Jökull


Jökull - 01.12.1985, Page 36

Jökull - 01.12.1985, Page 36
mass seems to have covered the Thingmúli volcano core and the surrounding high land to the E and partially drained through a zone of peripheral nunataks into the Grímsá valley, imparting a more westerly component into the flow of the other ice mass as the combined streams spread out. Other factors influencing the more westerly compo- nent in the overall ice movement could have been the strike of the Lagarfljót flexure, (Fig. 2), and the trend and frequency of the dykes associated with it, (Fig. 3). This more westerly component of ice flow is repre- sented, for instance, by a broad, swampy, col separating Fellaheiði from FTafrafeil, (Fig. 1), with ice-scoured features such as roches moutonnées, (Hvalbök), sepa- rated by shallow, steep-sided channels. The evidence of ice erosion in the valley cross-sec- tions is not entirely clear. Norðurdalur and Suðurdalur, (Fig. 1), are comparatively narrow and steep-sided and could possibly have contained valley glaciers, but against this, Norðurdalur in particular has overlapping rock spurs. These valleys join at the Múli promontory to form the upper part of Fljótsdalur, (Fig. 5), which also has some anomalous features. The valley floor is almost completely covered by alluvial deposits from Jökulsá í Fljótsdal and Kelduá and then, 8 km down valley, from the E tributary, Gilsá, forming the distal end of the delta at the lakehead. Drilling in the Gilsá alluvium has shown it to be 132 m deep and of Recent age, (Hjartar- son et al. 1981), suggesting that it is filling a trench in the valley floor which gives Lagarfljót a maximum depth of 111.5 m a little to the N, that is, some 90 m below sea- level, (Fig. 2). Although this trench is partly structural and associated with the Lagarfljót flexure, it could be partly caused by ice erosion. Smaller trenches occur down to 40 m further N, (Fig. 4), but the N end of the lake and the outflow are shallow for the 26 km down- stream to the sill of Lagarfoss at 19 m altitude. The valley sides of upper Fljótsdalur are marked on the W side by the steps of the Tröllkonustígur, running from Norðurdalur to Bessastaðaá, (Fig. 1). Individual basalt flows usually show up as irregularities on Ice- landic valley-sides, but here the steps are comparatively horizontal from front to back, where there is a sharp break of slope, while each step slopes up gently towards the N and W. The lowest fully visible step, rising from above the Valthjófsstaður farms is about 75 m in width, but an even wider step emerges from the alluvium of the valley floor a little further N and forms much of the cultivated ground of Skriðuklaustur. The higher steps are narrower and sometimes indistinct, but all bend concentrically into the Bessastaðaá ravine, where the main valley widens to the N, (Figs. 5 and 9). Fig. 3. General arrangement of dykes in the area, enlarged from Walker (1974), — 3. mynd. Lega ganga á rannsóknasvœðinu skv. Walker (1974). A dyke cuts transversely through the rock steps from the base near Valthjófsstaðir northwards to the top above Skriðuklaustur. It has not been eroded level with the steps and rises a metre or so above their level with a fairly smooth, rounded surface. A small ravine runs down the N side of the dyke, to some extent graded to its slope. The Múli promontory at the tributary valley junction is also stepped in profile. The point is a rock platform at about 100 m altitude, with some deposits on it, the nature of which I was unable to investigate. Smaller steps, similar to those in the main valley, run from Múli along the W valleyside of Suðurdalur. 34 JÖKULL 35. ÁR
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