Geomorphology of MURTOOs - a Triangular-shaped Subglacial Landforms Produced During Rapid Retreat of Continental Ice Sheets

High-resolution digital elevation models based on LiDAR (Light Detection and Ranging) reveals subglacial bedforms in great detail. We describe here an ice-sheet scale distribution and characteristics of a distinct type of glacial landform that occurs commonly in a terrain covered by the Scandinavian Ice Sheet (SIS) in the northern Hemisphere during the last ice ages. We refer to these triangular or V-shaped landforms as murtoos (singular, 'murtoo'). Murtoos have previously been mapped as dead-ice hummocks, but their distribution and geomorphic relationships with eskers, flutes, ribbed and De Geer moraines indicate that they were formed subglacially during times of climate warming and rapid retreat of the SIS when large amounts of meltwater was delivered to the bed.

Murtoos occur in patches in an ice-flow parallel paths or corridors that are of subglacial meltwater origin. Fields of murtoos are commonly 0.5-4 km wide and long and often contain from a few to a few tens of murtoos, typically between 10 and 50 murtoos. Individual murtoos are less than hectare in area, typically 30-200 m in length and 30-200 m in width, and the relief of murtoos is commonly less than 5 m, but in cases up to 10-15 m. The tip angle of triangular-shaped murtoos is generally between 50° and 90° and the longitudinal profile of murtoos is asymmetric; in most cases the distal slope is steeper (up to 25-30°) than the proximal slope. Murtoos are composed primarily of loose diamicton with some sorted sandy to silty beds, and are often characterized with numerous surface boulders.

Supported by sedimentological evidence, we propose that murtoos are associated with an evolving distributed drainage system that developed towards the ice-sheet interior to evacuate large volumes of subglacial meltwater, derived original form surface melt, in high pressure conditions prior to the development of flow in large meltwater tunnels. Their SIS scale distribution can be explained by ice-retreat rate and glacial dynamics as they occur in Sweden and Finland in areas with rapid ice retreat, especially during the Bölling-Alleröd and the Early Holocene. In addition, murtoos are completely absent from northern Scandinavia where the SIS was cold-based during much of the Weichselian glaciation.