The west wall of large-scale 'upwellings' that develop in the bottomside of the nighttime equatorial F layer becomes structured by the wind-driven gradient drift instability, the same process that leads to the formation of striations in barium ion clouds. Upwellings are initiated by wavelike perturbations with long spatial wavelengths (~400-km) and are amplified by the collisional Rayleigh-Taylor instability (and sometimes assisted by the gradient drift instability in the case of an upward moving F layer). The west wall structuring process is driven by an eastward neutral wind enhanced by reduced drag during the postsunset hours and a velocity shear in east-west bulk plasma drift in the bottomside F layer. West wall structures evolve in a manner analogous to primary plasma bubbles, i.e., secondary plasma bubbles grow from the west wall. Their characteristics are compared with those of the primary bubbles and discussed in the light of existing theories.