Characteristic features and formation processes of ribbed moraines

Ribbed moraine is a subglacially formed moraine landform type which can be found from Fennoscandia, Ireland, Iceland and North America. They occur as a group of moraine ridges which have oriented transversal to the latest ice flow direction. Theories for origin and formation of the ribbed moraines have been presented since 1950s but are still under debate. In this paper, characteristic features and formation theories of ribbed moraines will be discussed and reviewed with emphasis on the latest observations. Based on numerous published studies, it is evident that the formation of ribbed moraines has occurred during active ice movement under subglacial conditions, i) where ribbed moraines have typically formed extensive moraine ridge fields, ii) where the moraine ridges have a uniform transverse ridge orientation, iii) good and uniform orientation of pebbles in the moraine in the direction of the ice flow, and iv) an internal structure including active stratification, shear structures, sand banding, and v) enrichment of local fine material and rock fragments in the uppermost till layer. One notable feature, particularly observed in Finland, is the occurrence of large, angular boulders on the surface of ribbed moraines. Their composition varies rapidly, reflecting the composition of the underlying bedrock and indicating a very short glacial transport distance. Ribbed moraines can be typically found in the close vicinity of centres of the last continental glaciers. The formation process favoured certain subglacial condition in the contact zone of cold-based and warm-based glaciers. It seems that suitable formation conditions could have arisen as a result of rapid climate changes at the end of the cold Younger Dryas period.