Geological Survey of Finland, Special Paper 46 - arkisto.gsf.fi

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Geological Survey of Finland, Special Paper 46 Peter Johansson 50 THE ICE RETREAT IN THE FELL AREAS The ice margin started to retreat from the Younger Dryas End Moraines about 11,600 years ago (Saarnisto 2000, Saarnisto & Saarinen 2001). At the same time the highest felltops in northwestern and northern Lapland appeared from the ice as nunataks. In the northern part of Lapland, the ice margin retreated towards south-southwest and in the southern part of Lapland to northwest. The surroundings of the Lake Inari were deglaciated approximately 10,800 years ago (Kujansuu et al. 1998) (Fig. 3). Fig. 3. Recession of the ice margin in northern Finland. 1 = position of the ice margin, 2 = areas covered by ice-dammed lakes, 3 = Ancylus Lake, 4 = Younger Dryas End Moraine. After Johansson et al. (2005).
Fig. 4. Lateral drainage channels on the slopes of Jäkäläpää Fell in Inari. The ice margin reached the Lemmenjoki – Saariselkä fell range about 10,600 years ago. In the mountainous area the deglaciation was fi rst thinning of the ice. During the nunatak phase many of the overfl ow channels in the Saariselkä area were also formed as a result of meltwater erosion. As the ice sheet became thinner and its surface sank, the nunataks expanded into vast ice-free areas. The ice fl ow stagnated behind the nunataks, and fractures and crevasses were consequently formed at the margin of the ice sheet. The meltwaters penetrated under the ice margin, eroded subglacial chutes and deposited engorged eskers (Johansson 1994). The meltwater action at the boundary of the ice sheet and an exposed fell slope next to it produced series of shallow meltwater channels, i.e. lateral drainage channels (Fig. 4). The channels generally occur in groups, running side by side gently sloping down the fell side. These channels are of great importance in the study of deglaciation in minor scale, since they help to construct the position of the ice margin in a great detail, which gives a picture of the inclination of the surface of the ice sheet. Additionally they indicate that the ice margin was unbroken. In favourable places on the fell slopes channel systems comprising several tens of channels were formed in which the distance between the individu- Geological Survey of Finland, Special Paper 46 Late Weichselian deglaciation in Finnish Lapland al channels remains nearly constant. In these places the channels may have formed as a consequence of the annual thinning of the ice sheet and they can be used to document the rate of melting (Penttilä 1963, Johansson 1995). Their distribution indicates that the gradient of the ice surface was between 1.5 and 3:100 m and the ice margin thinned approximately 1.6 m per year. The rate of recession of the ice margin varied 130 – 170 m per year, approximately 140 m per year (Johansson 1995). In western Lapland the deglaciation rate was faster. Lateral drainage channels on the fellslopes in the Ylläs area show that the annual thinning was about 2.5 – 3 m and the ice retreat about 170 m per year (Abrahamsson 1974, Kujansuu 1967). The melting accelerated due to the decreasing mass of the continental ice sheet as the deglaciation proceeded and to warming of the climate, which also contributed to the acceleration. As the retreat of the ice margin continued, the southern edge of the Lemmenjoki – Saariselkä fell range became exposed. The ice margin was pressing against the fell range and it became undulated with tens of kilometres long ice lobes penetrating into the valleys of Lemmenjoki, Suomujoki and Anterijoki. At the end of this ice-lobe stage, the lobes turned passive and melted down after they had lost contact with the active ice sheet. 51
Page 1: GEOLOGICAL SURVEY OF FINLAND Specia
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Page 6 and 7: 4 CONTENTS Preface, Peter Johansson
Page 8 and 9: Geological Survey of Finland, Speci
Page 11 and 12: Applied Quaternary research in the
Page 13 and 14: DRUMLINS IN KUUSAMO AND RANUA In Ko
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