HOLOCENE LACUSTRINE ROCK PLATFORM AND QUATERNARY CRYOPLANATION OF ROCK SURFACES IN AND AROUND BERGEN, NORWAY

A shoreline cut in bedrock is recently discovered and mapped around the shores of Lake "Storavatnet" on the island of Osterøy, 25 km NE of Bergen, Norway. The lake consists of two connected parts, 5 and 7 km long and 3-900 m wide, with a natural lake level of 151,5 m a.s.l. The rock platform is sloping 5-10º towards the shore and the width varies from 0 to 20 m. The platform is developed in various kinds of bedrock, including well foliated Lower Paleozoic micaschist and amphibolite, as well as older granitic gneisses and quartzite. The weathered material is angular and some is still located in situ on the platform itself or on the lower slopes. Both shapes and sizes varies in relation to mineralogy and structure of the underlying bedrock with pebbles on the micaschist, tabular cobbles on the amphibolite and cubic cobbles to boulders on the gneissic rocks.

On several locations caves and notches are developed on the platform. Caves are preferentially located along fractures in the micaschist, particularly in fracture overlap zones. Unweathered glacial striation is found below the natural lake level while weathered glacial grooves sometimes can be found above the platform. The main part of the platform must have been formed by frost weathering along the lakeshore after termination of the oblique glacial rebound, some 6000 years ago. An older and 8-12 m higher rock platform, up to 1,5 km wide, is found between the two parts of the lake. Mapping by georadar show a relatively horizontal bedrock surface with some depressions filled with bogs or small lakes.

In the central part of the suburb Åsane, 10 km N of the City Centre of Bergen, an area of 1,5 x 3 km has a nearly horizontal bedrock surface only broken by four small lakes and some hilly bedrock remnants. Refraction seismic as well as georadar has been used in mapping the bedrock surface. The 4 km long river draining the area drops 2,5 m, and the bedrock surface along the river channel is almost horizontal and mainly older than the Holocene. The formation is thought to be similar to the cryoplanation of the rock surfaces on Osterøy island. Climatic investigations in the basin at Åsane show a temperature inversion situation in wintertime with intermittent cyclones giving precipitation and rise of the lake levels. Drifting of lake ice and main glacier is thought to be responsible for removing the weathered material.