The sporadic occurrence of layers of enhanced concentration of meteoric metals in the vicinity of the mesopause has been observed by lidar at many locations. These layers are much thinner than the background layer, last between a few minutes and many hours, and appear to be related to ionospheric sporadic E. A much more rare type of transient layer has been reported in the literature only once, by Kane et al. <2001>. The layers in question, observed in sodium by a lidar operating at Arecibo (18.3¿N, 66.7¿W), appear as C-shaped structures in the lidar height/time display. The structures, which have been observed at around 100 km, with durations around 30 min, appear not to be related to normal sporadic metals layers, and the Arecibo workers suggest that they might be produced by wave-breaking or Kelvin-Helmholtz billows. At S¿o Jos¿ dos Campos (23¿S, 46¿W) we observe C-structures in the sodium layer only very occasionally, but an analysis of their formation, together with simultaneous meteor winds measurements, suggests that the observed structures might be the result of wind-shear distortion of preexisting clouds of enhanced sodium concentration.