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Browsing by Subject "jäätikköjoki"

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  • Kivikko, Tommi (2022)
    Glacial meltwater potholes are cylindrical and often remarkably smooth-walled pits in the bedrock, formed as a result of the evorsion caused by eddy currents of water released by the melting of the continental glacier, and possibly also by the cavitation due to the meltwater rushing to a glacial moulin or crevasse. This study examines how the location, topography, orientation according to the direction of ice movement, bedrock and the distance from the fault lines and eskers explain the occurrence and abundance of potholes in the province of Uusimaa in southern Finland. This study also provides a unified database of potholes that have been found in Uusimaa, which have not been compiled to this extent before. The research material with location and characteristic data was collected in July-October 2020 and in May-June 2021. A total of 320 potholes were selected for the research group, of which 206 were determined from fieldwork and 114 from source data. There were a total of 116 sites around Uusimaa, where the number of potholes varied from one to twenty (1–20). Moreover, especially in the archipelago spreading off the southern coast of the study area, there are numerous potholes marked only in sporadic old articles. As some potholes were located in areas that were too difficult to reach or in private yards, for example, the work also required the use of databases and reports published by research institutes and municipalities. The above-mentioned sources were also utilized in the field when searching for potholes and later in slope and distance analyses. In addition, one hundred (100) comparison sites were drawn from somewhat evenly across the study area in order to interpret how the location and characteristic data of pothole sites differ from rock formations where potholes are not known to occur. The location and characteristic data of pothole and comparison sites were compiled into a material. Based on the data, diagrams as well as regression and correlation analyses were made to visualize the results and their interrelationships, and statistical models (generalized linear model, GLM) were used to model several variables simultaneously. The results showed that many physical geography-related factors simultaneously affect the occurrence and abundance of potholes. Since potholes are glaciofluvial landforms, they often occur in north-south or northwest-southeast queues because these directions are equivalent to the direction of ice movement. The proximity of bedrock faults of tectonic origin strongly explains the occurrence of potholes, as the faults, like glacial moulins and crevasses, were favourable locations for meltwater flows. On average, there are more potholes in the sites located on the lee sides (downstream sides) compared with those on the stoss sides or central parts, because there were cavities and cracks as well as more space between the ice and the rock in the rough and plucked lee sides. The thin, non-uniform soil and the abundance of rock outcrops have led to the discovery of many more potholes in the lower southern parts of the study area and especially in the archipelago than in the higher northern parts. A significant proportion of potholes, which were not known to be located near bedrock faults, were located in the archipelago. In contrast, inland potholes typically occurred in steeper and rugged terrain near the fault lines. However, there was a weak negative correlation between the proximity of the eskers and the occurrence of potholes, which is probably due to the fact that the formation of potholes requires faster flows in glacier rivers than the formation of eskers. Most of the potholes in Uusimaa are quite small, up to about 120 centimetres in diameter and depth, but approximately one in eight potholes were more than three (3) meters deep or wide. The steepness of the slope was most strongly associated with the occurrence of large potholes. Potholes formed on rock consisted of granodiorite were, on average, larger than potholes formed on microcline granite, probably due to the granodiorite minerals are more prone to dissolve.