De Geer moraines in Finland and the annual moraine problem

Abstract

The study has two main aims: (1) a description of the characterizing features of the morphology, material structure and mode of occurrence of De Geer moraines in Finland, and (2) conclusions about the depositional mechanism and the geo­chronological significance of the moraines. Thereby, it strives towards a defi­nition of the term 'De Geer moraine'. In doing so, both the previous term 'an­nual moraine' and the use of these landforms in deglaciation reconstructions are examined.

Main field investigation methods included petrographic, granulometric, mor­phometric and two‑ and three‑dimensional orientation analyses, carried out in the study area in Hyvinkää and other important occurrence areas. Distances between successive De Geer moraines in Finnish moraine fields were measured cartometrically, and the values later combined with corresponding information on moraines in, above all, Sweden and Canada. Seasonal rhythm in moraine dis­tances is found only exceptionally by comparison with clay varve data on the ice‑marginal rate of retreat. The common average distance between De Geer mo­raines in Finland, approximately 100 m, does not reflect successive positions of the ice margin during the course of deglaciation. Neither do the deposits show any other features typical of end moraines.

All De Geer moraines of any demarcated group of ridges were deposited al­most simultaneously during a quiescent phase of ice flow, as the ice subsided into a water‑soaked subglacial layer of till. The basal crevasses, formed during the previous quick flow of the ice mass, were filled by material resembling ground moraine being squeezed up from both sides into the cavity. The material redis­tribution resulted in very similar average distances in all moraine fields. Condi­tions were particularly favourable for so‑called glacial surges within a 10–15 km wide zone inside the ice margin in the vicinity of interlobate areas, exceed­ingly rich in melt‑waters. Long‑axis orientations of De Geer moraines thus indi­cate the influence of both substratum and subglacial drainage systems on basal crevasse development. The deposits – kinds of 'crevasse fillings' – were not deformed by subsequent ice flows, as the ice front was usually broken up by a considerable body of proglacial water.