Deglaciation pattern and dynamics in the Bolmen area, southwestern Sweden

Sammanfattning: The Late Weichselian deglaciation of an area in southwestern Sweden was reconstructed by geomorphological, sedimentological and lithostratigraphical methods. The morainic deposits were mapped and classified into three landsystems: Streamlined terrain, hummocky moraine and cover moraine. Streamlined terrain was assumed to have been formed subglacially by a wet-based ice sheet. Cover moraine was interpreted to consist of redeposited sediments which have melted out supraglacially from a debris-rich ice. Hummocky moraine was formed by supraglacial melt-out and flowage of glacial debris into depressions and subsequent inversion of the landscape due to melting of ice cores. A model is presented where difference in morphology between hummocky and cover moraine is explained by differences in distribution of glacial debris in the stagnant ice. This distribution depends on the flow regime of the ice before stagnating. Hummocky moraine developed where the ice had compressive flow before stagnating and glacial debris was elevated to high levels in the ice by thrusting and folding of debris-rich basal ice. Cover moraine was formed where the ice had an extending or steady state flow before stagnating and the debris-rich ice was concentrated near the base of the ice. An east-west trending end moraine was mapped and sedimentologically investigated. The moraine was found to be mainly composed of ”ice slope colluvium” deposited in front of an active ice front which had readvanced from the north. The moraine was connected to the west to a previously known end moraine named the Torup moraine and the combined moraine was named the Torup-Unnaryd moraine. A 40 km long southward continuation of the Gothenburg moraine on the Swedish west coast was found and based on geomorphological evidences it was correlated with the Torup-Unnaryd moraine. The total extension of this recession line supported by firm geomorphological evidences is 145 km.