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We analyze a time series of high resolution observations near the solar limb, obtained in Hα and the Mg b1 line. We identified arch-shaped dark mottles, which are thin, faint Hα structures observable under very good seeing conditions, best seen in Hα +0.75 Å. Their mean length is about 15″, their mean height about 6″ and indicative lifetimes is of the order of 5 min. They show negative (away from the observer) line-of-sight velocities. A possible interpretation is that material flows from the apex towards the feet of the arches.