Relict Blockstreams at Insteheia, Valldalen‐Tafjorden, Southern Norway: Their Nature and Schmidt Hammer Exposure Age

Two small relict blockstreams occur at Insteheia, a col at 910 m asl on the watershed between Valldalen and Tafjorden (Møre og Romsdal), southern Norway. Both blockstreams display morphological and sedimentological characteristics indicative of boulder accumulations that have moved downslope by solifluction, probably under a permafrost climatic regime. These comprise preferred orientation and dip patterns of boulders; inverse grading, with surface boulders overlying successively finer, well‐sorted cobble, pebble and fine‐grained (sand/silt dominated) sediment layers; imbrication, with the packing of small boulders behind larger boulders; and proximity to boulder‐strewn hillslopes whose constituent boulders (organised into lobes and terraces) feed downslope into the blockstreams. Schmidt hammer exposure ages indicate that the blockstreams were last active during the Younger Dryas Stadial‐Holocene transition. Blockstream development probably began at ~ 15 ka, following the Last Glacial Maximum, and lasted for ~ 5 ka. Since then any fine‐grained material within the near‐surface parts may have been progressively removed. The relatively rapid development of the blockstreams suggests that larger‐scale forms of considerably greater age in the southern hemisphere may also have formed rapidly and have been inactive for long periods. Copyright © 2016 John Wiley & Sons, Ltd.