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Have you ever thought that the world is upside down? For the geologist it sometimes is. Stand on the summit of Ben Lawers, the highest of the Breadalbane mountains, and look southwards over Loch
Tay and Loch Earn towards Ben Vorlich, Stuc a Chroin, Ben Ledi and the hills of the Glenartney Forest. It is then a fact that the rocks beneath your feet and all the outcrop in that wild country between you and
Callander, almost 20 miles away, are for all intents and purposes upside down. To understand this try a table-top experiment with a piece of carpet. You will know which way up it is because the top has the pile and
the underneath the canvas. While holding one end firm on the table, push he other end gradually towards it. The carpet will first buckle into a small fold that will rise higher and higher (depending on the strength of
the carpet) and then the fold will fall over the advancing carpet. As you continue to push, the fallen fold, or overfold, will roll over the advancing carpet while the sides or 'limbs' of the fold become longer and
longer.
 Now stop pushing and look at the fold you have produced. You will
see that the top layer or upper limb of the overfold is the right way up with the pile upwards. The next layer, or lower limb of the fold, has the canvas upwards and so is upside down. Below that the piece
of carpet you were pushing under the fold is, of course, the right way up. Were you to cut away the upper limb of the overfold you would expose the canvas back of the inverted lower limb.
In Breadalbane the inverted limb of the Tay overfold is about 20 miles in length and has been exposed by the removal of the upper limb of the fold by erosion. The root of the fold lies to
the north of Ben Lawers and its down turned snout is seen at Ben Ledi. What evidence do we have for this amazing conclusion? The rocks that are affected by the Tay overfold belong to
the 'Dalradian supergroup', which extends from Shetland, across Scotland, to the west coast of Ireland. By extensive study of these rocks over their whole outcrop, their succession has
been worked out and so we can recognise which are younger and which older. As work continues on the Dalradian so the relations of the major structures to one another become
clearer. The Dalradian rocks were deposited as marine sediments and where sedimentary structures are preserved it is sometimes possible to use such detailed features as grading in the
size of grains in a sandstone bed (they become finer upwards), or the structure of current bedding, to determine directly which way up the beds are.
The Dalradian strata took over 200 million years to accumulate and the rocks of Breadalbane are some of the younger of these, dating from around 600 million years ago. Around 510
million years ago earth movements folded the strata into major structures, such as the Tay overfold, and subjected the sediments to heat and pressure, changing mudstones into
sparkling schists, sandstones into quartzites and crystallising the limestones. Today the Ben Lawers schists lie far above the Loch Tay Limestone, which outcrops at the 1000-ft contour.
Geological evidence appears conclusive, however, that the Loch Tay Limestone is in fact younger than the Ben Lawers schists. Here it is an upside-down world. |
