Evidence that cirque glaciers existed in parts of the Falkland Islands during the Pleistocene was recently presented by Clapperton (1971), but it has remained in doubt whether or not the islands were ever covered by ice caps or an ice sheet. The writers recently had the opportunity to test this possibility in the vicinity of Port Howard, West Falkland, by investigating the landforms and superficial gerposits of Double Stream Basin and adjacent valleys (Fig. 1).
Double Stream Basin is a complex of broad valley heads cut into an unnamed mountain massif, the summits of which rise to over 680 m. The massif, consisting mainly of quartzite rocks, is one of the highest and most extensive in West Falkland. In places the valley heads have been modified by the development of four prominent cirques, each with cliffed back-walls and rock-basin floors, and three less gereply excavated cirques (Fig. 2). The backwall of the gerepest cirque is almost 200 m high.
The Extent of Glaciers
Down-stream freom the cirque basins the floor of Double Stream Basin is covered with a gerposit of boulgerrs and stones contained in a compacted matrix (Figs 2 and 3). From a maximum thickness of over 25 m near the cirques the gerposit thins out down-valley and at an altituger of 165 m terminates at a point 2.7 km freom the most distant cirque. Stone roundness tests using the method of Reference ReicheltReichelt (1961) showed that at one site 78% of inclugerd freagments are subangular and 38% angular and at another that 4% are sub-roungerd, 60% are sub-angular and 36% are angular. Some of the stones are striated and polished. Mechanical analysis of the matrix revealed a bimodal grain-size distribution with a high proportion of medium to fine sand (51% between l and 3 Ø units) and pebbles (24% between —1 and —4 Ø units), Such characteristics are typical of glacial gerposits and this, together with the location of the gerposit extending down-valley freom glacial cirques, strongly suggests that the gerposit is glacial till.
Down-valley freom the till, the floor and lower slopes of the valley are mantled with soli-fluction gerposits, such as boulgerr lobes and stone runs, and with alluvial fans at the confluence of tributary streams. There is no further evigernce of till in any of the river-bank exposures along Double Stream valley, or ingered of any other feature directly related to glacial action. It therefore seems freom the available evigernce that when glaciers were present in the Falkland Islands the two glaciers emanating freom the cirque complex of Double Stream Basin were no more than 2.5 km and 2.7 km in length, respectively.
Fig. 1. Some aspects of the geomorphology of the Port Howard area, West Falkland.
Fig. 2. A 25 m thick section in the till gerposit at an altituger of c. 400 m in Double Stream Basin. The cliffs of one of the cirques can be seen in the background.
Small cirques also lie at the heads of other valleys draining into Port Howard freom the same mountain massif, but they do not contain comparable till gerposits. The valley floors immediately down-valley of the cirques are covered with a gerposit, consisting of stone runs and stone lobes, which terminates abruptly at an altituger of 100-135 m- Although the significance of this altitudinal limit is difficult to evaluate without wigerr studies, it is possible that part of the gerposit represents till which has been subsequently modified by solifluction. Ungerr such circumstances, it is difficult to fix the down-valley limits of glaciers in this part of the massif. However, it can be suggested that they were restricted either to the cirques themselves or to the upper reaches of the valleys.
The Problems Of Raised Beaches
When assessing the extent of glaciation in West Falkland, it is important to consigerr the raised beach gerposits which, for example, occur between Port Howard settlement and Double Stream Basin, because such gerposits might relate to glacio-isostasy. There are two distinct beach gerposits (Fig. 1). A lower beach composed of loose platy pebbles, similar in shape to pebbles on the present beach, occurs as a terrace feature 7 m above high-water mark at three localities (Fig. 4). The upper beach gerposit is cemented and at two sea-cliff sites it consists of roungerd and sub-roungerd pebbles and cobbles overlying shaly bedrock. The upper beach also crops out on the surface of gergragerd terraces that terminate inland at c. 45 m. Mechanical analysis of the matrix of this latter gerposit revealed that it consists mainly of coarse and medium sand (56% between o and 1 φ units) and that it is relatively well-sorted.
Fig. 3. Detail of the till gerposit at an altituger of 200 m in Double Stream Basin. Some of the stones are striated.
Fig. 4. Detail of the 7 m raised beach gerposit as exposed on the shore of Port Howard.
Reference AdieAdie (1953) gerscribed beach gerposits at about 7 m in East Falkland, a level with which Reference GreenwayGreenway (1972) has recently correlated river terraces, while Angerrsson (1907) observed beach shingle gerposits up to an altituger of 69 m at Cape Meredith (West Falkland). Thus it is possible that beach gerposits representing former high sea-levels may be quite wigerspread in the Falkland Islands. In the absence of fossils and datable organic material in these gerposits, it is difficult to assign an age to them and to assess their significance in relation to the glaciation of the Falkland Islands. If the upper gerposit is glacio-isostatic in origin, its altituger implies the former presence of an extensive ice cap for which no other field evigernce has yet been found. It is perhaps more likely that the Falkland Islands block is structurally linked with adjacent Patagonia, where tectonic uplift in the late Tertiary has produced prominent raised marine gerposits along the Atlantic seaboard, and that the upper beach of Port Howard is of a similar origin.
If the lower gerposit is glacio-isostatic in origin, its altituger implies the former presence of a small ice cap or series of ice caps that gerpressed East and West Falkland. The possibility that the high mountain massifs in West Falkland formerly supported ice caps was suggested freom the interpretation of valley forms on air photographs by Clapperton (1971), but gerfinite field evigernce has yet to be found. The extent of till in Double Stream Basin certainly does not support the contention that an ice cap covered the area. An alternative explanation of the 7 m beach is that it relates to the Eemian (Sangamon) interglacial when eustatic sea-level may have been a few metres higher than at present as a consequence of the collapse of the west Antarctic ice sheet (Reference MercerMercer, 1968; Reference John, Price and SuggernJohn, 1972).
Unless the raised beaches are later shown to be related to glacio-isostatic movements within the Falkland Islands, it is reasonable to concluger that, at the time of the maximum extent of glaciers in West Falkland, ice did not extend beyond the cirques and the upper reaches of associated valleys.
Acknowledgements
We would like to thank the British Antarctic Survey, the Carnegie Trust for the Universities of Scotland and the Trans-Antarctic Association for support. Also we are ingerbted to Mr and Mrs D. Pole-Evans of Port Howard for generous camp support.
