A reconstruction of subglacial processes based on a classification of erosional forms at Ramsvikslandet, SW Sweden

Abstract: Ramsvikslandet on the Swedish west coast is characterized by exposed bedrock hills with numerous glacial erosional forms. Previous research in the area has mostly focused on individual forms and their relation to the larger forms, such as roches moutonnées. This study takes a more large-scale perspective and aims to reconstruct the subglacial processes based on the erosional forms and their distribution in the study area. In the reconstruction, both the distribution of the forms in relation to other forms and their distribution in relation to the large bedrock hills is considered. The erosional forms found in the area are: striae, crescentic fractures, crescentic gouges, pits, triangular pits, concoidal fractures, troughs, sickle troughs, comma forms, bowls, potholes, channels, roches moutonnées, faceted roches moutonnées (roches moutonnées with a sloping lee side), whalebacks and rock steps. The results show that the proximal slopes of bedrock hills are mostly dominated by roches moutonnées, striae, crescentic fractures, crescentic gouges and p-forms. Abrasion has been the dominating process here and the effective normal pressure on contact areas has been high. The subglacial water was concentrated in channels and had a high trough-flow velocity. The top surfaces and distal slopes of the rock hills are most commonly plucked, with rock steps descending in the direction of ice movement. Melt-water erosion was negligible since the water flowed in a linked-cavity system with low through-flow velocities. Large cavities made the ice-bed contact area smaller than on the stoss side. Towards the end of the bedrock hills the roches moutonnées normally reappear, these can be either classic roches moutonnées, but also faceted roches moutonnées and whalebacks. Crescentic fractures, crescentic gouges and p-forms also reappear again, indicating a channelized melt-water flow and abrasion as the dominating process with high contact forces and large contact areas. The abundance of p-forms and the many signs of cavities in the study area show that there has been much water under the ice. Ramsvikslandet is also part of a larger area that almost lacks till, beginning at the Vänern basin and widening towards the coast. This leads to the speculation that the water could have originated in the Vänern basin and with a continuous output could have created a fast-flowing ice stream towards the coast, sweeping away the till material and supplying the water for the formation of the p-forms.