Pre-crag ridges in southwestern Finland

Abstract

The pre-crag field in southwestern Finland includes about 90 streamlined pre-crag ridges sparsely distributed over an area of about 4000 km 2. These moraine ridges were developed on the proximal side of rock protuberances where they were controlled by glacial flow. As a result of recent mapping of Quaternary deposits on the 1: 20,000 scale, new pre-crag ridges have been discovered in the Salo map-sheet area (1 : 100,000) some kilometres north of the Third Salpausselkäice-marginal formation. The Salo area also includes the Karjanummi and Munamäki pre-crag ridges, which were investigated in detail using drilling, sampling, seismic refraction and reflection surveys and with ground-penetrating radar. Six other drilling profiles from the Salo area are reported for comparison. The pre-crag landforms are mainly composed of tills that form a complex sequence of sediments deposited during the Weichselian glaciation. The uppermost basal melt-out till unit was deposited during the late Weichselian deglaciation, when the ice sheet flowed southeastwards towards the Salpausselkäice-marginal formations. The lower lodgement till units were deposited during a phase of the Weichselian glacial episode. Reflection sounding of the 50 m thick Munamäki pre-crag ridge intimates that several till beds and intervening layers of sorted sediments may be present; the lowermost tills may have potentially been deposited during the glaciation preceding the Weichselian. The superimposed striae on bedrock surfaces in the same area imply that the older glacial flow trended towards the south. The longitudinal axes of the largest pre-crag formations are parallel to the glacial striae that trend almost N-S, whereas those of the smaller ridges coincide with the local younger NW-SE aligned striae.