Jökull


Jökull - 01.12.1987, Page 52

Jökull - 01.12.1987, Page 52
Figure 2: Aerial photograph (1960), of Nautadalur and its neighbouring rock glacier. © Copyright: Ice- land Geodetic Survey. 2. mynd. Flugmynd frá 1960 af Nautadal, sem sýnir grjótjökul (þelaurð). © Utgáfuréttur: Landmælingar íslands. Figure 3: Nautadalur rock glacier, as viewed from the corrie backwall. 3. mynd. Grjótjökullinn (þelaurðin) I Nautadal. Horft frá efstu rótum. glaciers formed and, as a result, they are a subject of some argument and conflicting interpretation. There have been few reports of rock glaciers in Ice- Iand (Einarsson 1968; Eyles 1978; Escritt 1976; Whalley 1974). Whalley (1974) suggested that the rock glaciers of Tröllaskagi shed much light on the forma- tion and significance of such features. This paper con- siders one rock glacier, located in Skjóldalur, Trölla- skagi (figure 1) on which monitoring has been in oper- ation over a period of eight years. ROCK GLACIER DEFINITION Martin and Whalley (in press) discuss the disputed terminology concerning the delimitation of rock gla- ciers and conclude that a morphological (hence non- genetic) definition is most consistent with existing literature and a general basis for definition is sug- gested as follows; 1. Rock glaciers are found in mountain regions which have, or have had, glacial/periglacial conditions. 2. they have an outward appearance of being com- posed of rock debris. 3. the extent of this rock debris is distinct both mar- ginally and terminally (though sometimes less so at its head). 4. they have a source area (or areas) i.e. a head, and a distinct snout which marks its maximum extent downslope. 5. they have, in many cases, flow-Iike features on their surface. The rock glacier is supposed to flow (or have flowed in the case of relict features) as a result of ice contained in some manner within the rock debris. 6. they may move; for active rock glaciers, at rates of approximately 2 — 3 orders of magnitude slower that for true glaciers (usually < 1 m/year). ROCK GLACIERS IN SKJÓLDALUR, TRÖLLASKAGI Mountains rising to 1300—1500m a.s.l. are found in Tröllaskagi which lies between Skagafjördur and Eyjafjördur. Much of the peninsula consists of basalt plateaux dissected by glacially modified valleys (Sig- bjarnarson 1983). Some of these valleys contain rock glaciers, identified by Griffey (in Escritt 1976). Many are located in corries with glacierettes at their heads suggesting that their origin may be related to glacial activity. Skjóldalur lies in the south of Tröllaskagi, running almost west—east, from the central watershed of the peninsula to the farm of Ysta-Gerði, 20km south of Akureyri (figure 1). On the southern side of Skjóldalur are four tributary valleys heading in corries. The west- ern most two contain both glaciers and rock glaciers. Nautadalur is the Iargest of these (figure 2) being deeply incised into the plateau which rises to just over 1300m at this point. The corrie is occupied by a small glacier (approx. area 0.28km2), that appears to merge downstream with a rock glacier, which resembles a heavily debris-covered glacier snout. In rock glacier terminology, it may be described as tongue-shaped (Wahrhafting and Cox, 1959) or valley floor (Outcalt 50
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