Nothing survives forever, not even rocks, not even the most rigid of structures. The erosion of that which has been solid is not necessarily bad: it can mean that something new is being created. The image is of ice floes, hardened in winter, dispersing in the warmth of spring; when the ice melts, a mighty river emerges.
Rigidity in the hearts of men tends to breed a separateness which can only be thawed by a greater force: typically, some strong spiritual stirring. The thawing of cold hearts brings good fortune.