Weathering Pits in the Lake Mjörn Area Northeast Of Göteborg

ABSTRACT

Pits, with a diameter up to 50 cm and a depth of max. 25 cm, often with overhanging walls, are found in the Lake Mjörn area northeast of Göteborg. The pits are concentrated to a near lake level zone between 3 m above lake level and 1.5 m below it. The pits are most frequent in tonalitic and amphibolitic rocks. Thin sections show that these rocks contain a larger amount of chlorite than the other rocks. The scattered chlorite makes the rocks less resistant against weathering. Thin sections show that the rock at the bottom of pits is not more weathered than the rock near the pits. Pits developed in glacial striae show that the weathering is Post-Glacial.

It is supposed that the formation of pits is initiated by the accumulation of water in small depressions like coarse glacial striae and joints or along foliation planes. The exclusive shore position of the pits indicates the importance of the intermittent wet occasions, their duration and frequency. Once the initial depression is formed and sufficient frequency of wetting and drying is at hand, the depression will have a generally higher weathering rate than the surrounding bedrock and a pit will be formed.