These are the Icicles of Misotsuchi in Oku-chichibu. They are built up very slowly by water dripping down. The best time to see these icicles is during the coldest part of the year, from about the middle of January to the middle of February. They're located in the in the Arakawa headwaters.
Icicles of Misotsuchi | Ootaki, Chichibu-Shi, Saitama-Ken | Chichibu