These are small conical piles of ash or ground rock. This is supposed to be mostly tephra from the last eruption of Katla in 1918, the volcano under the main glacier Myrdalsjökull. The ash/rock dust originally would have been fairly evenly throughout the snow or ice, but some kind of iterative process of differential melting would have concentrated it over the course of many seasons. This might have occurred by the black dust lowering the albedo, so that where it was more concentrated, the snow melted quicker in the summer, further concentrating the dust/ash in hollows or pits in the snow. The dust piles would be left in relief as the snow melted by warm air around them during warmer summer days. This process might be repeated after winter snowfall and summer melt over many years. We saw something similar on snowfields in Kerlingarfjall