Topography of the Pitlochry District

The rocks of this district are of very early formation, as the sandstone and other softer rocks, that at one time covered both hills and valleys, have been ground down and swept away by the ice of the glacial periods, leaving exposed the harder rocks of an earlier age.

The prevailing surface rock is now ‘Black Schist” which is a rough friable stone of a dark grey colour, and is spread all over the Crampian range, the hills to the west of Pitlochry being formed of it. It is really solidified mud, made from the weathering or erosion of rocks, perhaps by the ice of a very remote glacial period, and evidently deposited by water over a vast marine plain. This plain became broken up by violent earth movements, when the crust of the earth was thinner than it is now, and as the movements were of an undulating character, the plain was formed into ridges and valleys. In this district these earth waves seem to have gone from south west to north east, and the pressure caused by the movement was so great that nearly all the surface rocks became folded. Either when these earth movements were taking place, or shortly afterwards, a period of volcanic energy set in, and igneous rocks, in a molten state forced their way through fissures to the surface.

All our mountains, with one exception, have been formed by the eruption of igneous rocks, which became solidified as they cooled. The igneous rocks here are quartzite, granite, and basalt, and there is in addition some lava or epidiorite on the summit of Ben Vrackie.

Schiehallion, 3547 feet, and the Ben-y-Gloes, of which the highest is 3671 feet, are formed of quartzite. On Ben y-Gloe the quartzite is folded and refolded upon itself, showing that it must have emerged in a semi-liquid state. Quartzite is a crystalline rock of a bright silvery colour, and is closely allied to granite, of which it is an important constituent, and it has been noticed that a vein of granite becomes more quartzite in character as it ascends.

Ben Vrackie, 2757 feet, is the only mountain of volcanic origin in the Grampian range. The lower part is formed of a schist known as “Ben Lawers Phyllite,” and the upper part is of lava, which seems to have been ejected from a vent in the east shoulder of the hill. Beside the small lake at the foot of Ben Vrackie, there is a large detached mass of rock, of which the upper part is lava, and the lower is the native schist.

Ben Vuroch, 2961 feet, in the Strathardle district of Moulin Parish, consists wholly of granite, and is the only granite mountain in the Grampians. This vein of granite extends southwards to Dunfallandy, and westwards to Cairndhu, where the outcrop will be found on the crest of both hills. The whole of the ground within this triangle, which includes Moulin and Pitlochry, is occupied by a bed of Mica schist, or granitic gneiss as it is sometimes called, and most of the houses in Pitlochry have been built of this stone from Aldour Quarry.

Ben Lawers, 3984 feet, and the highest mountain in Perthshire, although outside the limits of this district, is deserving of notice, as it is the only mountain in Scotland that does not owe its origin or its height to an eruption of volcanic rock, as it is wholly formed of· schist. On Ben Lawers this schist takes the form of “Phyllite,” which is similar to black schist but lighter in colour, and is of more recent formation as it lies, above it, but it belongs to the same geological period. Iron crystaIs known as “Pyrites,” are found embedded in this Phyllite, and must have fallen into it before it hardened into rock. The rare Arctic flora, for which Ben Layers is noted, and which is not found elsewhere in Britain, is probably owing to its geological formation being the oldest in Britain.

There are many narrow veins throughout this district of what is usually termed “Whinstone” although that is a name that is not recognised in geology. It is an igneous rock of basaltic formation and of great toughness, and is invaluable as road metal. Beside these whinstone veins there, are usually extensive beds ofa dark blue rock like whinstone in appearance, but very brittle, and in consequence of little use either for roads or buildings, and being in layers or stratified it has got the name of “trap rock” but is more correctly termed “Hornblend Schist.” It seems to have been formed of disintegrated whinstone ‘as it consists of the same materials. The large rock at Portnacraig is an outcrop of this rock, and from Portnacraig it extends to the top of Fonab Hill and westwards to Cluniemore.

All schists are described as “Metamorphic,” which means “change of form,” that is, the rocks have altered from what they were originally. This has usually happened through erosion, the debris being carried by rivers to the sea or into lakes, where it became solidified by pressure, but this does not explain the change of Granite into Mica schist, or of Basalt into Hornblend schist. Two theories have been put forward, one is that the changes in these rocks have been caused by pressure, as the result of earth movements which had crushed the atoms or crystals of which these rocks were composed, and at the same time caused them to become stratified. Against that are two facts, that these meta­morphic rocks are often found to have been folded by earth pressure, as can be seen in the quarry of Hornblend schist at Balghulan near the Pitlochry Golf Course, and that stratified layers’ in the solid rock are sometimes several inches away from their original position, showing that the pressure was subsequent to the rocks becoming metamorphic.

The other theory which is now regarded as being more probable, is that these intrusive rocks came to the surface in a fluid state under water, that the water was at once converted into steam, which disintegrated the rock, with the result that the decomposed sediment was changed in character though its in. gredients were the same. The deposit being made under water was stratafied, and after consolidation became Mica schist or Hornblend schist. The black specks in granite are felspar, and are not present to any extent in Mica schist, because felspar, being soft, would remain suspended in the water till carried away and ultimately deposited as clay.

Another surface rock, which can be seen overlying the schist, is Limestone, and is found all through the Grampians. In this district it is known as the Loch Tay Limestone, that being the area where it is most abundant. There are extensive Limestone - deposits at Cluniemore, where there is a quarry, out of which Cluny Bridge was built, and from which Limestone was taken to be burnt into Lime in the local kilns. This vein crosses the Tummel and runs by Lettoch Farm, Balnakeilly, and Badvo to Strathardle.

In Glen Tilt the Limestone is so pure that it has become marble, and was at one time quarried. It was made into mantle-pieces which can still be seen in some of the older mansion houses of the district. Glen Tilt marble is of a pale green colour with darker veins through it, and is both durable and ornamental.

There is a curious attraction between Loch Tay Limestone and Hornblend schist, for which no explanation has as yet been found. Although they differ in their component parts and seem to have no affinity to each other, yet wherever Limestone is found there is always a bed of Hornblend schist either underlying it or in its immediate vicinity.

The only remaining surface rock to which attention might be drawn, is a dark schist studded with nodules holding small garnets, from which the rock is known as Garnetiferous schist. It is tbe most recent of the existing rocks in this district, as it overlies the Limestone, and can be seen in the Pass of Killiecrankie and in Glengirnaig.

There are numerous samples of this stone in the village gardens, where it has been set up on account of the curious shapes it sometimes assumes. There are no minerals in the district except that in some of the peat bogs there is a thin deposit of iron, which can be detected by the red appearance of the water; but the quantity of this ore is so small that no attempt seems to have been made to manufacture it. In Rannoch, where there were at one time extensive deposits of bog iron, a considerable industry was carried on 200 years ago, as the “Black Wood” provided fuel for smelting, and charcoal for tempering. Claymores of Rannoch manufacture are much prized as being the finest examples ot what, from the earliest tunes, has been regarded as the national weapon of the Celt.