Geology of Rannoch

Rannoch Net

Aberystwyth University Class of 1969 Geology Reunion

The Limestone pavement

Pictures from the Limestone Pavement

The limestone pavement is situated near Loch an Daimh on the Schiehallion road. It is Dalradian Limestone which is 600 million years old and has passed through the Caledonian orogeny. The pavement has deep channels called Grykes which have been worn by acid water flowing over and dissolving the calcium carbonate of the rocks. The ridges between Grykes are called Clints. The principal mineral in the rock is calcite but there are bands of mica, muscovite and biotite. Unlike the Carboniferous Limestone of Yorkshire and elsewhere in the Pennines the Dalradian is not fossiliferous but is a sedimentary limestone which has been indurated. Patches of the limestone occur between Rannoch and Glen Shee.

The limestone makes the soil calcium rich and provides a suitable habitat for certain lime-loving plants such as Rock Rose, Thyme, Wild Strawberry and Bottle Sedge. Rarer lime loving plants are Astragalus and Northern Felwort. The deep grykes are shelter for woodland plants where they are protected not only from the weather but also from grazing by sheep and deer. The whole area was at one time covered by the original Caledonian Pine Forest and the woodland plants are a remnant of this era. Dog's Mercury, Herb Bennet, Herb Robert, Ivy, Wood Anemone and Wood Sorrel are to be found in the Grykes.

On the surface of the pavement are two eratics of biotite schist which have been left there by the ice flowing from west to east. The eratics display honeycombe weathering where the iron-rich minerals have been differentially weathered.

Loch na Daimh

Limestone at Trinafour

There is another small limestone pavement on the ridge above Trinafour. The burn that descends to the old postoffice at Trinafour disappears and reappears, having dissolved its way through the limestone. It is calcium rich and delivers hard water to the houses in Trinafour. The fields along the side of the road through Glen Errochty demonstrate the effect of the limestone and are a habitat for globe flower and many orchids.

Roche Moutonee

A Roche Moutonee is a rock which has been shaped by ice flowing over it. The side from which the ice came is smooth which the side in the direction in which the ice departed is steep and has been plucked by the ice. The plucking is the result of freeze-thaw action enhanced by pressure changes generated by the flowing ice. This asymmetrical erosion allows glaciologists to trace the movements of the ice. The roche moutonne often has striations (scratches) inflicted by pebbles embedded in the undersurface of the ice. These too, allow the flow direction to be ascertained.

There is a large roche moutonee in a field close to the Kinloch Rannoch/Tummel Bridge road below the Wrights' house and opposite Dunalastair Water.

Kame deposit

Kame deposits are cone-shaped heaps of glacial sediment which was left behind when the ice melted and water flowed down through the ice. There is a large kame deposit next to the road junction between the road from Trinafour and Kinloch Rannoch and the road from Tummel Bridge. It has been used extensively as road grit.

Rannoch Moor Granodiorite

Granodiorite is a black and white rock which is very much akin to granite but in geological terms is slightly less acid. It is the rock which underlies Rannoch Moor and which provided the high platform for ice to flow from in the ice ages. There are numerous eratics to be found throughout Rannoch, diminishing in size and number from west to east. It has been extensively used as a building stone in Kinloch rannoch.

Truncated spurs

Truncated Spurs are ridges which have been cut off by glaciers flowing past. This leaves cliffs high above the valley. Craig Varr and Dhrum Glas are examples of truncated spurs. Their respective cliffs provide places for rock climbing for pupils from Rannoch School.

Schiehallion Boulder beds

The Schiehallion Boulder beds occur in irregular outcrops around the flanks of Schiehallion and then westwards to as far as Port Askaig on the Hebredean Island of Islay. It formed when floating ice melted and dropped pebbles and boulders of pink granite onto the sea floor where they became consolidated in the sediment. The Caledonian orogeny indurated the rock and recent erosion has led to the irregular exposures. The original granite probably came from somewhere in the vicinity of Greenland but the source has now disappeared.

Schiehallion Quartzite

The majority of Schiehallion consists of quartzite, a rock formed from an original sand by heat and pressure. Quartzite is a white or pinkish rock, in which the dominant mineral is quartz. Quartz is hard, being number seven on Mohs' scale of hardness. It is able to scratch glass since it is harder than glass. People often mistakenly think that Schiehallion is an extinct volcano because its shape is reminiscent of an active volcano. If fact it formed by sedimentary processes and the shape is the result of erosion by ice during the ice ages.

Dunalastair Mica Schist

There are rich deposits of mica schist on the Dunalastair Estate and there was a plan to exploit these commercially but it was decided that this would not be finacially viable. A track was made from near Dunalastair old house and some outcrops of the Mica schist were blasted to for some pleasing exposures overlooking the River Tummel opposite Macgregor's cave. Mica is a mineal that comes in two main forms - Muscovite and Biotite. Muscovite is a shiny silver, especially in sunlight where it reflect the light. In sheet for it is transparent and used to be used for sports car windows, furnace windows and house windows in Moscow. The latter fact gave rise to the name Muscovite. It has a very high melting point and it a good thermal insulator which is used in irons where other insulators would melt. Other uses include Christmas card glitter and anit-fouling paint for marine ships. Paint with mica in flakes off and prevents barnacles and other marine organisms from attaching to the vessel. Biotite is black and is seen in bands running throught the Dunalastair Mica schist. It often has garnets embedded in it. Mica schist with garnets in is known as garnetiferous mica schist, a most pleasing phrase.

Epidiorite at Killiecrankie and Ben y Vrackie

Epidiorite is a metamorphic rock which is formed when diorite dykes are subjected to great heat and pressure. The rock is studded with pink garnets. Garnets are semi-precious but these ones are too soft to be gemstones. The epidiorite dykes can be seen crossing the river at Killiecrankie in sinuous curves.

 

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