Morphology, Stratigraphy, and Genesis of Small Drumlins in Front of the Glacier Mýrdalsjökull, South Iceland

Abstract

AbstractInvestigations have been made of the geomorphology, internal structure, and till fabric of small drumlins in a drumlin field exposed in front of the retreating northern part of Mýrdalsjökull, Iceland. The drumlins either comprise irregular drumlin complexes or they show clearly the shape of typical drumlins with their highest points at the up-glacier ends of streamlined hills.The core of each drumlin consists either of undisturbed glacio-fluvial deposits or glacio-dynamically deformed drift. The distribution of the first type often has a characteristicen échelonarrangement, similar to the interstream areas in the upper reaches of outwash fans. The second type forms a pattern with a predominant trend conforming to the glacier margin; this type is superimposed on overridden ice-margin push-moraine ridges. It is shown that the material in the drumlin cores is only slightly eroded by the glacier that formed the drumlins. The core is mantled by subglacial lodgement till about 0.1–1.5 m in thickness.Seventeen fabric analyses of 25 clasts each were performed on material from within the till mantle. These analyses show a preferred long-axis orientation but indicate a considerable between-site variability ranging up to 45° even between samples taken a few metres apart. The preferred clast orientation for samples taken along the drumlin crests only deviates 0–10° from the regional ice-flow direction indicated by fluted moraines, whereas the deviation for samples from the flanks and the stoss-sides is up to 35°. However, the fabrics show a characteristic pattern relative to the shape of the drumlin; on their tops, the clast fabric parallels the ice-flow direction, whereas it tends to follow the contour direction on the flanks and stoss-sides.It is concluded that the cores of the drumlins consist of pre-existing deposits, whereas the mantles are composed of subglacial till. Regarding the processes involved, the material contained in the core is mainly eroded by pro-glacial melt-water streams and not by ice. During the subsequent flow of ice across the area, the most prominent terrain elements have acted as subglacial obstacles, leading to localized till deposition and drumlin formation. Thus, the drumlins were formed mainly by subglacial deposition of till but the obstacles acted as an essential factor favouring their initiation.