High-resolution digital elevation models of Finland and Sweden based on LiDAR (Light Detection and Ranging) reveal subglacial landforms in great detail. We describe the ice-sheet scale distribution and morphometric characteristics of a glacial landform that is distinctive in morphology and occurs commonly in the central parts of the former Scandinavian Ice Sheet, especially up-ice of the Younger Dryas end moraine zone. We refer to these triangular or V-shaped landforms as murtoos (singular, ‘murtoo’). Murtoos are typically 30–200 m in length and 30–200 m in width with a relief of commonly <5 m. Murtoos have straight and steep edges, a triangular tip oriented parallel to ice-flow direction, and an asymmetric longitudinal profile with a shorter, but steeper down-ice slope. The spatial distribution of murtoos and their geomorphic relation to other landforms indicate that they formed subglacially during times of climate warming and rapid retreat of the Scandinavian Ice Sheet when large amounts of meltwater were delivered to the bed. Murtoos are formed under warm-based ice and may be associated with a non-channelized subglacial hydraulic system that evacuated large discharges of subglacial water.

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Murtoos are recently discovered triangular-shaped subglacial landforms that form under warm-based ice and in association with significant subglacial meltwater flow. They appear in distinct fields and commonly occur in the area that was covered by the Fennoscandian Ice Sheet during glacial periods. Murtoos potentially represent a transition form from non-channelized to channelized subglacial drainage networks. In the present study, we analyse and classify murtoos and murtoo-related landforms in the Finnish area of the Fennoscandian Ice Sheet based on their characteristics and appearance in LiDAR-based digital elevations models. Combined with morphometric analyses, the observations suggest that five types of murtoos and murtoo-related landforms are common and widespread in Finland: (i) triangle-type murtoos (TTMs), (ii) chevron-type murtoos (CTMs), (iii) lobate-type murtoos (LTMs), (iv) murtoo-related ridges and escarpments (MREs), and (v) other murtoo-related polymorphous landforms (PMRs) that look like small mounds and ridges. The morphometric characteristics of the different types are described here in detail, and it is shown that they are spatially and geomorphologically related. In addition, we provide examples of murtoos other than the TTMs to demonstrate that different murtoo types and murtoo-related landforms are composed of similar sediments and architectural characteristics. The diversity of murtoo landforms and the transition between distinct murtoo types indicate rapid and complicated variations in the configuration of subglacial hydrology at different spatial and temporal scales. This study emphasizes the essential role of subglacial meltwater in the shaping of glacial landscapes and the redistribution of large volumes of sediments during the deglaciation of the Fennoscandian Ice Sheet.

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A novel high-resolution (2-4 m source and receiver spacing) reflection and refraction seismic survey was carried out for aquifer characterization and to confirm the existing depositional model of the interlobate esker of Virttaankangas, which is part of the Säkylänharju-Virttaankangas glaciofluvial esker-chain complex in southwest Finland. The interlobate esker complex hosting the managed aquifer recharge (MAR) plant is the source of the entire water supply for the city of Turku and its surrounding municipalities. An accurate delineation of the aquifer is therefore critical for long-term MAR planning and sustainable use of the esker resources. Moreover, an additional target was to resolve the poorly known stratigraphy of the 70-100-m-thick glacial deposits overlying a zone of fractured bedrock. Bedrock surface as well as fracture zones were confirmed through combined reflection seismic and refraction tomography results and further validated against existing borehole information. The highresolution seismic data proved successful in accurately delineating the esker cores and revealing complex stratigraphy from fan lobes to kettle holes, providing valuable information for potential new pumping wells. This study illustrates the potential of geophysical methods for fast and cost-effective esker studies, in particular the digital-based landstreamer and its combination with geophone-based wireless recorders, where the cover sediments are reasonably thick.

Mäkinen, J. 2014 (January): Sedimentology of an ice lobe margin esker with implications for the deglacial dynamics of the Finnish Lake District lobe trunk.We provide evidence for the subglacial to ice-marginal successive deposition of the Lohtaja-Kivijärvi ice lobe margin esker influenced by the changes in the meltwater delivery and proglacial water depth within the Finnish Lake District lobe trunk during the last deglaciation in Finland. The study is mostly based on the sedimentological data from the 100 km long esker chain with 15 logged sites. The long breaks in the lobe margin esker and the re-emerged deposition along the stable position of the subglacial meltwater route were related to the discontinuities and reappearances of the neighbouring eskers. This considerable variability in the meltwater discharge and debris transport under the described deglacial conditions cannot be explained by markedly decreased meltwater production due to palaeoclimatic factors or lack of debris within the trunk region. The primary control on the changes in meltwater availability and related esker deposition was thus due to the spatial and temporal changes in ice mass properties and shifting of the meltwater flow paths within the trunk. These changes were initiated by the topographically higher and partly supra-aquatic Suomenselkä watershed area with subsequent deepening of the proglacial water during the deglaciation. The understanding of the long-lived esker deposition along the former ice-stream trunk margin adds to the evaluation of palaeoglaciological reconstructions and geomorphologically based spatial models for ice-stream landscapes.

During the last millennia of the Weichselian glaciation (c. 11 700-14 000 cal BP) and the first millennium of the Holocene interglacial (c. 10 500-11 700 cal BP), melting of the ice sheet and subsequent subglacial drainage resulted in the formation of murtoos and murtoo-related landforms in the Fennoscandian Ice Sheet area in Finland and Sweden. Murtoos are composed of clay-poor diamictons produced by sediment-concentrated creep and material sorting in a subglacial environment under effective pressure close to zero, and potentially represent a transition form from non-channelized to channelized subglacial drainage networks. Here, we explore the geomorphology and substratum characteristics of the Lake Murtoo area, one of the locations where murtoos were initially found and described in SW Finland. Our interpretation suggests an interconnected development from inefficient distributed or semi-distributed cavities (murtoos) to efficient channelized drainage (esker) via erosional escarpments, all displayed in the same region. In addition, our excavation into a murtoo-related erosional escarpment (MRE) revealed subglacial erosional channels alongside MREs that were filled with fine-grained finely laminated sediments. We suggest that these channel fill sediments were deposited subglacially, because they are covered by flow till and show spatial and lithological characteristics that deviate significantly from the proglacial basin fill clay (varve) sediments in the area. The appearance of rhythmically laminated silt and clay sediments indicates sedimentation in a suspension-load-dominated environment in shallow subglacial channels with relatively low (but repetitiously changing) water flow velocities, possibly reflecting disconnected spill-over routes of subglacial drainage in small tributaries that were only periodically used due to water pressure changes in the up-ice direction.

The purpose of this paper is to describe the internal structure and composition of recently discovered subglacial landforms called “murtoos” in order to interpret their formative processes and depositional environment. So far, murtoos have only been reported from Finland and Sweden, but they probably exist in all areas covered by past ice sheets. Murtoos mainly occur in fields along subglacial meltwater routes or corridors in close relation to eskers and ribbed moraine tracts. Murtoos were excavated perpendicular to the long axes of the triangular murtoo heads in six locations. Murtoos were found to be composed of silt/clay‐poor, sandy and gravelly diamictons interbedded with sorted sediments, and are suggested to be produced by pulsed, highly sediment‐concentrated flows during weak glaciotectonic deformation, indicating effective pressure close to zero. Murtoos can be divided into three main depositional units: 1) the core, 2) the murtoo body and 3) the murtoo mantle. The initial deposition of murtoos took place in a network of low canals and conduits or cavities with fluctuating stream flow likely over 50 km from the ice margin. Murtoos reveal an increasing influence of subglacial meltwater flow in rapidly widening broad and low conduits with increasing sediment transport over short distances. The results of this work suggest that murtoos were formed time‐transgressively over yearly meltwater cycles in a semi‐distributed drainage system not recognised before, in which was high‐pressure porewater conditions with rapid mobilisation of subglacial saturated sediments, a critical factor in the development of semi‐efficient drainage. Murtoos are suggested as missing element between distributed and channelised drainage systems not included in current glaciohydrological models or even in the theoretical basis of glacial hydrology.