S. 2008. Stone run (block stream) formation in the Falkland Islands over several cold stages, deduced from cosmogenic isotope ( 10 Be and 26 Al) surface exposure dating.ABSTRACT: Cosmogenic isotope ( 10 Be and 26 Al) surface exposure dating has been applied to valley-axis and hillslope stone runs (relict periglacial block streams) and their source outcrops in the Falkland Islands, South Atlantic. The data indicate that stone runs are considerably older landforms than previously envisaged and afford no evidence that they are a product of the Last Glacial Maximum; the samples range in apparent 10 Be age from 42k to 731k yr BP, but some of these are minima. The results indicate that valley-axis stone runs may be up to 700-800k yr old, have simple exposure histories and are composite landforms that developed over several cold stages. Analyses of some hillslope and outcrop samples also demonstrate simple exposure histories with 10 Be ages from 42k to 658k yr BP. In contrast, isotopic ratios from other hillslope and outcrop samples reveal they have had a complex exposure history involving periods of burial or shielding; the samples range in 10 Be age from 59k to 569k yr BP and these are regarded as minimum age estimates. Larger stone runs may be older than smaller runs and there is a possibility that stone runs older than 800k yr exist in other parts of the Falklands. The assertion that glaciation in the Falklands was restricted to the highest uplands is supported by the data, and the potential for age determination of other boulder-strewn and bedrock landforms, using cosmogenic isotope analysis, in order to extend the geochronology of Quaternary events and processes is noted.

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Miniature sorted patterned ground features (nets and stripes) occur at an elevation of ca. 35 m in the L'Antioja valley, East Falkland. The features have developed following recent soil erosion and display many of the characteristics associated with the operation of frost-related processes in regolith materials. The patterns are currently active and indicate that in cool oceanic climates the distribution of frost-sorted patterns is controlled by the presence of bare ground on suitable regolith rather than by altitude.

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