Water and stone both flow. That the tephra cone (Eldborg) and the stream (Bólulækur) are the same colour on this June day is part of the mystery.
Both are coloured by the sky, which gains its colour by heated oxygen, which, to complete the pattern, is (more or less) on fire. The skill at recognizing these correspondences are one of the ways in which poetry adds to human knowledge of the world, and maintains it. Once you have made this realization, you will harm neither stream nor mountain.