Geology of the vitrified hill-fort Broborg in Uppland, Sweden

Abstract

Broborg is a hill-fort from the Migration Period, situated about 20 km southeast of Uppsala. Vitrification is found around almost the whole circumference of the inner rampart, with the notable exception of the entrance. Vitrification occurs along the inner face, forming a 100–150 cm wide, 40–70 cm deep layer consisting of amphibolite melt, penetrating and cementing blocks of gneissic granite. Amphibolite has been selectively enriched only in the vitrified parts. Temperature required for vitrification is about 1130°C, with low oxygen fugacities. Charcoal, rather than wood, is the most plausible fuel that has been used. Box-like structures, about two metres long, suggest vitrification in sections. All evidences indicate a constructive formation of the vitrified wall, employing forced draught (= bellows) to a confined space. Melting temperatures for vitrified products from several European forts are given.

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