A long period of exceptionally high temperatures in Finland in the summer of 2015 was associated with the formation of large ‘exfoliation’ or ‘sheeting’ fractures on Långören Island in the Finnish Archipelago. A video of the event shows sharp fractures forming along the edge of a thin bedrock sheet several meters across (Sarpaneva 2014), while total vertical strain of one section of the pop-up was up to 100 mm, and observations following the event indicate a slab of approximately 50 m2 was detached from the underlying bedrock. Long scratch marks (‘striations’) visible on the bedrock surface of the island are the result of boulders being dragged over the landscape during the last glacial period (>15 ka BP), hinting at the rarity of the recent events on the otherwise undamaged surfaces (Fig. 1).
Deglaciation of the region is bracketed by two moraine complexes dated at 15.2–13 ka BP and 13–11.5/6 ka BP, while flow lineations based on a reconstructed ice sheet topology and regional field mapping of drumlins indicate the present-day island was located on the eastern margin of a southward- flowing ice stream sourced in the center of the Weichselian Scandinavian ice sheet (Boulton et al. 2004, and references therein). The ice sheet in this region was around 2500 m thick during the estimated maximum for the Last Glacial Maximum at approximately 18.5 ka BP (Näslund 2006). Present-day isostatic uplift rates relative to sea level in the region are approximately 3.5–4 mm/a (Ekman 1996). Tidal variations in the archipelago only cause a few centimeters of sea level change, and as a result, the slow emergence of the now 3.5 m – 4 m. a.s.l. bedrock surface can reasonably accurately be extrapolated back to 0.8–1 ka BP.