The Edinburgh Geologist
by Bill Baird & John Smellie
The River North Esk runs from Roslin to Polton in a deep gorge cut into the Passage Group sandstones of the Carboniferous by meltwater, at least in part, during the period of the last deglaciation. The sides of the gorge in this area are up to 200 feet high and in several places almost vertical, It is not unusual under these conditions to find rockfalls and small landslides and there is evidence of several having taken place in this area during recent years. However, the sandstones in the gorge, although friable and containing thin shalv intercalations are horizontally bedded and therefore such falls as have occurred in the past have been of relatively small size and limited to areas where the exposed faces were waterlogged or undercut.
Just south of Polton the gorge widens out a little and at Springfield, on the flood plain of the river, are the buildings of an old paper mill (Springfield Mill). The buildings are on the east side of the river facing a high bank on the west. This area, called the Hewan Bank, is well wooded with many mature trees and it slopes towards the river at an angle of about forty five degrees. It is only when the geology of this slope is examined that one can understand why it should he prone to a major landslide while the much steeper walls of the gorge remain relatively undisturbed. In place of the horizontally bedded sandstones of the Passage Group there is a bank almost 200 feet high which seems to consist entirely of unconsolidated glacial sediments. After the landslide of December 1979, fresh exposures of the sediments showed that beds of glacial till occur beneath the lower slopes, but the upper and least stable part of the bank consists of loosely bound sand with clay partings capped, at the crest of the slope, by a thin layer of gravel. Moreover, the existence of a spring line half way up the slope must have greatly added to the overall instability of the of the area. All it required was a trigger.
There were several small slides prior to 1979 and one such slide is shown on old maps at the south end of the bank; this probably dates back to the last century. However, during the wet summer of 1979, an observer noticed that the floor of the ruined Hewan Cottage had split and part of the floor on the side neatest the river had dropped by about a foot relative to the other. Whether such movement was of a creeping type caused by waterlogging and the constant pull of gravity, or whether it was initiated by one of the small-magnitude earthquakes which are common in this district is uncertain, hut obviously the area was potentially unstable and had already begun to move during the latter half of the year. And then it rained in December. How it rained. By the end of the first week almost as much fell as would normally have fall en during the entire month. An inch of rain fell on the 7th of December and it was this which may have finally prepared the unstable sediments for movement. Still nothing happened, although it rained almost continuously on the 8th and again the following day and then something set the whole wet, soggy mass in motion. At 3.30 pm on Sunday 9th December a hundred thousand tons of debris plunged down the hill and across the river towards Springfield Mill.
David Tuddenham, the property officer at the mill, was in the store (A) at the time the slide took place and his impressions were of several heavy lorries going past just outside your living room window. It is of interest that he noted both noise and vibration. This has been confirmed by other observers and it strongly suggests that the slide moved with great speed. The site was examined immediately and it was discovered that the toe of the landslide had swept across the river and travelled some 50 yards further, reaching and overwhelming a large building (B) used as a store by Midlothian District Council. The mass of debris now on the east bank, estimated at 5000 tons and containing many full-grown trees uprooted and snapped like twigs blocked the river to a maximum depth of 25 feet, The river was in flood and rapidly built up a large lake behind the dam. There was now the possibility of much greater damage being caused by a dam burst.
The village of Polton now had its first bit of good luck that grim December day. Mr Tuddenham, who is also a civil engineer, rallied the Midlothian District Council staff and those from the other businesses in the various buildings at the mill. They all set to work with a will but already water was flowing around the dam. It flooded through some of the buildings nearest the river to a depth of 3 feet, Worse still, it was pouring down the mill access road and out towards a small estate of new houses (C) in Polton. At this point, the flood water was running parallel to the original river bed, separated from it only by a low but substantial stone wall. Something had to be done quickly. A large fork-lift truck was brought out of one of the buildings and used to smash a hole in the wall (D). With the wall down and a small diversionarv dam built, the river was channelled back to its old course. The danger was further reduced when the River North Esk itself managed to cut a channel through the temporary dam of tangled trees, sand and clay to resume its original bed.
With the help of a large dragline, work began clearing the remainder of the dam from the river and the east bank, even though it was realised that this could disturb parts of the slide higher up and possibly cause a further fall. Within a few days the major portion of the dam had been removed. A month later the site of the smashed building was fully cleared and the only evidence of the landslide was on the west bank of the river. It had been of a composite nature in that it involved both rotational downslope movement of large fairly cohesive blocks of tree-covered gravels and sandy soils in the upper portion, and several smaller mudflows from point sources below the spring line. Movement over the whole area of the slide was uneven. Near the middle of the slope this caused compression ridges and folds at right angles to the direction of movement. The head scarp showed a drop of between 20 and 30 feet and many similar scarps and transverse cracks were visible between it and the river. The sheer power exerted by the slide could be seen where cracks had occurred under the roots of mature trees, ripping them in two from the base of the trunk upwards. In other parts great roots could be seen strained from the ground like giant hawsers trying to anchor an ocean Liner in a storm.
With hindsight, it seems the most likely factor controlling the timing of the landslide was the unusually heavy rainfall, which occurred during the days immediately preceding the event. The situation was undoubtedly aggravated by the nature of the sand and gravel covering the slopes and by the presence of an active spring line midway up the bank. Earthquakes are unlikely to have been a contributing factor in this case since the official records of the period show only a small-magnitude tremor on the 4th December; the now well-known Boxing Day earthquake. which affected much of southern Scotland and northern England, occurred some two weeks after the Polton landslide. (Neilson, 1980).
The sketch map included with this article is a record of the landslide as it existed on 11th January 1980. Comparison of this with the bank as it currently exists will immediately show many changes, mostly caused by fresh falls and an overall creep down-slope. Many of these modifications probably passed quietly and unnoticed but a few were spectacular, such as the muddy slurry, witnessed at close hand by one of us (B-B.), which charged noisily and quickly down the steep 40 foot slope of the escarpment nearest the river at its north end. The Hewan Bank is obviously still very unstable and, short of expensive drainage and replanting work. it is difficult to see how this type of occurrence can be prevented in the future, It may be that the most economical method would be to quarry the remainder of the bank for sand and gravel, thus removing the unstable material.
The authors wish to acknowledge the help of the following for information used in this article:
The Superintendent, the Meteorological Office, Edinburgh. Seismology Unit, Institute of Geological Sciences, Edinburgh. The staff at Springfield Mill, in particular Mr Burnett, Mr Campbell and Mr Tuddenham.
Bolt, B. A., et al, 1975. Geological Hazards, Springer-Verlag. Berlin; Heidelberg, New York.
Edinburgh Evening News, 'Latest' Edition, Monday 10th December 1979. A front page article and photograph (photograph job no. 243552 a-d).
Neilson, C., 1980, The Recent Carlisle Area Earthquakes Edinburgh Geologist, 7, pp. 20-25.
Sissons, J. B., 1967, The Evolution of Scotland's Scenery, Oliver and Boyd, Edinburgh and London.
Figure. Location map
At the time of publication, Bill Baird worked for the Royal Scottish Museum in Chambers Street and John Smellie for the Institute of Geological Sciences in Murchison House.
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