Den fossila periglaciala morfologin i södra Finland

Abstract

Aartolahti; Toive: Den fossila periglaciala morfologin i södra Finland. Geografisk Tidsskrift 82: 74–77, Copenhagen, October 15, 1982.

Fossil patterned ground, boulder depressions, tundra polygons with ice-wedge casts, sand wedges, structures interpreted as periglacial involutions, periglacial dunes and strings on peat bogs have been found in southern Finland. The presence of ice-wedge casts and involutions on the top plateaus of Salpausselkä end moraines give evidence that permafrost conditions have prevailed here. These periglacial phenomena are dated to the Younger Dryas period and to the beginning of Preboreal period.

SUMMARY

Periglacial formations have been studied in southern Finland only during the last ten years. Fossil patterned ground formations (sorted and nonsorted nets, sorted polygons, sorted and nonsorted circles) and fossil boulder depressions occur more frequently in the interior of the country and in high places than on the coast or in lowlands. The development of these formations began immediately after the deglaciation of southern Finland and its emergence out of the water, and the process ceased or considerably weakened at the later half of the Yoldia sea stage about 9000 years ago.

Several ice-wedge casts and sand wedges, 2–3.5 m deep, have been found in stratified drift in gravel pits and road cuts on the plateaus of the First and Second Salpausselkä end moraines in southern Finland. Five different types of ice-wedge casts and sand wegdes have been distingquished on the basis of morphological and stratigraphical features. The distribution of ice-wedge casts and the regional differences in their size suggest, that ice-wedges were formed first on the First Salpausselkä 11 200–10 300 years ago and later on the Second Salpausselkä 10 500–10 300 years ago during the Younger Dryas period. Fossil tundra polygons have been found in connection with ice-wedge casts. Structures interpreted as periglacial involutions have been found at the top plateaus of the First Salpausselkä end moraine. These involutions conform to the »drip« or »plug« type but also resemble »stone pillars« based upon sediment type and structural shape. The structure of the formations suggest that they have been formed by vertical movements of material caused by freeze-thaw action. The presence of ice-wedge casts, fossil tundra polygons accompanying the ice-wedge casts and involutions on the top plateaus of Salpausselkä end moraines is the clear evidence of the past permafrost in mineral material during the Late-Pleistocene period.

Periglacial dune fields with fossilized parabolic dunes are common in southern Finland. These dunes date from the Late- and Post-Pleistocene periods, 11 000–8 000 years ago and were formed by the northwesterly winds. Transverse coastal dunes on recent coast of southern Finland were formed during the Little Ice Age, about 400–100 years ago, and they also may been held as periglacial formations. The strings of raised peat bogs, caused by hydrological, biological and frost processes appeared on bogs of southern Finland 3200 years ago.