Jökull


Jökull - 01.12.1967, Blaðsíða 50

Jökull - 01.12.1967, Blaðsíða 50
ernmost and biggest o£ alíj very indented and perhaps consisting of many small lakes. It ex- tends farther north than has been explored. To the east and north it is bounded by an amphitheatre of high mountain ranges cul- minating in the farthest north in a towering cone-shaped peak . . .” This lofty cone-shaped peak is in all probability the mountain Sydri- Kerling, which looking north-east lies open be- fore the eye, Fig. 2. At the present time there are only a few small lakes in the Tungnaá depression ancl surely no one would think of calling them Stórisjór. All these lakes, altogether four, are anonymous except Jökulvatn, which is situated at Hraunsskard. Prior to the drilling in the Tungnaá depression it was considered probable that the small lakes were remains of a bigger lake previously occupying the valley, having graduallv silted up with the sediment load in Tungnaá cluring the last thousands of years. Two jour- neys have been undertaken for an investigation of the thickness and nature of the sediments on the proposed dam site in the Tungnaá de- pression against the mouth of Lónakvísf with driffing. The forrner took place in November 1965, but a sufficient result could not be ob- tained then. Therefore, a new journey was undertaken on the 19th to 21st of October in 1966. The main emphasis was put on the effort of obtaining samples from the sediments of the Tungnaá depression, and also on an exploration of the depth to bedrock with sounding as time would allow. The drilling was performed by a group of six men under the leadership of G. Sigurdsson foreman and E. G. Vilmundardóttir geologist. As mentioned above the main purpose of the investigation was to obtain samples. They were intended to be taken with a sampler, which is a liollow cylinder open at the lower end wherein the samples are pressed, but the sediments proved to be so incoherent that the cylinder became emptied on the way up and only one sample was obtained by this method, from 2 m depth. Then the alternative of washing the samples up with water was turned to. It was done by press- ing down a casing and washing frorn inside it by the pumping down of water. Generally samples were taken at intervals of one m. Initi- ally material from an approximately 0.5 m interval was washed up and made to flow away, then the material from the next 0.5 m wTas collected into a 20 1 pail and left for a short time for sedimentation. Now the water was poured from above and the sample put into a plastic bag for preserving. The casing easily pierced down to a depth of 12 m simultane- ously with the washing out of it, but then it could not penetrate further down. Therefore, the samples from below that point were washed up without the protection of the casing. When 17 m depth was reached the sampling was given up because the hole began to cave in; yet bedrock had not been reached. Surely this method of sampling is rather rough, but it was the only applicable one. It is to be expected that by this method a considerable part of each sample got lost, especially the most coarse and also the most fine-grained material. Probably the samples therefore indicate a rather greater sand content than the sediment actu- ally contains. On the other hand it is likely that variation between individual samples is rather caused by different composition than by what was lost in their collecting, because the same method was used with all of them except the first sample. The samples do not seem to have become mixed with material from the walls of the hole as they were washed up. Such could not have taken place in the first 12 m protected by the casing, but the lowest sample clearly shows that no intermixing has taken place below that point either, as both white pumice and diatoms, which abound in the next sample above, are totally lacking here. The investigation of the samples can be divid- ed into three stages: 1) The samples were dried and sieved and a part of them gluecl to a special sheet of paste-board in order to get a general view of the stratigraphy of the sediments. 2) A petrographical and teprochronological analysis. 3) Investigation of diatoms. 1) This part of the investigations was per- formed in the Laboratory at Keldnaholt by S. Pálsson and G. Gudmundsson. A part of the sieved samples was glued to a sheet of paste- board, cf. Fig. 3, where the size groups of each 284 JÖKULL 17. ÁR
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Jökull

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