By conducting point-by-point inscription in a continuously moving slab of pure fused silica at the optimal depth (170 μm depth below the surface), we have fabricated a 250 nm period nanostructure with 30 nJ, 300 fs, 1 kHz pulses from a frequency-tripled Ti:sapphire laser. This is the smallest value for the inscribed period yet reported, and has been achieved with radical improvement in the quality of the inscribed nanostructures in comparison with previous reports.