Isrecessionen inom Uppsalaområdet

Abstract

Ice recession in the Uppsala region.

In connection with mapping for the Geological Survey of Sweden in the Uppsala region drillings were carried out at 17 localities (Fig. 1) through the glacial varved clay in order to discover the ice recession in this area. The device used for drilling was a soil sampler with metal foils, constructed by the Swedish Geotechnical Institute (W. Kjellman and co-workers 1950) and able to take undisturbed samples of lengths up to 11 m.

Varve measurements were carried out at all localities and used for connections with each other and with earlier measurements from here by Gerard De Geer (1940). The results are given in the graphs of Plate 3 and 4 and the table of Plate 5. Between all localities, situated in the Uppsala plain and the vallies associated with it, it is possible to find varve connections, but it is nearly impossible to do so in the area W of the town, probably owing to topographical conditions influencing the silty bottom streams of melt water from the ice front. Also in about 40 proximal varves, deposited in front of the ice tunnel opening, great irregularities in thickness are observed, owing to the same fact as mentioned above. In the author's opinion it is not possible to make far distant connections of varve series.

The map of Plate 2 shows the ice recession in the region with ice margins for every 10 years. During a period of about 90 years — between the years −900—−810 according to the timescale of Gerard de Geer—the ice front retreated from the Uppsala region. From the ice margins it is clear that the ice tunnel, now indicated of the immense glacifluvial deposits of the Uppsala esker, must have debouched into an ice ravine, a fact earlier stated by G. Lundqvist (1955). As it appears from the map the youngest glacial striae W of the esker have a NNW direction, but E of it a NNE direction, indicating an activity of the local ice fronts, formed by the ravine. Glacial striae and end moraines of the region (quoted from G. Lundqvist 1956) correspond very well with the ice margins found in this investigation.

In the upper part of the glacial clay of the Uppsala region about 20 to 30 cm above the last discernible microvarve, there is a zone maximally some 10 cm thick — the spotted zone — dotted with sand and gravel particles, mostly calcareous (Fig. 4). The problem of its origin has also been dealt with.

According to J. P. Gustafsson (1905) the calcareous material in the spotted zone was once deposited in the sea or lake by ice bergs from the Bay of Gävle, a region where the land ice was rich in such fragments. But the zone is not distributed at random as it would, naturally, be if the material was transported by floating ice, and it is found in every locality where the original strata is preserved, but not in glacial clay, deposited from ice rivers W of the Uppsala region. Its thickness also gradually diminishes with increasing distance from the esker (Table 1), and we never find the spotted zone together with ice floated, large till material which now and then occurs in the glacial clay.

The facts above suggest that the spotted zone is a synchronized phenomenon and limited to the sedimentation region of the Uppsala esker. As calculated in Fig. 5, it must have been deposited when the ice front was situated in the southern part of the Bay of Gävle, and the melt water from the ice river carried enormous quantities of calcareous fragments from the Silurian area in the bay. Further investigation of the sediment series N of the Uppsala region will probably provide a definite solution to the problem about the origin of the spotted zone.