Pillars in the Stream

十七日のつとめて、立つ。昔、しもつさの国に、まののてうといふ人住みけり。疋布（ひきぬの）を千むら、万むら織らせ、さらさせけるが家の跡とて、深き河を舟にて渡る。昔の門の柱のまだ残りたるとて、大きなる柱、河の中に四つたてり。人々歌読むを聞きて、心のうちに、

「朽ちもせぬ　　この河柱　　残らずは　　昔の跡を　　いかで知らまし」

On the morning of the 17th, we left. Long ago in Shimōsa there was a man they called "Rich Man Mano." They said the place where we crossed the deep river by boat was the very spot of the house where he had a hundred, a thousand, innumerable bolts of cloth woven and bleached. There were four huge pillars still in the river they said were the old door posts. While the others were composing poetry, I thought to myself,

"Were it not for these river-pillars still standing so long, how would we know anything about the past?"

Question: Does anyone know anything about the traditional manufacture of clothing? 織るoru definitely implies the weaving he had his hired help do, Mr. Mano, but 晒すsarasu? Is there some reason to expose newly woven cloth to the elements? "Bleach" doesn't quite satisfy me.

Notes: This "mura", acting as a kind of measuring word for counting vast, vast quantities of cloth, apparently comes from the mure/mura for "crowd, flock". I guess that makes sense, though I wish I had a better sense of the deep, deep original meaning of the word.