Náttúrufræðingurinn

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Náttúrufræðingurinn - 2004, Qupperneq 51

Náttúrufræðingurinn - 2004, Qupperneq 51
Tímarit Hins íslenska náttúrufræðifélags 9. mynd. Berggangur í Vatnsdalshólum. Horft til norðurs út Vatnsdal. - A dyke enclosed in theformation of Vatnsdalshólar, view to thc north. Ljósm./Photo: Höskuldur Búi Jónsson. berghlaupsins hægði á sér, leitaði miðhlutinn til hliðanna, og skýrir það skorumar um miðbik hólanna. Líklegt er að keilulögun stakra hóla sé orðin til vegna molnunar og veðrunar þeirra eftir að hlaupið féll, en þó má búast við að bergflykkin hafi molnað að hluta til við sjálft hlaupið. Vatnsdalshólar eru að öllum líkindum myndaðir eftir að sjór hvarf úr Vatnsdal fyrir um 9500 ámm og áður en súra öskulagið H5 féll fyrir 6100 ámm. Ekki er ljóst hvort austustu hólamir hafi mynd- ast um leið og meginþyrpingin eða hvort þeir hafi myndast seinna, en þeir eru ekki samhangandi við meginhólana og berggerð þeirra nokkuð önnur. Það er niðurstaða þessarar athug- unar að Vatnsdalshólar séu hvorki hluti af óhreyfðum berggmnni né mótaðir eða myndaðir af jöklum eða jafnvel hvort tveggja, eins og Agúst Guðmundsson10,11 hefur haldið fram. Ljóst er að hugmyndir Jakobs H. Líndals3 um að berghlaup hafi myndað hólana hafa staðist tímans tönn og þó svo að þær skýringar sem hér em settar fram séu að ein- hverju leyti frábmgðnar skýringum Jakobs, em niðurstöðumar að mestu leyti samhljóða. Niðurstaðan undir- strikar orð Sigurðar Þórarinssonar í formála bókarinnar Með huga og hamri, sem m.a. geymir safn jarð- fræðidagbóka Jakobs H. Líndals, en þar segir: „... em þær óbrotgjam minnisvarði glöggskyggni, skarp- skyggni og vísindalegum hugsunar- hætti þessa hógværa húnvetnska bónda."4 SUMMARY Was it a Rock Avalanche that formed Vatnsdalshólar in Northem Iceland? In this paper we discuss the formation of Vatnsdalshólar, which is a hummocky deposit in Vatnsdalur, northern Iceland (Fig. 1). It has been widely accepted that it was formed in a large rockslide (rock avalanche - bergsturz - sturzstrom).3,4'5 By comparing rock types in Vatnsdals- hólar with those exposed in Mt. Vatns- dalsfjall east of the Vatnsdalshólar form- ation, Jakob H. Líndal3,4 found a good correlation between what can be found in the northem part of Vatnsdalshólar and the northem part of the mountain. The same can be applied to the southem part (Fig. 2). This evidence, combined with the fact that glacially striated bedrock is exposed within the formation - stratigraphically undemeath it, and with the lack of sediment of glacial origin, prompted Líndal to suggest that Vatnsdalshólar were formed by a large rockslide. This has been contradicted in a recent study that suggests that Vatns- dalshólar are a part of the bedrock, with . .hummocky push moraines, modified by ablation of interior ice".11 The present work confirms Líndals suggestions.'4 The geology of the Vatnsdalur area is greatly influenced by an ancient volcanic centre. A map was made based on the studies of Annells,12 which shows the geology surrounding Vatnsdalshólar and Vatnsdalsfjall mountain (Fig. 3), a possible position of the main scarp of the rockslide has been included on the map. The distribution of rock types in Vatns- dalshólar has a striking pattem (Fig. 4) with basalt in the outer part and rhyolite in the middle. It is not likely that such a pattem can be of a glacial origin. In two places within Vatnsdalshólar glacially striated bedrock, that underlies the Vatnsdalshólar debris, shows that the Vatnsdalshólar formation is of post- glacial age. The surrounding hummocky terrain is made up of a deposit and not bedrock, since a glacier would have eroded the mass of the bedrock easily (Fig. 5). It is suggested that the pattem of rock types within the formation shows details of the original rock mass, where the rockslide originated, with rhyolite at the top and basalt at the bottom creating a pattem (Fig. 6). The morphology of Vatnsdalshólar is peculiar. In the outer- most part (south, west and north), there is a series of small hills (hummocks) and in the middle it has tumed into a more continuous mass (Fig. 7), with low val- leys in between. The reason for these divergent forms can be because of a dif- ference in the thickness, position of dif- ferent parts of rock types and the com- pressive and extensive forces within the moving mass (cf. Fig. 6). When individ- ual hills of the formation are studied, it can be seen that they are usually made up of one rock type, but when comprised of more rock types, layering is found (Fig. 8). This suggests tliat Vatnsdalshólar is not made up of glacier deposit, since the hills would be made up of a mixture of all the rock types and not in layers. Within the Vatnsdalshólar formation, there are several dykes (Fig. 9). The dis- tribution of these dykes is random, which suggests that they are not part of undisturbed bedrock and since they are present in Vatnsdalshólar it mles out the 137
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