Subglacial water-escape structures and till structures, Slettjokull, Iceland

Beyond Slettjokull, the north part of the ice cap Myrdalsjokull, south Icel and, an undulating till plain is dissected by meltwater streams. Throughout a transect from the ice margin to the 1900 ice-marginal moraine, a distanc e of about 1.2 km, finely laminated wedge-shaped water-escape structures (W ES) are present, originating in the till and disappearing down-ice into the underlying sands and fine gravels. A second set of much smaller and deform ed black WES occurring within the till has been produced by upward injectio n into the till as a response to stiffening of the till after dewatering by the downward developing WES. At present the toe of the glacier is frozen t o its bed and the frozen bed material is more than 2 m thick. During the re latively cold period around the turn of the century and until the late 1920 s, a similar submarginal body of permafrost must have existed. The sequence reveals that subglacial meltwater, attributed to drainage following the 19 18 Katla eruption, was blocked by a frozen glacier toe, causing the formati on of water-escape structures. Subsequently, glacier flow deformed the subs tratum and produced the threefold division of the till bed. The study has i mportant implications for theories envisaging subglacial floods because it demonstrates that it is very difficult for temperate glaciers with a tempor ary frozen bed in the toe region to retain subglacial meltwater. Copyright (C) 1999 John Wiley & Sons, Ltd.