From last summer (2013) hiking tour in Fjallabak in Iceland ("Behind the mountain") my group came where this nice outcrop is close to the mountain road Dómadalsleið and is south of the road in Dómadalsháls, location N64.02442 W19.15686. There are two very prominent double ash layers. Hekla was suspected but at least I was not sure and Hekla 3 and Hekla 4 was assumed possible.
After some discussion it became clear it is from Hekla but not from Hekla 3 and Hekla 4 but layers referred to as Hekla X and Hekla Y and belove them should be the layer Hekla Z a thicker layer but not visible or present for some reason in this outcrop. Those layers are from rather small eruptions in Hekla dated in the period 1850 – 2800 BP 1. and looking very similar to those layers investigated by Meara (2012) at Helliskvísl.
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Referecnes
1. Rhian Hedd Meara, 2012. Geochemical fingerprinting of Icelandic silicic Holocene tephra layers. Doctor of philosophy thesis from College of Science and Engineering at the University of Edinburgh.