Stranddünenwälle am Längsoszug Virttaankangas—Säkylänharju in SW-Finnland

Abstract

Seven swarms of well-developed fossil transverse dunes, 2–7 m high and 100–2 000 m long, were formed after the Boreal age (5 000–6 000 B. C.) on the slopes of the Virttaa—Säkylä esker system running in a SE-NW direction east of Lake Pyhäjärvi in SW-Finland. These orientated sand dunes consist of primary glaciofluvial material first sorted by littoral forces and then by aeolian activity. The position of these coastal dune ridges shows the direction of the ancient shore line. The 10 km 2 wide sand plateau of the Virttaankangas esker was deposited as a submarine delta in a broad crevasse in front of the stagnant margin of the melting continental ice cap. The steep-sloped Säkylä esker with its oblong dead-ice hollows, on the other hand, was mainly formed en- or subglacially in a narrow crevasse or tunnel under the thin, retreating ice. A dark layer of ashes with black-burned stubs in some dunes is an evidence of a fire which destroyed the pine forest on the esker complex for 100–150 years ago.