Weichselian and Holocene geology of Sør-Trøndelag and adjacent parts of Nord-Trøndelag county, Central Norway
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In Sør-Trøndelag and adjacent parts of Nord-Trøndelag a clayey till (10-25% in the clay fraction) is commonly found below 1-5 m of sandy till (1-5% in the clay fraction). The clayey till is mostly found in drumlins, in stoss-side positions, in deep narrow valleys transverse to the dominent ice-flow direction during the Late Weischselian and in depressions in the terrain. Till fabric indicates a WNW ice flow both for the clayey till and for the sandy till. Such an ice flow is also indicated by drumlins, fluted surfaces and glacial striae. In general there is a 0.1-1 m thick transitional zone between the two till layers. It is concluded that the clayey till is older than Late Weichselian.Radiocarbon dates of molluscs and gyttja indicate that the coastal area were deglaciated at c. 12,500 years B.P. When the inland ice reached rock thresholds in the outer parts of the fjords the ice regression was temporarily halted. A steep glacier gradient caused by calving led to minor glacial readvances, and terminal moraines were formed. After a few hundres years the calving continued further inland, interupted by glacial readvances when the Tautra ice-marginal deposits (10.800-10.500 years B.P.), The Hoklingen ice-marginal deposits (c.10.300 years B.P.) and the Vuku icemarginal deposits (c. 9800 years B.P.) were formed. The rapid calving left isolated ice caps in mountainous areas along the fjords. The last remnants of the inland ice were situated in depressions in the terrain in the southern and eastern parts of the investigated areas. As the ice culmination zone was situated to the east and south of the watershed, the drainage from a large area was directed towards Trondheims-fjorden.During the deglaciation Trøndelag was deeply submerged by the sea. The subsequent shoreline displacement led to an intense fluvial erosion, and sediments deposited during glaciation and deglaciation were transported farther down the valleys or into the fjords. This erosion, together, with leaching of saline pore water, reduced the stability of the remaining glaciomarine sediments, and numerous quick-clay slides have since occurred. These slides are characterised by an almost liquid clay as the disturbed shear strength is very low. The strong fluvial erosion also led to other types of clay slides and to slides in other superficial deposits.