This is the view from Loch na Bràthad, looking roughtly west : that’s the Atlantic Ocean over there. The colours of the hillside are entirely natural – though accentuated by the low winter sun. I’m thinking it likely that you’re eye will be drawn along that intriguing sweep of boulder-free grounds – marked by a swathe of paler grasses. Some of you might wonder if this is a man-made feature, such as an old drove-road long ago fallen into disuse. In fact it is a natural feature, adapted and extended by human ingenuity.
In the middle distance, towards the right, there is a steep-sided ‘valley’ or gully (in geological terms, a graben or half-graben) at the foot of the rocky slopes of the hill (rising another 200m or so to the right), and the broad raised bank feature referred to above. To the left of the bank, the ground first falls away relatively gently – with a smooth profile ; then it falls again steeply to a lower slope (just off the photo) – a great tumble of boulders and broken ground. This is the classic profile of a rotational slip, or landslide. In this case the failure appears to have occurred in a deep wedge-shaped deposit of loose boulders and clay that had accumulated at the foot of the hill, and made worse by steep, smooth-surfaced fault lines in the Lewisian Gneiss bedrock. (You can see exposed rock on the right of the photo). The rotational landslide may have been triggered by a sudden failure of the bedrock along a near-vertical fault. Not recently : unlikely to be less than 500 years ago, but it could have occured a great deal earlier.
The gully is steep sided, with streams flowing from both ends to where it reaches a depth of nearly three metres, and the streams break out of the gully through a break in the bank. On the map you can see these streams, marked in blue, between the words Coire Garbh and Coire nan Cuileag. They carry water draining from Loch na Bràthad, from the hillside above, and from the west too.
Within the graben, tall ferns in vivid greens cluster on the banks of the streams ; tall grasses remain upright – not flattened by winter storms ; the basal leaves of foxgloves await the arrival of spring ; low shrubby trees are entwined with wild honesuckle, and there are bird nests between the branches.
On the map, the blue lines of the streams, and the black lines beyond, correspond to the feature in the photo which I described as partly natural, partly man-made. The black lines are the man-made. On the ground the feature consists of raised bank of earth and stones, and a ditch on the uphill (north) side of the bank. Along the bank, or close to it, occasional steel posts indicate the line of a fence. Native islanders that I’ve spoken to about this feature explain that, back in the days before modern fencing, banks such as these were raised to mark the boundaries between estates or, as in this case, common grazings – to the north being the common grazings of Loch Boisdale, to the south the combined common grazings of Cille Bhrìghde and Taobh a Chaolais. Latterly – probably the late 1940s or the 1950s, the bank was supplanted by a fence, the wires of which are long gone, and some of the posts too. However, as an civil engineer, I notice something remarkable about the bank, which points intriguingly to a possible alternate explanation.
The effect of the natural landslide-gully, combined with its extension east to Loch na Bràthad and west to the ‘Sheepfold’, is to act as what is in land-drainage terms known as a ‘catch-water drain’. This is a drain that cuts across the contours, intercepting surface water running off the hill, and redirecting it to a reservoir or natural loch or watercourse. In some cases the purpose of such an arrangement is specifically to collect water in reservoirs for use as potable water (ie the public water supply) ; but for such drains of the mid 19thC or older the primary purpose may well be two-fold : to reduce the wetness of agricultural land below the catch-water drain ; and secondly to redirect the water towards a watermill, or to augment the supply of water to a canal or other navigation. Clever engineering like this is not an invention of the industrial age : it was practised by mediaeval monastries in developing their estates (there are some excellent examples in northern England). In this case, the catch-water drain redirects surface-water run-off to two lochs : to the west Loch Aiseabhat, and from there to Loch Smercleit and beyond to Tipperton ; and to the south, via Loch a’ Coire to Loch a’ Bhruga, and the sea at Cul-Phort. But why? This would have been very expensive work : not in materials, but as very considerable labour over a number of years, in a local economy that struggled to provide just the essentials of life.
My informed hunch is that this drain would have been constructed in the late 18thC at the direction and expense of MacDonald of Boisdale, the local laird for whom our walled garden was originally built around 1740-1742. His was the age of enlightenment, rationality and – above all – of agricultural improvement! Later – perhaps as late as the 1890 or even just before the Great War (the period when crofting townships and common grazings were being re-established in more-or-less their present-day form) the bank associated with this catch-water drain would have very conveniently adopted as boundary between two common grazings. After the Second World War, there were too few men left in the islands to maintain these boundaries as effective barriers to cattle and sheep, and with generous government grants available for fencing, the old bank of soil and stone was supplanted by steel posts and wire. By then, the original purpose of the bank and ditch had been forgotten.
Unfortunately, most of the historical papers of the Boisdale estate were lost in a fire, some time in the 20thC, so my ruminations on the intriguing features of the hillside above our walled garden are unlikely to be subjected to the scrutiny of proper historical research.