What Do We Know About the Limestone Around Gävle?

At Holmudden by Bönan, with perhaps the richest occurrences of loose limestone, test borings were also conducted. In the boring profile, beneath the quaternary layers (clay and moraine), followed an 11.6 metre thick blue-green, hard clay which was underlain by, among other things, a reddish sandstone. The interpretation of the layer sequence is as follows: the thick clay is of Lower Cambrian age, i.e. formed more than 500 million years ago, and the sandstone is Precambrian, i.e. younger than the bedrock but older than the Cambro-Silurian.

Beginning in the 1960s, a number of borings were conducted, including from the ice, in combination with geophysical and marine geological surveys with the aim of establishing the bedrock between Sweden and Finland. Today we have a completely different picture of the distribution of Cambro-Silurian bedrock in Scandinavia (figure 1). To previously known occurrences can now be added two very large areas between Sweden and Finland, one in the Bothnian Bay with only Cambrium and one in the Bothnian Sea with Cambrium and parts of Ordovician (figure 2).

LIME CONTENT IN THE SOIL

The latest continental ice sheet reached its greatest extent approximately 20,000 years ago. The ice sheet then covered all of Sweden and extended as far down as northern Germany. Following a marked climate improvement, rapid ice melt began, which meant that Sweden was ice-free approximately 8,500 years ago. The ice margin stood in the Gävle region somewhat less than 10,000 years ago.

The continental ice sheet was an enormous glacier that was plastic and slid across the earth's surface. The movement resulted from the weight of the ice and its flow from higher terrain to lower-lying areas. During its advance, the ice scraped the bedrock beneath it. In this way, ice striations were formed, which even today show the direction the ice moved. The ice also had the ability to carry material with it; gravel and stone froze to the underside of the ice and were transported long distances in the direction of ice movement. When the ice subsequently melted, the frozen material was deposited; the soil type moraine was formed in this way. The material transported by ice was subjected to great forces. Fragments of softer rock types were crushed down to fine-grained material such as silt and clay. More resistant rock types "survived" the ice's treatment better and came to be incorporated as stone, gravel and sand in the moraine or glacial outwash gravel.

Ice movement over Gävleborg County was predominantly northwest-southeast, i.e. the continental ice sheet came from the northwest and slid towards the southeast across the land. For some reason, the continental ice sheet veered as it slid across Gästrikland and the direction twisted from northwest-southeast to northeast-southwest, i.e. it turned in towards southern Gästrikland and northern Uppland. The ice striations speak their clear language. This twist in the ice's movement is believed to have occurred late, perhaps during the melting of the continental ice sheet.

The result of this late ice movement can be read in the form of elevated lime content in various soil types, particularly moraine and clay, in eastern and especially southeastern Gästrikland and adjacent parts of northern Uppland. In the eskers too, an elevated frequency of Cambro-Silurian rock types can be demonstrated in boulders and gravel. At Mackmyra, Cambrian sandstone has been found in glacial outwash gravel.

Put simply, one can say that the ice came from the north, slid out over the present bottom of the Bothnian Sea, picked up material from the soft limestone and slid in over the Gävle region and northern Uppland. As the ice slid across the landscape, it spread remains of limestone, ground down into fine constituents. One can say that a lee-side effect was created. On the lee side of the limestone area, lime material was dispersed. This "lee-side effect" can be demonstrated in other limestone areas in Sweden as well (figure 3).

Lime material can also be dispersed across the landscape in another way. During the final stages of the continental ice sheet, when the ice margin stood in our region, almost all of Gästrikland lay beneath water. When the continental ice sheet emptied into the sea, the ice's retreat was accelerated by so-called calving, i.e. the ice margin broke apart into larger or smaller icebergs. These could then drift across the landscape and when they ran aground or melted, leave their moraine material in new places. During the geological mapping around Gävle, limestone material was found in the clay out on Valboslättens plain and in adjacent areas.

An interesting question is how far westward in our landscape can elevated lime content be demonstrated? There are no investigations that give a clear answer to the question. A problem in this context concerns sampling. Experience from other areas in Sweden shows that the lime content in moraine varies with different depths, partly with regard to leaching processes in the soil. Moreover, the lime content varies depending on which grain size is analysed. Probably there runs a fairly distinct north-south boundary across Gästrikland, perhaps at or immediately west of Gävle.

SUMMARY

The soil around Gävle is strongly calcareous due to the occurrence of Cambro-Silurian bedrock on the bottom of the Bothnian Sea between Sweden and Finland. The highest lime contents occur in eastern and southeastern Gästrikland where blocks and stones of limestone can also be found, particularly on the islands in Gävle Bay.

Through the movement of the continental ice sheet across the land, lime has been dispersed across the landscape; the exact distribution is not known, but in general the lime content in both moraine and clay decreases rapidly the further east one goes.

SELECTED REFERENCES

Lundqvist, Jan, Geologi. Processer - landskap - naturresurser. Lund 1988.

Lindström, M., Lundqvist, J. & Lundqvist, T., Sveriges geologi från urtid till nutid. Lund 1991.

Berg och jord. Red. Curt Fredén. (Sveriges Nationalatlas). Höganäs 1994.

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INSECT COMPETITION

In the previous issue of Insectifera we announced a competition about which butterfly was depicted and what colour the scales had, and "why"?

Was the competition too difficult?
No correct answer has been received.

Since the answer is interesting from many "perspectives," the competition is hereby cancelled.

Some clues:

A large butterfly, 12-15 cm wingspan
A rainforest species, fairly common

Red and white cross-banded colony-living larvae

A dream for many to see this creature fly in the sun
The structure of the wing scales very interesting

See photo from scanning electron microscope in our previous issue in very high magnification and the image below of the same wing scale in somewhat lower magnification.

Answer to the editorial office of Insectifera:
Gästriklands Entomologiska Förening, Box 11056, 800 11 Gävle

Which butterfly bears these beautiful wing scales and what colour is it?