Stone run
The stone runs of the Falkland Islands are a remarkable feature of the landscape. Such stone runs, which are also called "stone river", "stone stream" or "stone sea" are found in very few places in the world. This is a conspicuous rock landform, the result of the erosion of particular rock varieties caused by myriad freezing-thawing cycles taking place in periglacial conditions during the last Ice Age.[1]
Stone runs are accumulations of boulders with no finer material between them. In the Falklands, they occur on slopes of between 1 and 10 degrees, and are the product of mass-movement and stone sorting during past periods of cold climate. They everywhere occur in association with poorly-sorted, clay-rich solifluction deposits [2]
Stone runs occur in southern Britain too, most notably at the Stiperstones in Shropshire.[3][4]
Falklands stone runs
An early description of the Falklands stone runs was given in Antoine-Joseph Pernety’s account of his exploration of the islands during the 1763–64 French expedition under Louis Antoine de Bougainville, which established the Port Saint Louis settlement on East Falkland. While crossing the neck between Baye Accaron (Berkeley Sound) and Baye Marville (Salvador Water) he described in detail two particular stone features he called ‘City Gates’ and ‘Amphitheatre’:
| “ | We were no less astonished at the sight of the infinite number of stones of all sizes thrown one upon another, and yet ranged as if they had been piled negligently to fill up some hollows. We admired with insatiable delight the prodigious works of Nature.[5] | ” |
Pernety’s observations were continued by young Charles Darwin, who visited the Falklands in 1833 and 1834:
| “ | Pernety has devoted several pages to the description of a Hill of Ruins, the successive strata of which he has justly compared to the seats of an amphitheatre. (...)
In many parts of the island the bottoms of the valleys are covered in an extraordinary manner by myriad great loose angular fragments of the quartz rock, forming "streams of stones." These have been mentioned with surprise by every voyager since the time of Pernety. 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. It is not possible to ascertain their thickness, but the water of small streamlets can be heard trickling through the stones many feet below the surface. The actual depth is probably great, because the crevices between the lower fragments must long ago have been filled up with sand. The width of these sheets of stones varies from a few hundred feet to a mile; but the peaty soil daily encroaches on the borders, and even forms islets wherever a few fragments happen to lie close together. In a valley south of Berkeley Sound, which some of our party called the "great valley of fragments," it was necessary to cross an uninterrupted band half a mile wide, by jumping from one pointed stone to another. So large were the fragments, that being overtaken by a shower of rain, I readily found shelter beneath one of them. Their little inclination is the most remarkable circumstance in these "streams of stones." On the hill-sides I have seen them sloping at an angle of ten degrees with the horizon; but in some of the level, broad-bottomed valleys, the inclination is only just sufficient to be clearly perceived. On so rugged a surface there was no means of measuring the angle; but to give a common illustration, I may say that the slope would not have checked the speed of an English mail-coach. In some places, a continuous stream of these fragments followed up the course of a valley, and even extended to the very crest of the hill. On these crests huge masses, exceeding in dimensions any small building, seemed to stand arrested in their headlong course: there, also, the curved strata of the archways lay piled on each other, like the ruins of some vast and ancient cathedral. (...) These scenes are on the spot rendered more striking by the contrast of the low, rounded forms of the neighbouring hills.[6] |
” |
The Falklands stone runs are made up of hard quartzite blocks. They are more widespread and larger on East Falkland, especially in the Wickham Heights area where the largest of them extend over three miles in length. Those on West Falkland and the minor islands are fewer in number and of smaller dimensions. Darwin's "great valley of fragments", subsequently renamed Princes Street Stone Run after Edinburgh's Princes Street[7] that was cobbled at the time, occupies a 2½ miles long and 450 yards wide shallow valley trending east-west. The feature is situated off the road to Port Louis, some 12 miles northwest of Stanley.[8][9][10][11]
Geology
The stone runs of the Falkland Islands are the best known for their exceptional diversity, size and abundance, though approaching them are the stone rivers of Vitosha Mountain in Bulgaria. The highly specific combination of particular climatic conditions and rock varieties that existed in each place during the Quaternary Period explains both the formation of stone runs in those two territories, and their absence in areas with otherwise comparable nature conditions.
The actual formation of stone runs involved no less than five processes: weathering, solifluction, frost heaving, frost sorting, and washing.[7] The stone runs are essentially different from moraines, rock glaciers, and rock flows or other rock phenomena involving the actual flow of rock blocks under stress that is sufficient to break down the cement or to cause crushing of the angularities and points of the boulders.[12] By contrast, the stone run boulders are fixed quite stably, providing for safer climbing and crossing of the run.
The present Falklands climate is quite similar to that of the Highlands in Britain, but the Highlands were completely glaciated during the Ice Age, which would not allow for the formation of stone runs. The Falkland Islands however were more temperate; the right periglacial conditions and rock composition were present to allow the weathering and freeze-thaw to create the runs.[13]
Outside links
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References
- ↑ Mather, Kirtley F. (1967). Source Book in Geology, 1900-1950. Harvard University Press. pp. 125–129.
- ↑ ALDISS, D T, and EDWARDS, E J. 1999. The Geology of the Falkland Islands. British Geological Survey Technical Report, WC/99/10.
- ↑ GOUDIE, A S, and PIGGOTT, N R. 1981. Quartzite tors, stone stripes, and slopes at the Stiperstones, Shropshire. Biuletyn Periglacjalny, Vol. 28, 47-56.
- ↑ CLARK, R. 1994. Tors, rock platforms and debris slopes at Stiperstones, Shropshire. Field Studies, Vol. 8, 451 - 472.
- ↑ Dom Pernety, Antoine-Joseph. Journal historique d'un voyage fait aux Iles Malouïnes en 1763 et 1764 pour les reconnoître et y former un établissement; et de deux Voyages au Détroit de Magellan, avec une Rélation sur les Patagons. Berlin: Etienne de Bourdeaux, 1769. 2 volumes, 704 pp. Online vol. 1 & vol. 2, in French. Abridged English version.
- ↑ Darwin, C.R. Journal of researches into the natural history and geology of the countries visited during the voyage of H.M.S. Beagle round the world, under the Command of Capt. Fitz Roy, R.N. 2nd edition. London: John Murray, 1845. Scanned by John van Wyhe 2006. pp.196-98.
- ↑ 7.0 7.1 Stone, Phillip. Periglacial Princes Street - 52° South. The Edinburgh Geologist. Issue No 35, 2000.
- ↑ Andersson, J.G. 1907. Contributions to the geology of the Falkland Islands. In: Wissenschaftliche Ergebnisse der Schwedischen Südpolar-Expedition 1901-1903, ed. O. Nordenskjöld, Stockholm, Vol. 3.
- ↑ Onshore Geology – the stone runs. British Geological Survey website.
- ↑ Aldiss, Donald and Phillip Stone. The Falkland Islands Stone Runs. Falklands Island Government and British Geological Survey Publication, 2001.
- ↑ Rosenbaum, M.S. Stone runs in the Falkland Islands. Geology Today. Vol. 12, 1996. pp. 151-54.
- ↑ Harris, Stuart A. Climatic Zonality of Periglacial Landforms in Mountain Areas. Arctic, Vol. 47, No. 2, June 1994, pp. 184-92.
- ↑ Vitosha Nature Park: Management Plan 2005-2014. Ministry of Environment and Waters, Sofia, 2005. (in Bulgarian).
