Subglacial water‐escape structures and till structures, sléttjökull, Iceland

Beyond Sléttjökull, the north part of the ice cap Mýrdalsjökull, south Iceland, an undulating till plain is dissected by meltwater streams. Throughout a transect from the ice margin to the 1900 ice‐marginal moraine, a distance of about 1.2 km, finely laminated wedge‐shaped water‐escape structures (WES) are present, originating in the till and disappearing down‐ice into the underlying sands and fine gravels. A second set of much smaller and deformed black WES occurring within the till has been produced by upward injection 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 to its bed and the frozen bed material is more than 2 m thick. During the relatively cold period around the turn of the century and until the late 1920s, a similar submarginal body of permafrost must have existed. The sequence reveals that subglacial meltwater, attributed to drainage following the 1918 Katla eruption, was blocked by a frozen glacier toe, causing the formation of water‐escape structures. Subsequently, glacier flow deformed the substratum and produced the threefold division of the till bed. The study has important implications for theories envisaging subglacial floods because it demonstrates that it is very difficult for temperate glaciers with a temporary frozen bed in the toe region to retain subglacial meltwater. Copyright © 1999 John Wiley & Sons, Ltd.