direction the water goes once it goes underground?

Small amounts of fluorescing dye is placed in the water by hydrologists as it goes underground, by sampling different springs or other water conduits around the area and testing for the dye over a period of time it is possible to build up a map of how the water flows underground and where it goes.

Page 105 – CHALLENGE YOURSELF 18: The groundwater in the Burren is classified as extremely vulnerable by the Geological Survey of Ireland, what particular characteristics make it so vulnerable?

Groundwater flowing underground in the Burren flows through cracks in the limestone which get wider over time as the water dissolves the rock. This means that water can actually flow very quickly underground and can go from the centre of the Burren to the coast or to springs or wells in less than 48 hours. Therefore any pollution on the surface will get transported unfiltered to these sites which makes them extremely vulnerable to pollution.