Friends of the Ochils Newsletter 17: Winter 2000

The geology of the Ochils

THE OCHILS consist predominantly of andesite and basalt lavas, dipping to the north under Strathallan and Strathearn. The dip, which can best be seen from the west or when climbing into the range from somewhere such as Alva, is picked out by more resistant bands, some of which are quite extensive. Though at their most impressive in the west, the lavas continue eastward to near Tayport, then under the Tay before finally dying out near Carnoustie.

The lavas were erupted during the late Silurian / early Devonian period: some 420 to 400 million years ago. By means of chemical analysis, it has been shown that the magma (the lava whilst still molten) was derived not only from the crust but also from the underlying mantle. It therefore comes from at least 35 kilometres down.

No evidence remains of the vents from which the lavas were erupted. However, in addition to lava, and especially along the front of Dumyat and the hills near Alva, there are large quantities of agglomerate, a rock consisting of angular blocks set in a fine matrix. As some of the blocks are quite large, they didn't travel far and so the vents may have been roughly along the line of the present Ochil Fault. They were produced by explosive activity and may have been helped along by water. The dip of the lavas means that some of the rain water falling on the Ochils seeps its way northward and ends up in bottles!

Whilst molten, the lavas contained much gas and fluid, some of which left cavities on solidification. Later, minerals were deposited in them. If this happened in pulses, then agates formed.

The other feature which makes the western Ochils so spectacular is the Ochil Fault. This is a dislocation traceable from west of Stirling to east of Dollar, where it splits, part going towards Arndean and the other branch continuing, much reduced, to near St Andrews. In geological terms the Ochil Fault is a "normal fault" - which means that the carboniferous rocks moved downwards.

The greatest movement on the fault was in the region of Alva and Tillicoultry. There may have been at least 7000ft of movement, though obviously not all at the one time, the movement taking place over an extended period at the end of the Carboniferous, some 300 million years ago. There is evidence to suggest that some carboniferous rocks did once lie on top of the lavas, but have long since eroded away.

After the faulting, molten material of a different composition to the lavas squeezed its way up parts of the fault, giving rise to the quartz dolerite (whinstone) that is still quarried at Tillicoultry. This squeezing occurred in the late Carboniferous / early Permian period, as with most of the mineralisation of the Ochils.

Seismic evidence would suggest that the earthquake activity covered in a previous article was not centred on the main fault as might be expected, but to the north and possibly on one of the faults which cut across the Ochils and give rise to the main "glens" and gullies seaming the face of Dumyat. It was along such faults that copper, iron and lead minerals were deposited.

The silver for which the Ochils are well known appears to be associated with the quartz dolerite intrusion, the vein taking advantage of a fault. The mine which started operations around 1711 encountered a large ore body containing "native" or metallic silver. Working this produced an average £400 worth of metal per week and an overall total of £40000-£50000 before the ore was exhausted. Cobalt ore for use in pottery was mined later on.

Gold has been panned in the burns near Glen Devon youth hostel: an acquaintance once found a "nugget" the size of the nail of her pinky. No gold-bearing mineral veins have been discovered, however, the gold having been deposited by stream activity, perhaps from glacially deposited material.

The ice age concludes the story. As a result of the Ochils having been covered by ice, spectacular melt-water channels (deep east-to-west gullies) cross the face of King's Seat. These were cut by water flowing at great pressure under the ice, which must have been well above the summits to produce the required pressure. The valley from Dollar through to Glen Devon, though following the line of a fault, was deeply eroded by the same melt water. Other much smaller channels run down the hillside towards Greenloaning.

J Kenneth Oakley

With regard to his earlier article, Mr Oakley adds: The shallow seismic activity under Forestmill and Clackmannan is indeed mining related, as stated, but is not due to shot firing. Once the coal cutter has stripped the face and moved on, the roof is deliberately collapsed, so reducing - I assume - the risk of subsidence.

Newsletter 17 Index