To the people who lived around it in the twelfth century, the great lake of Brecheiniog was not just a fishery. It was a sign that could be read. When the water of the lake, and of the Llynfi below it as far as the Wye opposite Glasbury, turned a deep green, people went to the old men of the country to ask what it meant. They were told it had happened before, a short time before Hywel ap Maredudd came and wasted the land. The colour came again, and war followed again, when the sons of Iestyn ravaged the province.
Gerald also heard that in his own day the lake had shown red, not all over but in thin streaks, as though blood were running through it in small channels. And in hard winters, when the lake froze, the ice gave out a horrible noise like a crowd of animals groaning. Gerald, who liked a marvel but also liked to seem reasonable, offered that this might just be the ice splitting and air working its way out.
Modern readers will think of algal blooms, which Llangorse still suffers from, and of iron-stained water after heavy rain. The name Gerald used for the lake, Clamosus or the noisy one, seems to have come from a misreading of the Welsh name of the river, Llynfi, as though it came from llef, a cry. The omen tradition is recorded here as he gave it.
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