The Edinburgh Geologist
Issue no 35

Periglacial Princes Street - 52° South

by Phil Stone


Imagine yourself on Princes Street in February. A stiff westerly wind rattles the hail around your collar and there's not a bus in sight. That might sound a familiar situation to many members of the Edinburgh Geological Society but this particular Princes Street is about 8000 miles away in the Falkland Islands. The name was transplanted by an émigré Scot but, apart from the fact that both versions run east-west, it is quite hard to see what similarities provoked the association with home (55° 56' North). The southern Princes Street is actually an enormous periglacial boulder field, the largest of the famous Falklands 'stone runs'. These are more-or-less flat-topped spreads of large quartzite blocks that fill many of the valleys or blanket hillsides. The apparently uniform surface level belies the more local structure since many boulders are balanced precariously and gaping holes open out downwards, whence rises the sound of running water. In many places parallel, linear zones of boulders, up to 5 m across, and more than 100 m long in places, alternate with similar-sized strips of vegetated ground. Repeated scores of times across the hillside these produce a bizarre, striped landscape.

The boulder fields and stripes certainly impressed and puzzled all of the early scientific visitors to the islands. One such was Charles Darwin who, in 1845, wrote in the second and enlarged edition of his Journal of Researches:

In many parts of the islands the bottoms of the valleys are covered in an extraordinary manner by myriads of great loose angular fragments of the quartz rock, forming 'streams of stones'... the blocks are not waterworn, their angles being only a little blunted; they vary in size from one or two feet in diameter to ten, or even more than twenty times as much. They are not thrown together into irregular piles, but are spread out into level sheets or great streams.

Darwin was probably describing the Princes Street boulder field when he wrote this. He certainly walked across it and a little later, after a visit to the Falklands by the 1901-1903 Swedish South Polar Expedition, Professor J. Andersson described it as the 'Darwin stone-river', although he also reported that 'an old Scottish shepherd... named with rustic humour this vast and almost impassable  accumulation of millions of huge quartzite blocks Princes Street'. Plenty of Scots were certainly involved in the exploration and colonisation of that part of the world and many family names are remembered geographically, together with the possibly generic Mount Jock. Perhaps surprisingly, there are very few other examples of transplanted Scottish place names, with Dunbar and Douglas about the only possible competitors for Princes Street.

Since the Darwin and Andersson accounts the vernacular term 'stone run' has been generally adopted to describe these distinctive Falklands features. However, no other stone run has managed to acquire its own unique name although the landform is pretty widespread and there are other sizeable examples. The stone runs occur across both of the main islands, West and East Falkland, and are principally associated with the outcrop of one particular rock type, quartzite. This occurs at two stratigraphical levels within the Port Stanley and Port Stephens formations. Falklands stratigraphy has only recently been formalised following a comprehensive survey programme carried out by the British Geological Survey on behalf of the Falkland Islands Government. The survey work was led by Don Aldiss, from BGS, assisted by Emma Edwards, a Falkland Islander and geology graduate; this article draws heavily on their work. Don and Emma established the Port Stanley Formation as the highest part of the largely Devonian West Falkland Group. The Formation consists of pale grey, very hard quartzites with subordinate, softer siltstone and rare mudstone. At the bottom of the West Falkland Group, the Port Stephens Formation contains somewhat similar but rather more feldspathic quartzites and may range down into the Silurian. The Port Stanley Formation forms much of the high ground on East Falkland where it gives rise to the most extensive stone runs. At some 4 km long and up to 400 m wide, Princes Street is the largest of these and lies about 20 km north-west of the Falkland Islands' capital, Stanley.

The Falklands stone runs seem to be unique in terms of their variety, size and abundance but similar, more restricted examples are known from elsewhere in the World.  Numerous theories have been proposed for their origin (Darwin put their formation down to earthquakes) but it is now generally accepted that the stone runs formed during the last Ice Age, about 26,000 to 13,500 years ago, as a result of intense frost action alternating with periods of thaw. During that time the Falklands were largely free of glaciers, with the exception of a few occupying small cirques on the highest hills, but the islands were subjected to a savage polar climate. Hence the stone runs are relict landforms, produced by mass-movement under periglacial conditions.

Many features are shared by all of the stone runs, regardless of their size, form or situation.  They are almost entirely composed of locally-derived quartzite blocks.  Most blocks seen at the surface are between 30 centimetres and two metres across, and rarely up to five metres long; the range of block size can vary locally, however.  Some blocks in stone runs are equant, but most are tabular or elongate, their shape and size reflecting the common distribution of joints and bedding planes in nearby quartzite exposures.  The blocks tend to be fairly angular, with only slightly rounded corners and little other sign of abrasion. In many stone runs the blocks are randomly arranged but some display a marked fabric in which the tabular blocks are packed together on edge. These blocks tend to be orientated parallel to the slope and are usually seen near the margins of a stone run. Excavations show that the largest boulders form the top part of the stone run with the size of the blocks then decreasing downwards. This attribute was utilised by the British troops advancing against the occupying Argentineans during the 1982 conflict. The soldiers realised that if a large block could be displaced from the top of a stone run, then the smaller cobbles underneath could be readily removed to create a commodious dug-out. This exercise was made easier by the absence of matrix material right down to the base of the stone run, where the basal blocks lie abruptly on unsorted regolith or, more rarely, on bedrock.  Although boulders in the upper parts of undisturbed stone runs are uniformly pale grey in colour, lower down in the stone run, where they have been in long-term contact with water but have otherwise been protected from the weather, the boulders and cobbles are invariably stained by iron oxides. This was the disadvantage of the stone-run dug-out; the piles of excavated orange-brown cobbles gave away its position!

During the recent mapping project in the Falkland Islands Don Aldiss had ample opportunity to study the stone runs and concluded that at least five processes were involved in their formation: weathering, solifluction, frost-heave, frost-sorting and washing.  In addition to the hard, white quartzite, both the Port Stanley Formation and the Port Stephens Formation include feldspathic sandstones, with some siltstones and mudstones.  These latter rock types would be readily broken down, by frost and chemical weathering, to sand and clay whereas the hard quartzite would survive as large boulders.   As this unconsolidated mixture was subjected to repeated freezing and thawing it would gradually creep downhill, a process known as solifluction.  At the same time, frost heave would tend to move the quartzite blocks towards the surface of the deposit, and frost-sorting would cause them to be grouped together. On level ground, frost-sorting can give rise to polygonal patterns but even on the slightest slope these become elongated and pass into stripes. The width of the stripes generally increases with clast size and so the availability of abundant large boulders was probably crucial for development of the exceptionally large Falklands examples. Between the stripes and beyond the limits of the boulder fields isolated blocks lie in a heterogeneous mixture of clay, sand and angular pebbles. There is barely any gradation between the two deposit types, nor is there any difference in their surface levels beyond that created by the vegetation cover over unsorted ground.

The complete removal of matrix from the stone runs is the outstanding enigma of their formation. Much of it was probably washed out progressively by rain or by streams flowing within the stone run, but Don Aldiss considers that the earlier, gradual replacement of the clay-rich matrix by ice also played a part.  He points out that rock tends to have a higher thermal conductivity and a lower heat capacity than an adjacent moist, fine-grained matrix.  So, as the ground freezes, the freezing front will advance more rapidly through the blocks than the matrix. Ice formed under these conditions around the blocks will effectively push away the fine-grained matrix and the smaller stones. The combination of repeated freeze-sorting and washing during periods of thaw eventually generated ice-bound concentrations of large blocks; these became the stone runs. As the climate became milder the intervening areas still underlain by the heterogeneous, clay-rich, solifluction regolith were preferentially colonised by plants. In this way the vegetation now accentuates very effectively the patterns originally produced during the Ice Age.

Finally, some corrective action is necessary. The first two sentences of this article might have given the impression that summer in the Falkland Islands leaves something to be desired. The odd hail squall straight from the Antarctic is certainly a possibility but, in general, Falklands weather has received an unjustifiably bad press. It all started with Darwin who wrote of 'miserable islands... with a desolate and wretched aspect', but anyone familiar with summer in the Hebrides would feel quite at home, though the Falklands are drier and windier. They also have one other attribute to gladden the heart of any Scottish field geologist ? nae midges! What's more, Princes Street - 52° South, has no bus lanes, traffic wardens, burger bars, Big Issue vendors...

For more information on the Falkland Islands stone runs you could try:

Aldiss, D.T. & Edwards, E.J. 1999. The geology of the Falkland Islands, British Geological Survey Technical Report WC/10/99, pp. 97-103.

Clark, R., Edwards, E., Luxton, S., Shipp, T. & Wilson, P. 1995. Geology in the Falkland Islands, Geology Today, Volume 11, pp. 217-223.

Rosenbaum, M. 1996. Stone runs in the Falkland Islands, Geology Today, Volume 12, pp. 151-154.

The new 1:250 000 solid geology map for the Falkland Islands (on two sheets) is available through the British Geological Survey or from the Department of Mineral Resources, Ross Road, Stanley, Falkland Islands. Price £20. Highly recommended.

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