Crieff Geology

In the last chapter of Sir Walter Scott's The Fair Maid of Perth Conachar plunges, shrieking, into the roaring waters of Campsie Linn. Scott described the Linn, between Stanley and Stobhall, as:- "the place where the princely River Tay rushes tumultuously over a range of basaltic rocks, which intercepts the current like a dike erected by human hands" Sir Walter was more of a geologist than he cared to admit and here used the word dike in its geological sense, as well as in its common meaning. He probably did so knowingly because he would have seen the drawing of a geological dyke at or near Campsie Linn made some 40 years earlier by John Clerk of Elgin, the father of his close friend Will Clerk and companion of James Hutton on his geological excursions between 1785 and 1788. In 1802, Professor John Playfair drew attention to the fact that whinstone veins of this kind were called in Scotland Dikes, one of many words that Scotland has contributed to the geological vocabulary. The late Dr. J. E. Richey, Director in Scotland of the Geological Survey, wrote in 1935 that it is fitting that in Scotland, where the term dyke was first applied, there should occur more numerous and more varied dyke-assemblages than are known from any other area of equal size. For the geologist, a dyke represents a crack in the pre-existing rocks, up which molten rock has flowed and there congealed. If the dyke rock is more resistant to weathering than the rocks it has penetrated through, it will stand up as a wall-like feature, whereas if less resistant it will appear ditch-like. Columnar jointing is often developed at right angles to the dyke walls, which are the cooling surfaces, and is best developed towards the centre of the dyke. Towards the walls, the dyke rock becomes chilled and finer grained, while the country rocks with which it is in contact may show signs of baking. The orientations of dykes are controlled by tensional forces in the crustal rocks that have caused the cracks, and they vary from one intrusive episode to another. For example, at one stage the forces that folded the rocks of southern England and South Wales along east-west axles created tensional cracks in the dominantly east-west direction in northern England and Scotland. Tensional cracks in the northwest-southeast direction were formed less than 60 million years ago in connection with the central volcanic complexes of the Hebrides, which resulted in the swarms of dykes that extend across southern Scotland and northern England. Professor Holmes has cited an example of one such complex of dykes in Arran, where the southern coast has 525 dykes along a 15-mile stretch. This has resulted in an extention of the earth's crust of one mile in fifteen. Strathmore exhibits the largest and longest dykes known in Scotland, which have been extensively quarried for roadstone. They belong to the east-west group composed of quartz-dolerite rock emplaced 292 million years ago. The longest extends from Fife and can be traced to the south of Perth and Crieff to Loch Fyne; a distance of 80 miles. It is probably this dyke - or one running parallel to it - which James Hutton traced for 30 miles of its length two centuries ago. The widest dykes have a breadth of 150ft, but there are many that are shorter and narrower. In Strathmore the dykes cut sandstone and lava flows of the Old Red Sandstone period. Where they penetrate sandstone they form ramparts that have been used as the rocky foundations on which defences, such as Drummond Castle, have been founded. The wall-like appearance of these great dykes can be seen to advantage east and west of Drummond Castle and most easily in the nature trail at Bennybeg, where the dyke is crossed by the A822 Muthill-Crieff road. East-west dykes also form spectacular features where they are cut by rivers, such as those seen in the Tay at Broadgreen near Cargill or Campsie Linn and Thistlebridge near Stanley. Sometimes the course of the river is deflected by the dyke before it is finally breached, as at broadgreen. © Scottish Towns