Ribbed moraine stratigraphy and formation in southern Finnish Lapland

Abstract

AbstractCharacteristics of ribbed moraines, the dominating moraine type in southern Finnish Lapland, have been studied in detail. The ridges are composed of several till units, of which the bottommost units consist of mature basal tills and the surficial parts are enriched with local, short‐transport rock fragments and boulders in till and at the surface of ridges. As a result of this re‐examination a two‐step model of the formation process of ribbed moraines is presented. In the first stage, while cold‐based conditions prevailed, both the bottommost part of the ice sheet and the frozen, substrate fractured under compressive ice flow. Following glacial transport of fractured blocks and formation of the transverse ridge morphology, erosion between the ridges continued owing to freeze–thaw process under variable pressure conditions. In the areas with a low pre‐existing till sheet, the process caused quarrying of the bedrock surface and subsequent deposition of rock fragments and boulders under high pressure on the next ridge. The most suitable conditions for ribbed moraine formation existed during Late Weichselian deglaciation, after the Younger Dryas when the climate warmed very quickly, leading to an imbalance between a warm glacier surface and a cold base. Copyright © 2006 John Wiley & Sons, Ltd.